UNIT 4 Electricity and magnetism

Question 1

magnetic forces are due to

Answer 1

due to interactions between magnetic fields

Two bar magnets can repel or attract

Question 2

left-hand = find force

right hand = find current

Question 3

exam tip

Answer 3

• adding arrows pointing away from the north pole and towards the south pole
• making sure the magnetic field lines are further apart as the distance from the magnet increases

Question 4

the relative strength of a magnetic field is represented by

Answer 4

represented by the spacing of the magnetic field lines

Question 5

Properties of magnets

types of magnets

Answer 5

• ends of a magnet are called poles
• have two poles: a north and a south
• Permanent magnets
  Induced (temporary) magnets

Question 6

Permanent Magnets

Answer 6

• made out of permanent magnetic materials, for example steel
• will produce its own magnetic field
  It will not lose its magnetism

Question 7

Temporary (Induced) Magnets

Answer 7

• When magnetic material is placed in a magnetic field, the material can TEMPORARILY be turned into a MAGNET: INDUCED magnetism

steel obj -> can be magnetised and will remain magnetic for a while

• other obj, such as electromagnets or transformers (;;soft iron) will be demagnetised as soon as the cause of the induced magnetism is removed

Question 8

When magnetism is induced on a material:

Answer 8

• One end of the material will become a north pole, the other end south pole
• Magnetic materials will always be attracted to a permanent magnet; so end of the material closest to the magnet will have the OPPOSITE pole to magnet’s pole closest to the material
• When magnetic material removed from magnetic field it will lose most/all of its magnetism quickly

Question 9

there are positive and negative charges

what repels what

FOR MAGNETIC materials (that aren’t magnets) - attracted to either pole, doesn’t matter if N or S

Answer 9

positive charges repel other positive charges,

negative charges repel other negative charges,

but positive charges attract negative charges
LIKE REPEL. OPP ATTRACT

Question 10

define magnetic material

Only a magnet can ____ another magnet

[Non-magnetic materials do not experience a force when placed in a magnetic field]

Answer 10

• experience a force when placed in a magnetic field
• attracted to a magnet when unmagnetised
• can be magnetised to form a magnet

only magnet can repel

Question 11

electrical charge, Q, measured in

charge is measured in

Answer 11

Coulomb (C)

Question 12

charging of solids by friction involves
ONLY

Answer 12

only a transfer of negative charge (electrons)

Question 13

electric field as …

• shown by electric field lines

TOWARDS NEG. CHARGES

Answer 13

a region in which an
electric charge experiences a force

• charged object creates an electric field around itself; experiences electrostatic force
• Fields lines always point AWAY from positive charges and towards negative charges

Question 14

direction of an electric field at a
point is …

Answer 14

the direction of the force on a positive
charge at that point

[i.e. “Field lines show the direction that a positive charge would experience if it was at that point”]

• in demonstrations it is always electrons (negative charges) which are free to move according to that force

Question 15

• strength of an electric field depends on the distance from the object creating the field:

Answer 15

[!!] field is strongest close to the charged object - shown by the field lines being closer together

[!!!] the field becomes weaker further away from the charged object - shown by the field lines becoming further apart

Question 16

Electric Field Patterns

• objs in electric field experience an electrostatic force [force=vector ∴ the direction of this force depends on whether the charges are the same or opposite]

The force is either ATTRACTIVE or REPULSIVE,
meaning…

Answer 16

• if charges are same (both pos./both neg.), this force = repulsive & the second charged object will move away from the charge creating the field
• if the charges are the opposite (negative and positive), this force will be ATTRACTIVE and the second charged object will move TOWARD the CHARGE CREATING THE FIELD

Question 17

size of the force depends on the strength of the field at that point

so FORCE BECOMES

Answer 17

• STRONGER as the distance between the two charged objects DECREASES

& weaker - as the distance between the two charged objects increases

[relatshp between strength of the force and the distance applies to both the force of attraction and force of repulsion, i.e.
two negative charges brought close together will have a stronger repulsive force than if they were far apart]

Question 18

complete recap:

1. field lines around a POINT CHARGE
2. field lines - Between Two Oppositely Charged Parallel Conducting Plates [uniform electric field]
3. field lines - Around a Charged Conducting Sphere [demo’ed using a Van der Graaff Generator, using streamers, small pieces of paper, polystyrene beads, aluminium foil containers]

Answer 18

1. field lines going towards neg. away pos., evenly spaced, with arrow showing direction (towards neg)
2. • field lines are:
     Directed from the positive to the negative plate
     Parallel
     Straight lines
3. field lines are - symmetrical, as with a point charge
   - bc the charges on the surface of the sphere are evenly distributed.
   charges are the same, so they repel.
   the surface is conducting, allowing them to move

so just arrows pointing away from pos. evenly spaced around circle

Question 19

electric current is related to the

happens bc…

Answer 19

flow
of charge

two oppositely charged conductors are connected together (by a length of wire), charge will flow between the two conductors

Question 20

direct current (d.c.) - straight horizontal line
and alternating current (a.c.) - parabola up and down dipping below x-axis too

difference?

voltage y-axis, time x-axis

Answer 20

direct: charge flow in 1 direction while in alternating the direction of charge flow changes regularly

Question 21

dc

ELECTRONS. NEG TO POS

[state in circuit whether current/electron flow is clockwise or anticlockwise]

Answer 21

produced when using dry cells and batteries (and sometimes generators, although these are usually ac)

The electrons flow in one direction only, from the negative terminal to the positive terminal

Question 22

ac

Answer 22

typically comes from mains electricity and generators

• needed for use in transformers in the National Grid

The direction of electron flow changes direction regularly
A typical frequency for the reversal of ac current in mains electricity is 50 Hz

Question 23

Define electric current

I = Q/t
Q = It

Answer 23

the [amount of] charge passing a
point [in a circuit] per unit time [per second]

charge/second CHARGE PER SECOND

Question 24

current eqn

Answer 24

I = Q/t
Current (A) = charge (C)/time (s)

current = power [of appliance] / [mains] voltage

Question 25

conventional current is from…

Answer 25

CONVENTIONAL current is from positive to negative

and that the flow of free electrons is
from negative to positive
clockwise/anticlockwise in circuit?

Question 26

🎨 Ammeters should always be connected in SERIES with the part of the circuit you wish to measure the current through

• bc ammeters measure the amount of charge passing through them per unit time, so the ammeter has to be in series so that all the charge flows through it

Question 27

AMMETER - analogue: Always double check exactly where the marker is before an experiment, if not at zero, you will need to subtract this from all your measurements. They should be checked for zero errors before using

• are also subject to parallax error
  Always read the meter from a position directly perpendicular to the scale

Answer 27

digital:

can measure very small currents, in mA or µA
show measurements as digits and are more accurate than analogue displays

• easy to use because they give a specific value and are capable of displaying more precise values
• may ‘flicker’ back and forth between values and a judgement must be made as to which to write down

Digital ammeters should be checked for zero error. Make sure the reading is zero before starting an experiment, or subtract the “zero” value from the end results

Question 28

Define electromotive force (e.m.f.) - voltage supplied by a power supply

Answer 28

the electrical work done by a source in moving a unit charge around a complete circuit

-measured in volts (V)

Question 29

eqn for emf

Answer 29

E = W/Q
same for voltage!

emf = converted energy [work] / positive charge

Question 30

Define potential difference (p.d.)

p.d. between two points is
measured in volts (V)

Answer 30

the work done by a unit charge passing through a component

Question 31

eqn for p.d.

Answer 31

V = W/Q

voltage (v) = work (J)/charge (C)

Question 32

🎨 Voltmeters are connected in PARALLEL with the component being tested

The potential difference is the difference in electrical potential between two points, therefore the voltmeter has to be connected to two points in the circuit

Question 33

VOLTMETER - analogue:

Analogue voltmeters subject to PARALLAX ERROR

• ALWAYS read the meter from a position directly perpendicular to the scale
  Typical ranges are 0.1-1.0 V and 0-5.0 V for analogue voltmeters although they can vary
• ALWAYS double check exactly where the marker is before use in an experiment, if not at zero, you will need to subtract this from all your measurements

Answer 33

digital: [SAME AS OTHER]

can measure very small potential differences, in mV or µV
- more accurate, easy to use,
- judgement must be made as to which to write down, check for zero error, make sure the reading is zero before starting an experiment, or subtract the “zero” value from the end results

Question 34

define resistance

eqn for Resistance

Answer 34

Resistance is the opposition to current

For a given potential difference, the higher the resistance, the lower the current
Therefore resistors are used in circuits to control the current

-> R = V/I

Question 35

ohm’s law

Answer 35

Current is directly proportional to potential difference as long as the temperature remains constant

Question 36

consequences of ohm’s law

Answer 36

• resistors are used in circuits to control either 1) the current in branches of the circuit (through certain components), or 2) the potential difference across certain components

due to consequences of ohm’s law -

The current in an electrical conductor decreases as its resistance increases (for a constant p.d.)

The p.d. across an electrical conductor increases as its resistance increases (for a constant current)

Question 37

following relationship for a
metallic electrical conductor:

Answer 37

(a) resistance is directly proportional to length - R inc., L inc.

