4. Electricty (16-21)

Question 1

Describe the forces between magnetic poles and between magnets and magnetic materials

Answer 1

magnetic forces are due to interactions between magnetic fields.

magnets have two poles (ends)- north and south.
like poles repel and unlike poles attract (force).
magnets experience attraction and repulsion.

Question 2

Describe induced magnetism

Answer 2

when a magnetic material is only magnetised when placed in a magnetic field (brought close to the pole of a permanent magnet).

A permanent magnet can attract or repel another permanent magnet. it can also attract other unmagnetised magnetic materials. ex- a bar magnet can attract steel pins, fridge magnet, paper clips. these are made of magnetic materials but are not magnetised all the time.

when the permanent magnet is removed the pin will return to its unmagnetised state or it may retain a small amount of magnetism.

Question 3

properties of temporary magnets and the properties of permanent magnets

Answer 3

1. temporary magnets (made of soft iron)- easy to magnetise and demagnetise- readily loses its magnetism.

cores for electromagnets, transformer, radio aerials

2. permanent magnets (made of steel)- difficult to magnetise and demagnetise- retains magnetism well.

permanent magnets, compass needle, loudspeaker magnets

Question 4

difference between magnetic and non-magnetic materials

Answer 4

MM- experience a force when placed in a magnetic field.
are attracted to a magnet and can be magnetised

NM- no experience of force when placed in a magnetic field.
not attracted by a magnet and cannot be magnetised

Question 5

Describe a magnetic field and state what determines drawing magnetic field lines

Answer 5

1. a region in which a magnetic pole experiences a force.
2. the direction of a magnetic field at a point is “the direction of the force on the N pole” of a magnet at that point- N to S with arrows.
3. the relative strength of a magnetic field is represented by the spacing of the magnetic field lines-

MF stronger at poles- MFL closest together.

become weaker as distance from magnet increases as lines are getting further apart.

equal spacing bw MFL- MF has same strength

Question 6

What happens when two magnets are placed near each other

Answer 6

when two bar magnets are placed close together, their magnetic fields interact & produce a new line of magnetic lines of force. The pattern indicates whether the magnets will attract or repel.

Question 7

Describe the plotting of magnetic field lines with a compass or iron filings and the use of a compass to determine the direction of the magnetic field

Answer 7

Iron filings- place a piece of paper under a magnet. sprinkle iron filings on top of paper & tap it for it settle. they will line up in field.

Plotting compass- Use a permanent magnet kept on paper.
mark point of compass needle.
move compass to other side of dot.
align needle with dot & put a dot on other side of needle.
start again from another position next to magnet.
MF- north to south and arrows

Question 8

Describe the uses of permanent magnets and electromagnets

Answer 8

Permanent magnet- compass, fridge magnet, school lab, toy

Electromagnet- electric doorbells, loudspeakers, electric motors, relays, transformers

Question 9

Principles of charges

Answer 9

positive charges repel other positive charges, negative charges repel other negative charges, but positive charges attract negative charges.

charge is measured in coulombs

Question 10

How is charging caused

Answer 10

charging of solids is caused by friction & it involves only a transfer of negative charge (electrons). Positive charge is not transferred as they are in bound to nucleus so can’t move freely.

if polythene rod becomes negatively charged then electrons are transferred from cloth to rod.

gain e- negative
lose e- positive

Question 11

Describe an electric field

Answer 11

an electric field as a region in which an electric charge experiences a force.

the direction of an electric field at a point is the direction of the force on a positive charge at that point.

a charged object can affect both charged and uncharged objects, without touching them as it exerts a force on it.
it creates an electric field around itself. force of attraction and repulsion will become stronger as the distance between two charge object decreases & vice versa.

Question 12

Describe simple electric field patterns

Answer 12

(a) around a point charge- arrow pointing outwards

(b) around a charged conducting sphere- arrows pointing outwards

(c) between two oppositely charged parallel conducting plates- +ve to -ve arrow parallel straight and curved at edges

Question 13

why can charge MOVE through electrical conductors but not insulators

Answer 13

in insulators, e are tightly bound to their atoms and not easily removed. in conductors, e are free to move bw atoms.

insulators become charged as if on end becomes charged and the other one will be uncharged as e- can’t move.

conductors do not become charged as e- are free to move so they will flow through the object, through hand, through earth. it can only be charged if it is held by an insulating handle- the charge will spread evenly through conductor.