(b) resistance is inversely proportional to
cross-sectional area - R inc., CS-A dec.

Question 38

I-V Graphs for Ohmic Resistors, Filament Lamps & Diodes [current on y-axis]

resistor: straight line with positive correlation, diagonal through origin

filament lamp: curved like ⁔/⁀ through origin

Answer 38

• because the resistor has a constant resistance
• for filament lamps,

The current INCREASES at a proportionally SLOWER rate than the potential difference

Question 39

The current INCREASES at a proportionally SLOWER rate than the potential difference

BECAUSE

Answer 39

The current causes the filament in the lamp to heat up

As the filament gets hot, its resistance increases

This opposes the current, causing it to increase at a slower rate

Question 40

I-V Graph for a Diode
I
FORWARD bias: looks like ╯, shows a sharp increase in voltage and current

reverse BIAS: where its horizontal+ straight line along x-axis like ____, graph shows a flat line where current is zero at all voltages

BECAUSE ;;

Answer 40

A diode is a non-ohmic conductor that allows current to flow in one direction only

The direction is shown by the triangular arrow of the diode symbol = forward bias

In the reverse direction, the diode has very high resistance, and therefore no current flows = reverse bias

Question 41

length & effect on R - directly proportional

cross-sectional area & its effect on R - inversely proportional

Answer 41

LENGTH - If the wire is longer, each electron will collide with more ions and so there will be more resistance:

SO: The longer a wire, the greater its resistance

AREA - If the wire is thicker (greater diameter) there is more space for the electrons and so more electrons can flow:

THEREFORE: The thicker a wire, the smaller its resistance

Question 42

electric circuits transfer…

Answer 42

transfer energy
from a source of electrical energy, such as an
electrical cell or mains supply, to the circuit
components and then into the surroundings

Question 43

eqn for electrical power

Answer 43

P = IV

Question 44

eqn for electrical energy

Answer 44

E = IVt
joules = amps x volts x seconds = ELECTRICAL energy

Question 45

The amount of energy an appliance transfers depends on:

• e.g.,

A 1 kW iron uses the same amount of energy in 1 hour as a 2 kW iron would use in 30 minutes
A 100 W heater uses the same amount of energy in 30 hours as a 3000 W heater does in 1 hour

Answer 45

📌 how long the appliance is switched on for

📌 the power of the appliance

Question 46

Define the kilowatt-hour (kWh)

eqn?

1kW = ___W

Answer 46

A unit of energy equivalent to one kilowatt of power expended for one hour

E = Pt

Where [e & p are × 10^3]
E = energy (kWh)
P = power (kW)
t = time (h)

[kilo (k) means 1000, so 1 kW = 1000 W]

Question 47

POWER ratings tell

Answer 47

The amount of energy transferred (by electrical work) to the device every second

Question 48

Since the usual unit of energy is joules (J), this is the 1 W in 1 s

Therefore:
1kWh =1000W x 3600s = 3.6 x 10^6 J

Since 1 kW = 1000 W and 1 h = 3600 s

📍 HOW TO CONVERT btwn Joules & kWh

Answer 48

kWh x (3.6 x 10^6) = J

J / (3.6x10^6) = kWh

Question 49

Energy Transfer in Electrical Circuits

• domestic appliances transfer energy from batteries
• household appliances transfer energy from the AC mains [mostly]

-[electric] to the kinetic energy of an electric motor [motors in vacuums, fridges, washmachin.)

electric to thermal ;; or heating devices: toasters, kettles, radiators

Question 50

circuit w/ closed-loop, i.e. a SERIES CIRCUIT [!!]

the CURRENT is the same value at any point/all components in a closed-loop have the same current

because …

& amount of current flowing around a series circuit depends on:
- The voltage of the power source
- The resistance of the components in the circuit

Answer 50

bc the number of electrons per second that passes through one part of the circuit is the same number that passes through any other part

&& & inc. voltage of power source drives more current around the circuit
So, decreasing the voltage of the power source reduces the current

• INC. the number of COMPONENT in the circuit INC. the total RESISTANCE
  Hence LESS CURRENT flows through the circuit

Question 51

SERIES CIRCUIT - POTENTIAL DIFFERENCE.

Answer 51

• combined EMF is equal to the sum of their individual EMFs; sum of potential differences across the components is equal to the total EMF of the power supply

Question 52

Current in Parallel Circuits [consists of two or more components attached along separate branches of the circuit]

advantages of parallel;;

what happens to current in parallel?

Answer 52

• the components can be individually controlled, using their own switches
• if one component stops working the others will continue to function

;; the current splits up, so the current in each branch will be smaller than the current from the power supply

Question 53

Potential Difference in Parallel Circuits?

Answer 53

potential difference across each component connected in parallel is the same

Question 54

At a JUNCTION in a PARALLEL circuit (where two or more wires meet), the current is conserved, MEANING…

Answer 54

the amount of current flowing into the junction is equal to the amount of current flowing out of it

bc CHARGE is conserved

current does this bc it is the flow of electrons:

📌 electrons are physical matter – they cannot be created or destroyed.
📌 the total number of electrons (and hence current) going around a circuit must remain the same.
📌 when the electrons reach a junction, however, some of them will go one way and the rest will go the other

Question 55

is current always split equally in parallel circuits

Answer 55

the current does not always split equally – often there will be more current in some branches than in others

The current in each branch will only be identical if the REISTANCE of the components along each branch are IDENTICAL

Question 56

resistors in series -

When two or more components are connected in series:

Answer 56

The combined resistance of the components is equal to the sum of individual resistances

I.E.;; add them together

Question 57

combined resistance

of 2 resistors

in parallel

is… ____ than that of either one alone.

eqn:

Answer 57

the combined resistance of two
resistors in parallel is less than that of either resistor by itself

[If two resistors of equal resistance are connected in parallel, then the combined resistance will halve]

;;; EQN! 1/R = 1/R1 + 1/R2

Question 58

Determining Resistance in Parallel

To calculate the resistance:
First find the value of 1/R (by adding 1/R1 + 1/R2)

Next find the value of R by using the reciprocal button on your calculator (labelled either x-1 or 1/x, depending on your calculator)

Question 59

Recall and use in calculations, the fact that:

3 facts

Answer 59

(a) the sum of the currents entering a junction in a parallel circuit is equal to the sum of the currents that leave the junction

Question 60

b)

Answer 60

(b) the total p.d. across the components in
a series circuit is equal to the sum of the
individual p.d.s across each component

Question 61

c)

Answer 61

c) the p.d. across an arrangement of parallel resistances is the same as the p.d. across one branch in the arrangement of the parallel resistances

Question 62

Explain that the sum of the currents into a
junction is…

Answer 62

the same as the sum of the currents
out of the junction

Question 63

p.d. across conductor…

Answer 63

the p.d. across an electrical conductor
increases as its resistance increases for a constant current

Question 64

the equation for two resistors used
as a potential divider

📌 potential divider eqn

• • potential divider…,
    [used widely in volume controls and sensory circuits using LDRs and thermistors,]

Answer 64

R1/R2 = V1/V2

📌 V out = R2 / (R1+R2) x V in

[numerator has to be the resistance of the resistor over/next to/in series with V out]
V in = power supply voltage

+ - are circuits which produce an output voltage as a fraction of its input voltage

• have two main purposes:
  To provide a variable potential difference
  To enable a specific potential difference to be chosen
  To split the potential difference of a power source between two or more components

Question 65

Variable Potential Dividers

When two resistors are connected in series,…

Answer 65

the potential difference across the power source is shared between them

Question 66

potential difference across each resistor depends…

Answer 66

… depends upon its resistance:

• The resistor with the LARGEST RESISTANCE will have a GREATER POTENTIAL DIFFERENCE than the other one [from V=IR]
• If the resistance of one of the resistors is increased, it will get a greater share of the potential difference, whilst the other resistor will get a smaller share

Question 67

what is a potentiometer

Answer 67

• a kind of variable resistor
• a single component that (in its simplest form) consists of a coil of wire with a sliding contact, midway along it

w/ a terminal A and terminal B

Question 68

sliding contact

Answer 68

has the effect of separating the potentiometer into two parts – an upper part and a lower part – both of which have different resistances

Question 69

Moving the slider (as seen in variable resistor symbol) changes the resistances (& ∴ potential differences) of the upper and lower parts of the potentiometer

Answer 69

If slider moved upwards, the resistance of the lower part will increase

& ∴ the potential difference across it will also increase

Question 70

Resistors as Potential Dividers

• Kirchhoff’s Second Law …

Answer 70

When two resistors are connected in series, the potential difference across the power source is divided between them

Question 71

📍 input voltage V in is applied to the top and bottom of the series resistors

📍 The output voltage V out is measured from the centre to the bottom of resistor R2

📍 In potential divider circuits, the p.d across a component is proportional to its resistance from V = IR

Question 72

When thinking about potential dividers, remember that the higher the resistance the more energy it will take to ‘push the current through’ and therefore the higher the potential difference.

This means that if a component (often shown as a voltmeter in questions) needs to be switched on by a change such as increased light or temperature, then the resistor it is in parallel with needs to become larger compared to the other resistor.

Question 73

a mains circuit consists of

explain why a switch must be connected to the
live wire for the circuit to be switched off safely

Answer 73

consists of a live wire
(line wire), a neutral wire and an earth wire

Question 74

magnets

Answer 74

magnets have two poles: a north and a south
- objects which experience attraction and repulsion

Question 75

explain attraction & repulsion

Answer 75

Like poles repel (push each other apart)

Unlike poles attract (move towards each other)

Question 76

what are magnetic materials

Answer 76

• experience a force when placed in a magnetic field
• are attracted to a magnet when unmagnetised
• can be magnetised to form a magnet
• only a magnet can repel another magnet

Question 77

Series

Answer 77

Rt = r1 + r2 + r3 …

Question 78

Parallel

Answer 78

1/Rt = 1/r1 + 1/r2 + 1/r3…

1/Rt = 1/4
Rt = 4
add in series
4 + 5 + 3 = 12 ohms

so then I = V/R = 24/12 = 2 amps

Question 79

Kirchoff’s second law

• applies to voltage drops across components in a circuit

Answer 79

• states that around any closed loop in a circuit, the directed sum of potential differences across components is 0

Question 80

the magnetic field is always in the direction from North to South and current is always in the direction of a positive terminal to a negative terminal.

Answer 80

mag field - N to S

• current pos to neg

Question 81

uses of permanent magnets (steel, stay magnetised)

Answer 81

📍 Compasses: humans use for navigation, since needle always points north

📍 School lab experiments: magnets used in school science demos - permanent magnets

📍 Toys: toy trains and trucks often have magnets which attach the carriages or trailers to the engine or cab

📍 Fridge magnets: made either of flexible magnetic material or by sticking a magnet to the back of something

Question 82

uses of Electromagnets (use electricity to create magnet from a current-carrying wire; have adva. of being magnetised and demagnetised at the flick of a switch; can be switched on and off; soft iron; easily become a temporary magnet)

Answer 82

📍 MRI scanners: hospitals; large, cylindrical machine using powerful electromagnets to produce diagnostic images of the organs of the body

📍 Speakers (loudspeakers), microphones and earphones: used in phones and laptops use electromagnets to sense or send soundwaves

📍 Recycling: bc steel is a magnetic material it can be easily separated from other metals and materials using electromagnets. Once recovered the steel is re-used and recycled, reducing mining for iron ore and processing ore into steel

📍 Mag-Lev Trains: the ability of Mag-Lev trains to hover above the rails is due to them being repelled by large electromagnets on the train and track. This reduces friction and allows speeds of nearly 400 miles per hour

Question 83

magnetic elements

Answer 83

iron, steel (iron alloy), cobalt, nickel

Question 84

test whether a material is a magnet

Answer 84

• material brought close to a known magnet

repelled by the known magnet then the material itself is a magnet

If it can only be attracted and not repelled then it is a magnetic material

Question 85

define: magnetic field

(which all magnets are surrounded by)

Answer 85

The region around a magnet where a force acts on another magnet or on a magnetic material

(such as iron, steel, cobalt and nickel)

Question 86

define uniform magnetic field

Answer 86

• one that has the same strength and direction at all points

to show mag. fie. has same strength at all points - MUST: equal spacing between all magnetic field lines; same distance apart between the gaps of the poles

& acting in the same direction at all points - MUST: an arrow on each magnetic field line going from the north pole to the south pole

This field can be determined by using plotting compasses that will point from north to south or by using iron filings

Question 87

uniform magnetic field - NORTH to south

2 bar magnets can be used to produce this;
Point opposite poles (north and south) of the two magnets a few centimetres apart

Answer 87

A uniform magnetic field will be produced in the gaps between opposite poles

[Outside that gap, the field will not be uniform]

Question 88

Magnetic Field Lines

Answer 88

• used to represent the strength and direction of a magnetic field; direction of the magnetic field is shown using arrows

RULES - Magnetic field lines:

📍 Always go from north to south (indicated by an arrow midway along the line)
📍 Must never touch or cross other field lines

Question 89

Magnetic Field Around a Bar Magnet

Answer 89

• magnetic field is strongest at the poles
  This is where the magnetic field lines are closest together
• The magnetic field becomes weaker as the distance from the magnet increases
  This is because the magnetic field lines are getting further apart

Question 90

Magnetic Field Strength

• strength of the magnetic field is shown by the spacing of the magnetic field lines,

as in… ;;

Answer 90

If the magnetic field lines are CLOSE together then the magnetic field will be STRONG

If the magnetic field lines are far apart then the magnetic field will be weak

Question 91

describe a method of plotting the magnetic field around a bar magnet

1st way: Using Iron Filings

Answer 91

Place a piece of paper on top of the magnet
Gently sprinkle iron filings on top of the paper
Now carefully tap the paper to allow the iron filings to settle on the field lines

• surrounds, circles

Question 92

2nd: compass

Answer 92

Place the magnet on top of a piece of paper; draw around magnet

Draw a dot at one end of the magnet (near its corner)

Place a plotting compass next to the dot, so that one end of the needle of the compass points towards the dot

Use a pencil to draw a new dot at the other side of the compass needle

Now move the compass so that it points towards the new dot, and repeat the above process

Keep repeating until you have a chain of dots going from one end of the magnet to the other. Then remove the compass, and link the dots using a smooth curve – the magnetic field line

📍 The direction of the field line is the same as the direction of the plotting compass

You can now repeat the whole process several times to create several other magnetic field lines

Question 93

obj to use to know which end of magnet is north pole; explain

Answer 93

While both the spoon and horseshoe will be attracted to the magnet, only the compass will be able to identify which of the two poles is the north pole

[!!] AS the north arrow in the compass will align with the magnetic field of the magnet.

Question 94

Demonstrating Electrostatic Charges

• Electrostatic repulsion is caused by the _____ between charges

Answer 94

FORCE btwn charges

Question 95

simple exp. showing the production of electrostatic charges by FRICTION, insulating solids (e.g. plastics) are given a charge

• done using friction to transfer electrons from the surface. Removing electrons

which means…

Answer 95

by REMOVING electrons, which have negative charge, the insulator is left with a POSITIVE charge

Question 96

Method

Answer 96

Suspend one of the insulating materials using a CRADLE and a length of string so that the material can rotate freely

Rub one end of the material using a cloth (in order to GIVE it a CHARGE)

Now take a second piece of insulating material and charge that by rubbing with a cloth

Hold the CHARGED end of the second piece close to the charged end of the first piece:

• If the first piece rotates away (is repelled) from the second piece then the materials have the same charge
• If the first piece moved towards (is attracted to) the second piece then they have opposite charges

results: observations

Question 97

when desc a demo what should you do

Answer 97

should state a conclusion – in other words, explain what you expect to happen and what it means.

Question 98

what rods used

Answer 98

there will be friction between the cloth and the rods
only insulating materials can be charged by friction
glass and plastic rods should be chosen as they are insulating materials & copper and steel rods are made of conductive materials

Question 99

Investigating Conductors & Insulators

key difference

Answer 99

Conductors allow charge carriers to freely move. Insulators do not allow charge carriers to move

• reasons for this are to do with their internal structure

Question 100

Conductors

Answer 100

a material that allows charge (usually electrons) to flow through it easily

e.g., Silver, Copper, Aluminium, Steel (metals)

• made up of positively charged metal ions with their outermost electrons delocalised, ∴ the electrons are free to move
• Current is the rate of flow of charged particles. So, the more easily electrons are able to flow, the better the conductor

Question 101

insulators (rubber, plastic, glass, wood ALTHOUGH wood allows some charge to pass through them - conduct a little in the form of static electricity)

Answer 101

a material that has no free charges, hence does not allow the flow of charge through them very easily

Question 102

Investigating Electrical Conductors & Insulators

• Gold-leaf Electroscope (GLE); to distinguish btwn cond. and insul., used to demonstrate charge - CONSISTS OF…

[or as alt: electronic charge detector]

Answer 102

• A metal plate attached to one end of a metal rod
• At the other end of the rod a very thin leaf of gold foil is attached
• The rod is held by an insulating collar inside a box with glass sides, allowing the gold leaf to both be seen and protected from draughts

Question 103

GLE CHARGED,

rod and leaf…

Answer 103

When the GLE is charged, the plate, rod and gold leaf have the same charge (either positive or negative)

Since the rod and leaf have the same charge, they repel, and the leaf sticks out to the side

When the rod and leaf are discharged (are neutral) the leaf hangs down

Question 104

To Test Electrical Conductors and insulators

Answer 104

Charge the plate of the GLE so that the gold leaf stands clear of the rod

Carefully touch the plate of the GLE with the items being tested, for example:

Metals (wire, paperclip, scissor blades),
Non-metals (paper, fingers, glass, graphite), Plastics (plastic ruler, the handles of the scissors, finger in a plastic sandwich bag), Comparisons (wet cloth, dry cloth; finger and finger in a plastic sandwich bag)

Record the observations each time

Leaf falls: material is a good conductor

Leaf remains in place: object is a poor conductor (good insulator)

Leaf falls slowly: material is a poor conductor

Question 105

power
p.d.
current

SO electrical power: rate of change of work done; energy transferred per second in an electrical component

Answer 105

rate of doing work
work done per unit charge
rate of flow of charge

Question 106

eqns

Answer 106

P = E or W / t
P = IV

P = I^2R. P = V^2 /R.
“Twinkle Twinkle Little Star, Power equals I squared R”

Question 107

Circuit Components

• Power supplies

Answer 107

Cells, batteries, power supplies and generators all

SUPPLY CURRENT

to the circuit

Question 108

Resistors

Answer 108

Potential dividers, fixed and variable resistors, thermistors and light-dependent resistors (LDRs) are all used to

CONTROL CURRENT

Question 109

Meters

Answer 109

Ammeters and voltmeters are used to MEASURE the current and potential difference

• ammeters are always connected in series
• voltmeters are always connected in parallel

Question 110

Electromagnetic Components

Answer 110

Magnetising coils, relays and transformers USE ELECTROMAGNETIC EFFECTS

Relays use a SMALL current in ONE CIRCUIT to SWITCH on a much LARGER CURRENT in ANOTHER

Transformers ‘step up’ and ‘step down’ current and potential difference

Question 111

Fuses

• line right thru rectangle

Answer 111

Protect expensive components from current surges and act as a safety measure against fire

Question 112

Thermistors - [ _/ thru rectangle]

a non-ohmic conductor and a temperature-dependent resistor!