Question 14

Describe an experiment to distinguish between electrical conductors and insulators

Answer 14

connect the cell and lamp in a simple circuit to make the lamp light.

make a gap in the circuit by removing a wire.

using the crocodile clips, attach a material into the gap in the circuit.

if the lamp lights, it is a conductor and if it does not, it is an insulator.

Question 15

Define electric current

Answer 15

the charge passing a point per unit time/
the rate at which electric charge passes a point in a circuit

I = Q/t

Question 16

Describe the use of ammeters (analogue and digital) with different ranges

Answer 16

analogue- has a needle, which moves across a scale. you have to make a judgement of the position of the needle against the scale. 0.1-1A & 1-5A

digital- gives a direct read-out in figures. there is no judgement involved in taking a reading. can measure small current

Question 17

Describe electrical conduction and conventional electrons

Answer 17

conventional current is from positive to negative and that the flow of free electrons is from negative to positive

Question 18

difference between direct current (d.c.) and alternating current (a.c.)

Answer 18

(d.c.)- electric current that flows in the same direction all the time. dry cells and batteries and sometimes generators

(a.c.)- electric current that periodically changes direction. mains electricity and generators, transformers

Question 19

Define electromotive force (e.m.f.) & potential difference (p.d.)

Answer 19

the electrical work done by a source in moving a unit charge around a complete circuit. It is measured in volts (V).

E = W/Q

the work done by a unit charge passing through a component. p.d. between two points is measured in volts (V)

V = W/Q

Question 20

Describe the use of voltmeters (analogue and digital) with different ranges

Answer 20

analogue- has a needle, which moves across a scale. you have to make a judgement of the position of the needle against the scale. range- 0.1-1V & 1-5V

digital- gives a direct read-out in figures. there is no judgement involved in taking a reading. range- can measure small p.d.

Question 21

explain the current–voltage graphs for a resistor of constant resistance, a filament lamp and a diode

Answer 21

voltage on x-axis & current on y-axis

1. ohmic resistor- straight line- directly proportional because resistor has constant resistance.
2. filament lamp- at low voltage I-V directly proportional. at higher voltages, the graph starts to curve over because current increases slowly- if I inc, e- flow inc through wire. no. of collision bw e- and lattice inc. kinetic energy transferred to internal energy. more lattice vibration so more resistance.
3. diode- acts as a switch & allows current to flow in one direction only.
   arrow indicates direction of conventional current.
   is a semiconductor- behaves like an insulator until enough p.d. given to make it behave as a conductor

Question 22

State, qualitatively, the relationship of the resistance of a metallic wire to its length and to its cross-sectional area

Answer 22

(a) resistance is directly proportional to length

(b) resistance is inversely proportional to cross-sectional area

Question 23

How is energy transferred from electric circuits

Answer 23

electric circuits 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 24

Equation for electrical power and electrical energy

Answer 24

P = IV

E = IVT

Question 25

Define the kilowatt-hour (kWh)

Answer 25

energy transferred in one hour at a rate of transfer of 1 kW

Question 26

circuit components—

Answer 26

cells, batteries, power supplies, generators,
potential dividers, switches,

resistors (fixed and variable), heaters, thermistors (NTC only),
light-dependent resistors (LDRs),

lamps, motors, ammeters, voltmeters, magnetising coils,
transformers, fuses and relays, diodes and light-emitting diodes (LEDs)

Question 27

Current and p.d. in series and parallel circuit

Answer 27

1. current at every point in a series circuit is the same

the sum of the currents entering a junction in a parallel circuit is equal to the sum of the currents that leave the junction. for a parallel circuit, the current from the source is larger than the current in each branch

2. 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. for a series circuit, the voltage from the source is larger than the voltage in each branch

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 resistance

Question 28

resistance in series and parallel circuit

Answer 28

S- R= R1+R2+R3

P- combined resistance of two resistors in parallel is less than that of either resistor by itself.

1/R= 1/R1+1/R2- effective resistance for TWO resistors in parallel

Question 29

State the advantages of connecting lamps in parallel in a lighting circuit

Answer 29

lamps in parallel circuit have normal brightness or are brighter than lamps in series circuit

if one lamp fails, the other lamp is still lit (because each lamp is provided with its own switch so it can be operated separately).