[opposite]

Answer 112

the resistance of a thermistor changes depending on its temperature

• as temperature increases the resistance of a thermistor decreases - high temp, low resistance

temp dec., resistance inc. - low temp, high resistance

Question 113

Light-dependent Resistors (LDR) -

a non-ohmic conductor and sensory resistor

Answer 113

Its resistance automatically changes depending on the light energy falling onto it (illumination)

As the light intensity increases, the resistance of an LDR decreases - more light, lower resistance

less light, higher resistance

Question 115

Diodes

[triangle arrow with line at tip, and line NOT going thru it but sometimes does]

Answer 115

a component that only allows a current in one direction

Question 116

If a diode is connected to an a.c. (alternating current) power supply…

Answer 116

it will only allow a current half of the time

• called rectification

Question 117

using 2 diodes, lamp & power supply, draw a circuit diagram that will provide the lamp with a rectified current

Answer 117

a.c. at top, diode with line at bottom, lamp, then diode with line at top

when pos. terminal is on the left, the diode stops the flow of current.

rectified current/circuit. lamp receives current every 2nd cycle or half the time

Question 118

using 2 diodes, lamp & power supply, draw a circuit diagram that will provide the lamp with a rectified current

Answer 118

a.c. at top, diode with line at bottom, lamp, then diode with line at top

when pos. terminal is on the left, the diode stops the flow of current.

rectified current/circuit. lamp receives current every 2nd cycle or half the time

Question 119

[card 72]; potentially lethal …

Mains electricity - 50 volts can be serious hazard. COMMON HAZARDS;;

Answer 119

📍 Damaged Insulation – If someone touches an exposed piece of wire, they could be subjected to a lethal shock

📍 Overheating of cables – Passing too much current through too small a wire (or leaving a long length of wire tightly coiled) can lead to the wire overheating. This could cause a fire or melt the insulations, exposing live wires

📍 Damp conditions – If moisture comes into contact with live wires, the moisture could conduct electricity either causing a short circuit within a device (which could cause a fire) or posing an electrocution risk

📍 Excess current from overloading of plugs, extension leads, single and multiple sockets when using a mains supply - If plugs or sockets become overloaded due to plugging in too many components the heat created can cause fires

Question 120

what is Mains electricity

Answer 120

the electricity generated by power stations and transported around the country through the National Grid

• we connect to the mains when plugging in an appliance such as a phone charger or kettle
• is an alternating current (a.c.) supply ;; does not have positive and negative sides to the power source - equivalent to positive and negative are called live and neutral and these form either end of the electrical circuit
• has a frequency of 50 Hz and a potential difference of about 230 V [vary 50-60Hz]

A frequency of 50 Hz means the direction of the current changes back and forth 50 times every second

Question 121

neutral, earth, live wires, fuse, cable grip, wires leading to metal cased appliance

wires purpose?

Answer 121

live and neutral wires deliver the electricity to the device.

The Earth wire is for safety

Question 122

Insulation & Double Insulation

Answer 122

conducting part of a wire is usually made of copper or some other metal
If this comes into contact with a person, this poses a risk of electrocution

For this reason, wires are covered with an insulating material, such as rubber

• Insulation around the wires themselves
  A non-metallic case that acts as a second layer of insulation = double insulation if there’s a metal case

Question 123

Double insulated appliances do not require…

Answer 123

an earth wire or have been designed so that the earth wire cannot touch the metal casing

Question 124

Earthing;

earth wire is an additional safety wire that can reduce risk of electrocution

Answer 124

Many electrical appliances have metal cases = potential safety hazard: If a live wire (inside appliance) came into contact with the case, the case would become electrified and anyone who touched it would risk being electrocuted

If this happens:
📍 The earth wire provides a low resistance path to the earth
📍 It causes a surge of current in the earth wire and hence also in the live wire
📍 The high current through the fuse causes it to melt and break
📍 This cuts off the supply of electricity to the appliance, making it safe

Question 125

fuse - a glass cylinder which contains a thin metal wire.

(typically 3A, 5A and 13A)

Answer 125

safety device designed to cut off the flow of electricity to an appliance if the current becomes too large (due to a fault or a surge)

• come in a variety of sizes so know how much current an appliance needs. always choose the next size up [e.g. 3.1 amp appliance - 3 amp too small, 13 amp too big, 5 amp fuse good]

Question 126

If the current in the wire becomes too large…

Answer 126

The wire heats up and melts

This causes the wire to break, breaking the circuit and stopping the current

Question 127

trip switch [found in the Consumer Box (where the electricity enters the building)]

how it works?

Answer 127

When the current is too high the switch ‘trips’ (automatically flicks to the off position)

This stops current flowing in that circuit

Question 128

Induced EMF

A conductor, such as a wire, cuts through a magnetic field’s lines

The direction of a magnetic field through a coil changes and conductor is stationary in it

used in:
Electrical generators which convert mechanical energy to electrical energy
Transformers which are used in electrical power transmission

Answer 128

an EMF will be induced in a conductor [wire] if there is

• RELATIVE MOVEMENT between the conductor [wire] and the magnetic field.

📍 For an electrical conductor moving in a fixed magnetic field, the conductor (e.g wire) cuts through the fields lines, inducing an EMF in the wire.

• also be induced if the conductor is stationary in a changing magnetic field

Question 129

e.g - When the magnet enters the coil, the field lines cut through the turns/wire, inducing an EMF

Answer 129

For a fixed conductor in a changing magnetic filed

📍 As the magnet moved through the conductor (e.g. a coil), the field lines cut through the turns on the conductor (each individual wire)

📍 This induces an EMF in the coil

Question 130

how to measure emf

Answer 130

A sensitive voltmeter can be used to measure the size of the induced EMF

If the conductor is part of a complete circuit then a current is induced in the conductor

This can be detected by an ammeter

Question 131

no deflection of needle in voltmeter

Answer 131

C is correct because there the magnet is stationary

This means there is no relative movement between the coil and the magnetic field, therefore there are no magnetic field lines being cut

If the magnetic field lines are not being cut then there will not be a potential difference induced

Question 132

Lenz’s Law -

Answer 132

The direction of an induced potential difference always opposes the change that produces it

• means that any magnetic field created by the potential difference will act so that it tries to stop the wire or magnet from moving

Question 133

Demonstrating Lenz’s Law

Answer 133

If a magnet is pushed north end first into a coil of wire then the end of the coil closest to the magnet will become a north pole

📍 Explanation

Due to the generator effect, a potential difference will be induced in the coil

The induced potential difference always opposes the change that produces it

The coil will apply a force to oppose the magnet being pushed into the coil

Therefore, the end of the coil closest to the magnet will become a north pole

This means it will repel the north pole of the magnet

Question 134

then…

Answer 134

If a magnet is now pulled away from the coil of wire then the end of the coil closest to the magnet will become a south pole

📍 Explanation:

Due to the generator effect, a potential difference will be induced in the coil

The induced potential difference always opposes the change that produces it

The coil will apply a force to oppose the magnet being pulled away from the coil

Therefore, the end of the coil closest to the magnet will become a south pole

This means it will attract the north pole of the magnet

Question 135

Right-Hand Dynamo Rule

^ When moving a wire through a magnetic field, the direction of the induced EMF can be worked out by using THIS RULE.

THUMB - THRUST [direction wire]
FIRST FINGER - FIELD
SECOND FINGER - CURRENT

Answer 135

Start by pointing the first finger (on the right hand) in the direction of the field

Next, point the thumb in the direction that the wire is moving in

The Second finger will now be pointing in the direction of the current (or, strictly speaking, the EMF)

Question 136

Demonstrating Induction

Experiment 1: Moving a magnet through a coil

Answer 136

When a coil is connected to a sensitive voltmeter, a bar magnet can be moved in and out of the coil to induce an EMF

• expected results are:

📍 When the bar magnet is NOT MOVING, the voltmeter shows a ZERO reading

📍 When the bar magnet is held STILL inside, or OUTSIDE , the coil, there is NO CUTTING of magnetic FIELD LINES, so, there is NO EMF induced

📍 When the bar magnet begins to move inside the coil, there is a READING on the voltmeter. As the bar magnet moves, its magnetic field lines ‘cut through’ the coil. This induces an EMF within the coil, shown momentarily by the reading on the voltmeter

📍 When the bar magnet is taken back out of the coil, an e.m.f is induced in the opposite direction (a result of Lenz’s law)
As the magnet changes direction, the direction of the current changes
The voltmeter will momentarily show a READING with the OPPOSITE SIGN

📍 Increasing the SPEED of the magnet induces an e.m.f with a HIGHER MAGNITUDE
The direction of the electric current, and e.m.f, induced in the conductor is such that it opposes the change that produces it = Lenz’s law

Question 137

Factors that will increase the induced EMF are

1. when moving a magnet through a coil
2. when moving a wire through a magnet

Answer 137

1. Moving the magnet faster through the coil

📍📍 [USE THIS WORDING:] Adding more turns to the coil

Increasing the strength of the bar magnet

📌 2.
Increasing the length of the wire

Moving the wire between the magnets faster

Increasing the STRENGTH of the magnets

Question 138

Experiment 2: Moving a wire through a magnet

When a long wire is connected to a voltmeter and moved between two magnets, an EMF is induced

The pattern of a magnetic field in a wire can be investigated using this set up
[no current flowing through the wire to start with]

Answer 138

expected results are:

📍 When the wire is not moving, the voltmeter shows a zero reading. When the wire is held still inside, or outside, the magnets, the rate of change of flux is zero, so, there is no EMF induced

📍 As the wire is moved through between the magnets, an EMF is induced within the wire, shown momentarily by the reading on the voltmeter. As the wire moves, it ‘cuts through’ the magnetic field lines of the magnet, generating a change in magnetic flux

📍 When the wire is taken back out of the magnet, an EMF is induced in the opposite direction. As the wire changes direction, the direction of the current changes
The voltmeter will momentarily show a reading with the opposite sign

📍 As before, the direction of the electric current, and e.m.f, induced in the conductor is such that it opposes the change that produces it

Question 139

magnitude (size) of the induced EMF is determined by:

Answer 139

the speed at which the wire, coil or magnet is moved

the number of turns on the coils of wire

the size of the coils

the strength of the magnetic field

Question 140

direction of the induced potential difference is determined by:

Answer 140

The orientation of the poles of the magnet [Reversing the direction in which the wire, coil or magnet is moved]

Question 141

speed at which the wire, coil or magnet is moved:

Answer 141

Increasing the speed will increase the rate at which the magnetic field lines are cut

This will increase the induced potential difference

Question 142

number of turns on the coils in the wire:

Answer 142

INC. the number of turns on the COILS in the wire will INC. the potential difference induced

• bc each coil will cut through the magnetic field lines and the total potential difference induced will be the result of all of the coils cutting the magnetic field lines

Question 143

size of the coils:

Answer 143

Increasing the area of the coils will increase the potential difference induced

This is because there will be more wire to cut through the magnetic field lines

Question 144

strength of the magnetic field:

Answer 144

Increasing the strength of the magnetic field will increase the potential difference induced

Question 145

Simple A.C Generators;

generator effect can be used to generate ALTERNATING CURRENT in an alternator, varying in size and direction as the coil rotates & emf induced when coil rotates in the external magnetic field

[simple alternator: type of generator that produces an alternating current; a rotating coil in a magnetic field connected to slip rings]

Answer 145

📍 a rectangular coil is forced to spin in a uniform magnetic field; coil is connected to a centre-reading meter by metal brushes that press on two metal slip rings

📍 The slip rings and brushes provide a continuous connection between the coil and the meter

📍 The coil turns in one direction:
1) The pointer defects first one way, then the opposite way, and then back again
This is because the coil cuts through the magnetic field lines and an EMF, and therefore current, is induced in the coil

📍 The pointer deflects in both directions because the current in the circuit repeatedly changes direction as the coil spins. This is because the induced EMF in the coil repeatedly changes its direction. This continues on as long as the coil keeps turning in the same direction

The induced EMF and the current alternate because they repeatedly change direction

Question 146

difference btwn motor and generator

Answer 146

A motor takes in electricity and turns it into motion.

A generator takes in motion and generates electricity.

Question 147

Graphs for A.C. Generators

Answer 147

When the number of field lines through the coil is at a maximum, induced e.m.f. is at a minimum; number of field lines through the coil is at a maximum and induced e.m.f. is zero in this position

When the number of field lines through the coil is at a minimum, induced e.m.f. is at a maximum; no field lines pass through the centre of the coil

Question 148

an alternating current can be produced by:

Answer 148

A coil rotating in a magnetic field

A magnet rotating within a coil

Both will induce an e.m.f. in the coil as they both ensure the coil will experience a changing magnetic field.

Question 149

Magnetic Field Around Wires & Solenoids -

occurs when…

Answer 149

When a current flows through a conducting wire [any wire that has current flowing through it] a magnetic field is produced around the wire

no current = no magnetic field

-The shape and direction of the magnetic field can be investigated using plotting compasses. The compasses would produce a magnetic field lines pattern that would like look circles anticlockwise - made up of concentric circles

Question 150

circular field pattern indicates that…

Answer 150

the magnetic field around a current-carrying wire has no poles

Question 151

As the distance from the wire increases…

Answer 151

the circles get further apart

shows that the magnetic field is strongest closest to the wire and gets weaker as the distance from the wire increases

• Increasing the amount of current flowing through the wire = increases the strength of the magnetic field = field lines will become closer together

right-hand thumb rule can be used to work out the direction of the magnetic field around the wire

dot = current out, cross = current in

Question 152

Reversing the direction in which the current flows through the wire will …

Answer 152

reverse the direction of the magnetic field

Question 153

Magnetic Field Around a Solenoid

Answer 153

When a wire is looped into a coil, the magnetic field lines CIRCLE AROUND each part of the COIL , passing through the centre of it

• To INC. the STRENGTH of the magnetic field around the wire it should be COILED to form a SOLENOID

The magnetic field around the solenoid is similar to that of a bar magnet

Question 154

magnetic field inside the solenoid is strong and uniform

HOW DO the ends of the solenoid behave?

[Look for a coil / solenoid - this is going to act as an electromagnet
Look for a piece of iron - this will be attracted to the solenoid]

Answer 154

One end of the solenoid behaves like the north pole of a magnet; the other side behaves like the south pole

To work out the polarity of each end of the solenoid it needs to be viewed from the end

If the current is travelling around in a clockwise direction then it is the south pole

If the current is travelling around in an anticlockwise direction then it is the north pole

If the current changes direction then the north and south poles will be reversed

If there is no current flowing through the wire then there will be no magnetic field produced around or through the solenoid

Question 155

Magnetic Effects of Changing Current -

solenoid can be used as an electromagnet by…

Answer 155

adding a soft iron core ;; solenoid wrapped around a soft iron core

iron core will become an induced magnet when current is flowing through the coils

magnetic field produced from the solenoid and the iron core will create a much stronger magnet overall

magnetic field produced by the electromagnet can be switched on and off

📍 When the current is flowing there will be a magnetic field produced around the electromagnet

📍 When the current is switched off there will be no magnetic field produced around the electromagnet

Question 156

as always… changing the direction of the current also changes …

Answer 156

the direction of the magnetic field produced by the iron core

Question 157

Factors Affecting Magnetic Field Strength

Answer 157

📌 The strength of the magnetic field produced around a solenoid can be increased by:

Increasing the size of the current which is flowing through the wire

Increasing the number of coils

Adding an iron core through the centre of the coils

📌 The strength of an electromagnet can be changed by:

Increasing the current will increase the magnetic field produced around the electromagnet

Decreasing the current will decrease the magnetic field produced around the electromagnet

Question 158

Applications of the Magnetic Effect

Answer 158

📍 Relay circuits (utilised in electric bells, electronic locks, scrapyard cranes etc)

📍 Loudspeakers & headphones

Question 159

Electromagnets are commonly used in-

RELAY CIRCUITS

what are relays? what do relay circuits consist of?