Question 30

Describe the action of a variable potential divider AND equation for two resistors used as a potential divider

Answer 30

Part of a circuit consisting of two resistors connected in series to obtain a smaller voltage than supplied.

Splits emf of a power source between two components/resistors connected in series in proportion to resistances of the two components or resistors.

R1/R2 = V1/V2

Question 31

State the hazards when using a mains supply

Answer 31

(a) damaged insulation- emit poisonous fumes, catch fire, lethal shock

(b) overheating cables- insulation melt, fumes

(c) damp conditions- water conducts electricity, electrocution risk, short circuit

(d) excess current from overloading of plugs, extension leads, single and multiple sockets- heat up and catch fire

Question 32

Describe all types of wires and explain in which a switch must be connected to

Answer 32

a mains circuit consists of a live wire- carries current from wall socket to electrical appliances, a neutral wire- carries current back to socket and an earth wire- safety- protects the circuit & cabling for double-insulated appliances.

a switch must be connected to the live wire for the circuit to be switched off safely- current could still pass into the appliance if the switch was off & lead to to fire or someone being electrocuted if they touched a faulty appliance.

Question 33

use and operation of double-insulation, earthing, trip switches and fuses

Answer 33

1. double insulation- no need of earthing as electrically insulating material- so electric current can’t pass through it. a live conductor will not touch the outer case
2. fuse- contains a thin section of wire, designed to melt and break the circuit if the current gets above a certain value.
3. trip switch- includes a switch that opens (trips) when a current exceeds a certain value
4. earthed- the earth wire provides a low resistance electrical path to ground and reduces the chances of a fatal electric shock.

a fuse without an earth wire protects the circuit and the cabling for a double-insulated appliance

Question 34

Magnetic effect of a current

Answer 34

Describe the pattern and direction of the magnetic field due to currents in straight wires
and in solenoids- right hand grip rule

Describe how the magnetic effect of a current is used in
relays- switches that open and close via action of electromagnets. electrical circuits contain an electromagnet who’s magnetic field is induced and attracts switch on a second circuit near to it

loudspeakers- electrical to sound due to motor effect- has a coil wrapped around one pole of magnet. ac passes so direction of mf changes, force exerted on coil so it oscillates creating sound waves.

State the qualitative variation of the strength of the magnetic field around straight wires and
solenoids- closer and stronger near wire and poles and further and weaker

no of coils
size of current
add iron core

Question 35

AC generators and em induction

Answer 35

-a conductor moving across a magnetic field or a changing magnetic field can induce an e.m.f. in the conductor

converts mechanical energy to electrical energy.`

direction of an induced e.m.f. opposes the change causing it

RHR- directions of force, field and induced current

e.m.f. against time- sin graph
peaks- horizontal
troughs- horizontal- 180 degree
zeros of the e.m.f.- vertical

factors affecting the magnitude of an induced e.m.f.-
(a) the number of turns on the coil
(b) size of coil
(c) the strength of the magnetic field

a.c. generator (rotating coil or rotating magnet)
slip rings- allows current to flow from to and from coil of ac generator
and brushes- rub against slip rings so have same emf as coil

Question 36

DC motors

Answer 36

-a force acts on a current-carrying conductor in a magnetic field

converts electrical energy to mechanical energy.

reversing direction of force: (a) the current, (b) the direction of the field

LHR- force, magnetic field and current

• Know that a current-carrying coil in a magnetic field may experience a turning effect and that the
  turning effect is increased by increasing:

(a) the number of turns on the coil
(b) the current
(c) the strength of the magnetic field

split-ring commutator- reverses direction of current in coil every half turn so it rotates in same direction
brushes- transmits current from battery to coil without breaking wires

Question 37

transformer- what, how, why

Answer 37

alternating current in (primary coil)

(current in primary generates) changing magnetic field

iron core concentrates (magnetic) field OR iron core transfers (magnetic) field (to secondary coil)

secondary coil is in alternating / changing (magnetic) field OR secondary coil cuts (magnetic) field

e.m.f. induced (in secondary coil)

• step up, step down- increase or decrease voltage
  Vp/Vs = Np/Ns

100% efficient same power- IpVp = IsVs

Question 38

State the advantages of high-voltage transmission

Answer 38

P=I^2R
to explain why power losses in cables are smaller when the voltage is greater