Answer 159

relays are switches that open and close via the action of an electromagnet

• An electrical circuit containing an electromagnet
• A second circuit with a switch which is near to the electromagnet in the first circuit

Question 160

how they are used

Answer 160

When a current flows through Circuit 1, a magnetic field is induced around the coil
The magnetic field attracts the switch, causing it to pivot and close the contacts in Circuit 2. This allows a current to flow in Circuit 2

When no current flows through Circuit 1, the magnetic force stops. The electromagnet stops attracting the switch. The current in Circuit 2 stops flowing

Question 161

real life use

Answer 161

Scrapyard cranes utilise relay circuits to function:

When the electromagnet is switched on it will attract magnetic materials

When the electromagnet is switched off it will drop the magnetic materials

Question 162

Electric bells also utilise relay circuits to function:

As the current alternates, the metal arm strikes the bell and drops repeatedly to produce the ringing effect

Answer 162

When the button K [right button] is pressed:

A current passes through the electromagnet E creating a magnetic field

This attracted the iron armature A, causing the hammer to strike the bell B

The movement of the armature breaks the circuit at T

This stops the current, destroying the magnetic field and so the armature returns to its previous position

This re-establishes the circuit, and the whole process starts again

Question 163

Loudspeakers & Headphones

Answer 163

• convert electrical signals into sound
• work due to the motor effect
• loudspeaker consists of a coil of wire which is wrapped around one pole of a permanent magnet
• An alternating current passes through the coil of the loudspeaker, creates a changing magnetic field around the coil

Question 164

As the current is constantly changing direction, the direction of the magnetic field will be constantly changing

The magnetic field produced around the coil interacts with the field from the permanent magnet

The interacting magnetic fields will exert a force on the coil. The direction of the force at any instant can be determined using Fleming’s left-hand rule

Answer 164

As the magnetic field is constantly changing direction, the force exerted on the coil will constantly change direction. This makes the coil oscillate

The oscillating coil causes the speaker cone to oscillate. This makes the air oscillate, creating sound waves

Question 165

Investigating the Field Around a Wire - The magnetic field patterns due to currents in straight wires and in solenoids can be investigated using:

Answer 165

A thick wire

A solenoid (a wire wrapped into a coil) - for example, a metal slinky

Cell, ammeter, variable resistor and connecting wires

Cardboard with holes (the holes must be large enough for the wire to fit through)

Clamp stand

Iron filings or a compass

• Spread the iron filings uniformly on the cardboard and place the magnetic needle on the board

Tap the cardboard slightly and observe the orientation of iron filings

Question 166

When the current direction is reversed, the compasses point…

Answer 166

point in the opposite direction showing that the direction of the field reverses when the current reverses

Question 167

Experiment 1: Plotting the magnetic field around a wire

Answer 167

📍 Attach the thick wire through a hole in the middle of the cardboard and secure it to the clamp stand

Secure the wire vertically so it sits perpendicularly to the cardboard

Attach the ends of the wire to a series circuit containing the variable resistor and ammeter on either side of the cell

📍📍 Using plotting compasses:

Place plotting compasses on the card and draw dots at each end of the needle once it settles

Make sure to draw an arrow to show the direction of the field at different points

Move the compass so that it points away from the new dot, and repeat the process above

Keep repeating the previous process until there is a chain of dots on the card

Then remove the compass, or compasses, and link the dots using a smooth curve – this will be the magnetic field line

Repeat the whole process several times to create several other magnetic field lines

📍 📍📍 Using iron filings:

If using iron filings, simply pour the filings onto the cards and gently shake the card until the filings settle in the pattern of the magnetic field around the wire

Question 168

Experiment 2: Plotting the magnetic field around a solenoid

Answer 168

📍 Attach the thick wire through a hole on one side of the cardboard and loop it through a hole on the other side of the cardboard and secure it to the clamp stand

Secure the wire so it forms a circular loop around the cardboard

Attach the ends of the wire to a series circuit containing the variable resistor and ammeter on either side of the cell

📍 📍 Using plotting compasses:

Follow the procedure outlined in Experiment 1

Note: this can be carried out using a solenoid, but since a solenoid is essentially many circular loops, the pattern around a circular loop can be extended to give the pattern around a solenoid

📍 📍 📍 Using iron filings and a solenoid:

Take a solenoid (a metal slinky works well for this) and thread it through pre-made holes in a piece of card

Pour the filings onto the card and gently shake the card until the filings settle in the pattern of the magnetic field around the solenoid

Question 169

MOTOR EFFECT;; Force on a Current-Carrying Conductor (that produces its own magnetic field)

it will only experience a force when…

Answer 169

only experience a force if the current through it is perpendicular to the direction of the magnetic field lines
- current-carrying wire -> in a magnetic field -> experience force if wire perpendicular

direction of the FORCE is determined by FLEMING’S LEFT HAND RULE

Question 170

Two ways to reverse the direction of the force (and therefore, the copper rod) are by reversing…

Answer 170

The direction of the current

The direction of the magnetic field

Question 171

Left Hand Rule

direction of the force (aka the thrust) on a current carrying wire DEPENDS on the direction of the current and the direction of the magnetic field…

all three PERPENDICULAR to each other

Answer 171

• point to field first
• find direction of current
• determine direction of force

📌 RMBR: magnetic field is always in the direction from North to South AND current is always in the direction of a positive terminal to a negative terminal.

Question 172

Charged Particles in a Magnetic Field

current-carrying wire is placed in a magnetic field, it will experience a force if the wire is perpendicular BECAUSE…

Answer 172

bc the magnetic field exerts a force on each individual electron flowing through the wire [THE ELECTRON; in the case of a electron in a magnetic field the second finger points in the opposite direction to the direction of motion

when a charged particle (an electron) passes through a magnetic field, the field can exert a force on the particle, causing it to be DEFLECTED BY THE FIELD

The force is always at 90 degrees to both the direction of travel and the magnetic field lines

Question 173

particle is travelling _____, it experiences a _____ force

Answer 173

📍 If the particle is travelling perpendicular to the field lines:
It will experience the maximum force

📍 If the particle is travelling parallel to the field lines:
It will experience no force

📍 If the particle is travelling at an angle to the field lines:
It will experience a small force

Question 174

Electric Motors…

motor effect can be used to…

Answer 174

to create a simple d.c. electric motor. Simple dc motor consists: of a coil of wire (which is free to rotate) positioned in a uniform magnetic field

force on a current-carrying coil is used to make it rotate in a single direction

Question 175

current flows through coil…

Answer 175

As current flows through the coil, it produces a magnetic field which interacts with the uniform external magnetic field, so a force is exerted on the wire

Forces act in opposite directions on each side of the coil, causing a turning effect so it rotates [On the blue side of the coil, current travels towards the cell so the force acts upwards (using Fleming’s left-hand rule). On the black side, current flows away from the cell so the force acts downwards]

The greater the turning effect on the coil, the faster it will turn

The turning effect is increased by increasing:
The number of turns on the coil
The current in the coil
The strength of the magnetic field

Question 176

when in a dc motor is a complete circuit with a cell formed

Answer 176

when the coil of wire is horizontal,

Question 177

desc the setup

Answer 177

The coil is attached to a split ring (a circular tube of metal split in two) ;; split ring connects the coil to the flow of current

This split ring is connected in a circuit with the cell via contact with conducting carbon brushes

Question 178

what happens when the coil has rotated 90 degrees

Answer 178

Once the coil has rotated 90°, the split ring is no longer in contact with the brushes

No current flows through the coil so no forces act

• No force acts on the coil when vertical, as the split ring is not in contact with the brushes

Question 179

Even though no force acts,…

Answer 179

📍 the momentum of the coil causes the coil to continue to rotate slightly; current still flows anticlockwise and the forces still cause rotation in the same direction

📍 The split ring reconnects with the carbon brushes and current flows through the coil again; Now the blue side is on the right and the black side is on the left

📍 Current still flows toward the cell on the left and away from the cell on the right, even though the coil has flipped

📍 The black side of the coil experiences an upward force on the left and the blue side experiences a downward force on the right [FLIPS]

📍 The coil continues to rotate in the same direction, forming a continuously spinning motor

Question 180

Factors Affecting the D.C Motor

Answer 180

📍 The speed at which the coil rotates can be increased by:
Increasing the current
Use a stronger magnet

📍 The direction of rotation of coil in the d.c. motor can be changed by:
Reversing the direction of the current supply
Reversing the direction of the magnetic field by reversing the poles of the magnet

📍 The force supplied by the motor can be increased by:
Increasing the current in the coil
Increasing the strength of the magnetic field
Adding more turns to the coil

Question 181

dc motor set up - Determine whether the coil will be rotating clockwise or anticlockwise.

Answer 181

STEPS:

1. Draw arrows to show the direction of the magnetic field lines - from the north pole of the magnet to the south pole of the magnet
2. Draw arrows to show the direction the current is flowing in the coils. Current will flow from the positive terminal [LONG LINE ON LEFT] of the battery to the negative terminal
3. Use Fleming’s left hand rule to determine the direction of the force on each side of the coil ; direction of the force on each side of the coil, these should be in opposite directions because the directions of the current through each side are opposite.

Start by pointing your First Finger in the direction of the (magnetic) Field

Now rotate your hand around the first finger so that the seCond finger points in the direction of the Current

The THumb will now be pointing in the direction of the THrust (the force)

4. Use the force arrows to determine the direction of rotation

The coil will be turning clockwise

Question 182

transformer

Answer 182

a device used to change the value of an alternating potential difference or current

using the generator effect

Question 183

structure - a basic transformer consists of:

Answer 183

A primary coil [first] - copper (conductive, cost efficient)
A secondary coil
A soft IRON core [arrow pointing at top]

Question 184

why is iron used

Answer 184

easily magnetised

Question 185

Operation of a Transformer

Answer 185

📍 An alternating current is supplied to the primary coil.

📍 The current is continually changing direction. This means it will produce a changing magnetic field around the primary coil

📍 The iron core is easily magnetised, so the changing magnetic field passes through it

📍 As a result, there is now a changing magnetic field inside the secondary coil

📍 This changing field cuts through the secondary coil and induces a potential difference

📍 As the magnetic field is continually changing the potential difference induced will be alternating

📍 The alternating potential difference will have the same frequency as the alternating current supplied to the primary coil

📍 If the secondary coil is part of a complete circuit it will cause an alternating current to flow

Question 186

step-up transformer does what?

Answer 186

📍 increases the potential difference of a power source

A step-up transformer has more turns on the secondary coil than on the primary coil (Ns > Np)

Question 187

step-down transformer…

Answer 187

📍 decreases the potential difference of a power source

A step-down transformer has fewer turns on the secondary coil than on the primary coil (Ns < Np)

Question 188

output potential difference (voltage) of a transformer depends on:

Answer 188

number of turns on the primary and secondary coils

The input potential difference (voltage)

Question 189

OUTPUT POTENTIAL DIFFERENCE of a TRANSFORMER

📍 EQUATION

Answer 189

potential difference across primary coil / potential difference across secondary coil = number of turns on primary coil / number of turns on secondary coil

Vp / Vs = Np / Ns

• can be flipped to have secondary on top
• shows that ratio of the potential differences across the primary and secondary coils of a transformer is equal to the ratio of the number of turns on each coil

Question 190

example ques

an input voltage of 10V supplied to primary coil. output voltage of 40V produced across secondary coil. 10V supply at primary coil now replaced with 40V.

new output voltage across secondary coil =?

Answer 190

ratio of primary to secondary turns
10/40 = 0.25.

input voltage changes but number of turns on primary coil/secondary stays the SAME

40 / Vs = 0.25
Vs = 40/0.25 = 160V

Question 191

2nd example ques

output current =?
power =?

Answer 191

input current =0.050A
input power = I x V = 0.050 x 230 = 11.5W

input power = output power
11.5W = output power.

P out = Is x Vs
Vs = 6.0V output.

11.5 = Is x 6
Is = 1.9A

Question 192

NOTE to rmbr

Answer 192

individual loops of wire going around each side of the transformer should be referred to as turns and not coils.

Question 193

Transformer Efficiency - ideally, 100% efficient, meaning input power = output power

limitation of them is… bc…

Answer 193

although transformers can increase the voltage of a power source,

due to the law of conservation of energy, they cannot increase the power output

Question 194

if a transformer is 100% efficient then:

EQN

Answer 194

Vp × Ip = Vs × Is

Question 195

ALSO: eqn

Answer 195

Ps = Vp × Ip

[Ps = output power (power produced in secondary coil) in Watts (W)]

Question 196

HOW TO PARSE QUES:

A transformer in a travel adapter steps up a 115 V ac mains electricity supply to the 230 V needed for a hair dryer. A current of 5 A flows through the hairdryer.

Answer 196

Voltage in primary coil, Vp = 115 V
Voltage in secondary coil, Vs = 230 V
Current in secondary coil, Is = 5 A

Question 197

advantages of High Voltage Transmission

;; Electricity is transmitted at high voltage, reducing the current and hence power loss in the cables

makes the transfer of electrical energy through the wires more efficient

Answer 197

When electricity is transmitted over large distances, the current in the wires heats them, resulting in energy loss

To transmit the same amount of power as the input power, the potential difference at which the electricity is transmitted should be increased

so a smaller current is being transmitted through the power lines

bc P = IV, so if V increases, I must decrease to transmit the same power

📌 A smaller current flowing through the power lines results in less heat being produced in the wire so REDUCES THE ENERGY LOSS IN THE POWER LINES

Question 198

Calculating Power Losses

eqns

Answer 198

P = I^2R

E = P x t [[the power is the energy lost per second, the total energy lost in a time t ]]

Vs/Vp = Ns/Np

IpVp = IsVs

Question 199

P = I2R.

The power (energy per second) lost

Answer 199

When a current passes through a wire, the current creates a heating effect which means the wires warm up

This means they lose electrical energy as heat which reduced the efficiency of the transformer

This is due to electrical resistance

Question 200

“faraday’s law”

Answer 200

• Whenever a conductor is placed in a varying magnetic field, an electromotive force is induced. If the conductor circuit is closed, a current is induced, which is called induced current.

-

Question 201

simple summary of

1. generator effect
2. motor effect

Answer 201

1. generator effect: the creation of a potential difference (voltage), therefore an induced current (flowing electrons) when a wire experiences a changing magnetic field; occurs when a coiled wire is wrapped around a magnet.
2. motor effect: A wire carrying a current creates a magnetic field. This can interact with another magnetic field, causing a force that pushes the wire at right angles;

putting a current-carrying wire between the poles of a horseshoe magnet; deflect

Question 202

relay

Answer 202

A relay is made of an electromagnet coil and a magnetic switch. I

If a current flows through a coil, a magnetic field will be produced. This can attract a switch in a separate circuit, causing it to flow and allowing a current to pass through the circuit.

Question 203

in a generator, MECHANICAL energy is used to rotate a magnet inside a set of stationary windings of wire. As the magnet spins, it changes the magnetic field inside the wires.

Changing magnetic field induces a flow of electric current in the wire

Question 204

electromagnetic induction

Answer 204

a current produced because of voltage production (electromotive force) due to a changing magnetic field.

Question 205

magnetic flux

Answer 205

a measure of the number of field lines passing through a region of space.

Question 206

State and explain the effect of this change [resistance increasing] on the power of the lamp.

Answer 206

Power (of lamp) decreases

P = IV and current in lamp decreases. OR P = V2 /R

Question 207

State the name and purpose of component X.

Answer 207

variable resistor AND

control or limit the voltage across heater

Question 208

Some time after the heater is switched on, the ammeter reading is seen to have decreased.

Suggest why this happens. [2]

Answer 208

📌 “Energy of battery used up. Resistance (of heater) increases. Current decreases.”

[battery going flat / energy of battery used up OR V less OR more/increased resistance (of heater)

use of relationship between I and V or R OR the current decreases]

Question 209

The temperature of the thermistor is increased so that its resistance decreases.

State the effect of this change in resistance on the current through the 500Ω resistor.

Explain your answer. [2]

Answer 209

“Current increases. Total resistance (of circuit) decreases.”

📍 (more current in circuit so) current (in 500Ω resistor) increases

📍 resistance of PARALLEL COMBINATION decreases
OR total resistance (of circuit) decreases

Question 210

A variable resistor is included so that the current in the circuit may be changed.
Suggest an advantage of being able to change the current [1]

Answer 210

different results OR graph can be plotted

OR to ensure wire does not overheat

Question 211

• aluminium cable replaced with a new aluminium cable of the same length. Current remains at 250 A.
• diameter of the new cable is double the diameter of the original cable.

[DIAMETER DOUBLES = POWER LOST DECREASES TO 1/4 OF POWER in kW]

State and explain how the power loss is affected by this change. [3]

Answer 211

• POWER loss reduced
• resistance decreases/REDUCED
• power lost decreases to a quarter OR (P =)19kW / 18.75 kW [75 x 0.25 = 18.75kW]

📌 WRITE IN kW AND WATTS IN SIMPLE ANSWER QUES. 📌

Question 212

thermistor - heat detector

transistor - switch

Question 213

In Fig. 9.1, lamp A is not glowing brightly.

Suggest and explain what could be done to component B to make lamp A glow brightly. [4]

Answer 213

📌 “Increase light intensity of B. Resistance of B decreases. Greater share of voltage. More current flows through lamp.” 📌

increase light intensity / brightness of B
resistance (of B) decreases [cao]
voltage at mid-point increases OR greater (share of) voltage
(more) current flows (through lamp)

Question 214

State what is meant by the direction of an electric field.

Answer 214

direction of the force on a positive charge

Question 215

• by an electric field.

Answer 215

A region in which a force acts upon an (electric) charge/charged object

Question 216

Suggest, in terms of forces, why the oil drop does not move up or down. [2]

Answer 216

Upward force (on drop) due to electric field/ charge on plates = weight of drop

Upward force on drop = downward force on drop. no resultant force on drop

Question 217

Without losing any of its charge, the oil drop begins to evaporate.

State and explain what happens to the oil drop. [2]

Answer 217

Drop moves upwards

Weight/mass of drop decreases OR downward force decreases

Question 218

Describe what happens to the charges in the metal paint on the ball as the positively charged
rod is brought close to the ball.

Answer 218

electrons / negative charges move towards the rod

Question 219

The ball is attracted towards the charged rod.

Explain why this happens, given that the ball is uncharged.

Answer 219

negative charges (are) close(r) (to the rod)

attraction between opposite charges greater than repulsion between like charges

Question 220

e.m.f what is meant

Answer 220

energy transferred per coulomb

Question 221

drawing -

Answer 221

Straight parallel lines from upper to lower plate B1
B1
At least 3 lines drawn. All lines drawn equally spaced,
approximately symmetrical with respect to plates
Arrows downwards [3]

++ at least three horizontal, parallel lines evenly spaced (ignore edge effects) B1
arrows pointing left to right

+++ right hand half of ball has more + signs than – signs
AND left hand half of ball has more – signs than + signs
equal numbers of + and – signs

📌📌📌 negative charge(s) on left [bcoppositecharges] AND positive charge(s) on right M1
equal number of positive and negative charges AND number of each ≤ 7

Question 222

An earthed metal wire is touched against the smaller metal sphere.

State and explain what happens to the charge on the smaller sphere.

Answer 222

electrons (/ negative charges) flow from Earth/ on to sphere

total charge negative OR (some) protons / positive charges cancelled

Question 223

Explain, in terms of their structures, why the metal wire is an electrical conductor but the
plastic stand is an electrical insulator.

Answer 223

metal contains free (delocalised) electrons

electrons in plastic not free to move

Question 224

Fig. 7.2 shows a metal sphere on an insulating support.

A student has available two rods, one charged positively and one charged negatively. Using one of these rods, she gives the sphere a uniform negative charge by induction.

State which rod she chooses, and describe the procedure she follows. [4]

Answer 224

📍 Use positively charged rod

📍 Place rod close to surface of sphere

[Touch sphere (briefly) with finger /] 📍 Briefly connect sphere to earth

📍 Remove charged rod

Question 225

State the direction of the force exerted on the vertical rod. [2]

Answer 225

to the right or left

to the right

Question 226

State what happens to the ammeter reading if the 1 Ω resistor is replaced by a 3 Ω resistor.

Answer 226

ammeter reading reduces

Question 227

) Suggest why B is connected to a relay, rather than directly to the lamp.

Answer 227

B cannot provide enough power / current for lamp, or equivalent

OR allows remote lamp

Question 228

📌 The safety expert recommends that normal domestic light switches, as shown in Fig. 9.1,
are replaced.

Explain why this recommendation is made.

📍 Suggest how the lights should be switched on and off.

Answer 228

📌 current could flow through switch due to dampness. water (good) conductor

danger of electrocution

📍 pull switch with long cord of insulating material
OR normal switch outside workroom

Question 229

Suggest how fuel flowing through the hose can cause a large build-up of electric
charge on the aircraft.

Answer 229

friction with hose

charge moved to aircraft from hose
[or rubber insulates]

Question 230

The aircraft is refuelled on a particular day when the tyres and wheels are wet.

Explain why there will be no large build-up of charge in this case.

Answer 230

(water conducts) charge away/to ground ;

OR owtte: earthing ; [of case/outside]

Question 231

Explain what the hazard might be if the heater is not earthed.

Answer 231

electrocution

live wire touches case

Question 232

A change to the environment around component B (thermistor) causes component A (light-emitting diode) to emit light.

State the environmental change.

ii) Explain your answer to (i)

Answer 232

temperature (decreases)

ii)

change of resistance causes a change of temperature, they both increase

voltage of left of LED increases 📌OR 📌higher voltage across thermistor

current flows through to light LED, enough voltage

Question 233

wire (current) downwards, into the page

direction of magnetic field? at right wire

force on right wire?

Answer 233

MAGNETIC field -> DOWN, ‘towards the bottom of the page’

force - left

RMBR HOW TO USE LEFT HAND RULE FOR FORCE.

Question 234

State and explain whether there is also a force on wire X.

Answer 234

There is a force on X

due to the (magnetic) field of Y
OR the (magnetic) fields due to X and Y interacting

Question 235

When switch S is first closed, the needle of the galvanometer deflects briefly, then returns to
zero.
Explain why the brief deflection occurs

Answer 235

Change in current is brief, occurs as switch closes

Changing magnetic field links with secondary coil
field lines cut coil

Causes induced voltage and thus current

Question 236

i) Explain the final position of the brass pointer.

(ii) Explain the final position of the magnet.

Answer 236

i) no forces or effect on needle

ii) needle aligns with field
[N or S pole attracted along field line]

Question 237

Using your knowledge of investigating the magnetic field around a bar magnet,

suggest an experiment or experiments to confirm that you have drawn the correct pattern and direction in (a). [4]

Answer 237

use of compass/suspended small magnet

observe needle/magnet on one field line

observe needle/magnet on another field line

mark on card OR needle/magnet shows direction of field

📌OORRR;;

(sprinkle) iron filings
tap card
alignment of iron filings show field
use compass to show field direction

Question 238

A second current-carrying wire is inserted vertically through the card at Y.

Suggest why there is now a force on the wire at X. [2]

Answer 238

wire X is in a magnetic field

current carrying conductor in field interact

Question 239

On Fig. 10.1, draw lines to show the pattern of the magnetic field due to the current.

Fig shows a variable resistor (rheostat) and a solenoid (long coil) connected to a
battery. [2]

Answer 239

at least two vertical lines inside the coil

at least four lines (two left, two right) outside the coil of correct shape

Question 240

State the feature of the pattern of the magnetic field lines that indicates the strength
of the magnetic field at particular points.

Answer 240

lines closer where field is stronger

Question 241

State and explain the effect on the magnetic field of increasing the resistance of the
variable resistor.

Answer 241

reduces (strength of) field

(increasing the resistance) reduces the current

Question 242

State and explain the effect on this path of reversing the current in the coils. [2]

Answer 242

curves in opposite direction to (c)(i)

magnetic field reversed

Question 243

The student drops the magnet so that it falls through the solenoid.
State and explain what would be observed on the milliammeter

(i) as the magnet enters the solenoid, [2]

(ii) as the magnet speeds up inside the solenoid [2]

Answer 243

i) 📌 (milliammeter) deflects/shows reading [there is a current]

📌 field lines cut / emf induced

ii) greater deflection/current

rate of change of flux (linkage) is greater. more magnetic field lines cutting coil (per second)

Question 244

As the magnet passes into the coil in part (a), the coil exerts a force on the magnet even though there is no contact between them.

(i) State the direction of this force.

(ii) Explain how this force is caused. [3]

Answer 244

i) UPWARDS /opposite to magnet’s direction of travel

ii) current (in coil) causes a magnetic field

force caused by overlapping (magnetic) fields

Question 245

When XY is moved slowly upwards the needle of the voltmeter shows a small deflection.

(i) State how XY must be moved to produce a larger deflection in the opposite
direction.

(ii) XY is now rotated about its central point by raising X and lowering Y. Explain why no deflection is observed.

Answer 245

i) In the opposite direction
Faster

ii) No voltage/current induced

Currents (induced) in each half of XY are equal and in opposite directions / oppose each other

Question 246

lower end of the magnet is pushed down into the upper end of the coil and
held at rest.

During the movement, an e.m.f. is induced in the coil. The meter shows a deflection to the right and then returns to zero.

Explain why this e.m.f. is induced.

Answer 246

(magnetic) field (lines) of magnet cut by turns / coil

Question 247

State what happens to the needle of the meter when

1. the magnet is released from rest and is pulled up by the spring, [1]
2. the magnet continues to oscillate up and down, moving in and out of the coil with each oscillation. [1]

Answer 247

1. (needle of meter) deflects to the left (and returns to zero)
2. deflects to right and left (alternately)

Question 248

magnet is released and it falls through the solenoid. During the initial stage of the fall, the sensitive ammeter shows a small deflection to the left.

(a) Explain why the ammeter shows a deflection.

Answer 248

(magnetic) field (lines) of magnet cuts coils (of solenoid)

Question 249

magnet passes the middle point of the solenoid and continues to fall. It reaches
position Y.

Describe and explain what is observed on the ammeter as the magnet falls from the middle point of the solenoid to position Y. [4]

Answer 249

meter deflects in opposite direction

deflection is greater (than initially) OR for shorter time

magnet moving faster

more field lines cut per second

Question 250

Suggest two changes to the apparatus that would increase the initial deflection of the ammeter. [2]

Answer 250

stronger magnet

• use a solenoid (of same length) with more turns

Question 251

Explain why there is a current in the lamp. [4]

Answer 251

(alternating current causes) magnetic field in core

magnetic field changes

field cuts secondary coil

e.m.f. induced (and current flows in lamp)

Question 252

output of an a.c. generator in a power station is 5000 V. A transformer increases the voltage to 115 000 V before the electrical power is transmitted to a
distant town.

(a) State and explain, using a relevant equation, one advantage of transmitting electrical power
at a high voltage. [3]

Answer 252

P = I^2 R

less energy lost
smaller current

Question 253

The transformer contains two coils, the primary coil and the secondary coil.

(i) State the other main component of a transformer and the material from which it is made.

ii) State the component in the transformer to which the a.c. generator is connected.

c) Transformers within the town reduce the voltage to 230 V.
Suggest one reason for this.

Answer 253

i) iron core

ii) connected to primary coil

c) safer OR less insulation needed

Question 254

i) Explain why the wheel turns when the switch is closed.

ii) Describe how the split-ring commutator on an electric motor works

Answer 254

i) current in spoke in magnetic field

causes force on spoke/wheel

ii) brushes connect to other split ring every half turn/coil vertical

reverses direction of current every half turn/coil vertical

Question 255

Explain what is meant by electromagnetic induction.

Answer 255

when magnetic field cuts by coil

current/e.m.f caused

Question 256

A cathode ray is a beam of electrons.
Suggest one way of controlling the number of electrons in the beam.

Answer 256

change current in filament/cathode/heater