Electricity and Magnetism

Question 1

Conservation of Charge

Answer 1

charge can neither be created nor destroyed
i.e. both objects will gain equal but opposite charges if rubbed together

Question 1

Electrical Conductors/ Insulators

Answer 1

a material that allows a flow of charge through it is called an electrical conductor
if charge cannot flow through a material it is called an electrical insulator

Question 2

Electric Field Definition

Answer 2

a region of space where a charged object experiences a force due to its charge

Question 3

What do electric field lines represent?

Answer 3

the closer the field lines, the stronger the field
the direction of the field shows the force that would act on a positive test charge placed at that point

Question 4

Coulombs Law

Answer 4

the force between two point charges is:
- proportional to the product of their charges
- inversely proportional to the square of their separation

Question 5

Electric Field Strength (E) Definition

Answer 5

the force per unit charge experienced by a small positive test charge placed in the field

Question 6

Potential Difference between two points Definition

Answer 6

the work done (or energy transferred) (W) when one unit of charge (q) moves between two points.”

Question 7

Electronvolt

Answer 7

one electronvolt is the energy an electron would gain by being accelerated by a potential difference of 1V

Question 8

Current Definition

Answer 8

the rate of flow of electric charge

Question 9

Electron Flow

Answer 9

electrons flow from the negative terminal of the power source around the circuit to the positive terminal

Question 10

Conventional Current

Answer 10

current is said to flow in opposite direction (positive to negative)

Question 11

Drift Velocity

Answer 11

is the speed at which electrons move due to the current

Question 12

What causes drift velocity?

Answer 12

• within a conductor electrons have high random speeds
• an electric field will exert a force on the electrons causing them to accelerate in a direction opposite to the direction of the field
• so the net velocity of the electrons is in the opposite direction of the electric field

Question 13

Series Circuit

Answer 13

the current is the same around the circuit
the sum of the voltages (potential differences) across the components adds up to the supply voltage

Question 14

Parallel Circuits

Answer 14

the current splits up between the branches
the voltage (potential difference) across each branch is the same

Question 15

Measuring Current

Answer 15

an ammeter is used
it must be connected in series with the component that is having the current flowing through it
an ideal ammeter has a resistance of zero so it does not effect the current in the circuit in which it is connected

Question 16

Measuring Potential Difference

Answer 16

a voltmeter is used
it must be connected in parallel with the component that is having the potential difference measured
an ideal voltmeter would have an infinite resistance so it does not draw any current from the circuit to which it is connected

Question 17

Resistance Definition

Answer 17

“the ratio of potential difference across the component to current flowing through the component.”
i.e. a high resistance needs a large potential difference for a current to flow

Question 18

Resistor Definition

Answer 18

a component whose resistance is constant over a range of potential differences (and currents)

Question 19

Ohm’s Law

Answer 19

“the current flowing through a piece of metal is proportional to the potential difference across it providing the temperature remains constant.”

Question 20

Resistivity

Answer 20

is a property of a substance that gives a measure of how the substance opposes the flow of electric current

Question 21

Potential Dividers

Answer 21

two resistors in a series circuit will divide up the voltage between them in proportion to their resistances

Question 22

Potentiometer

Answer 22

a single resistor that can be used to divide up the voltage
the output voltage to a device connected in parallel can be varied by adjusting the proportion of the potentiometer the device is connected to

Question 23

Light Dependent Resistors

Answer 23

as the light intensity decreases the resistance of the LDR increases

Using light to turn something on:
* connected in series
- light intensity increases
- R of LDR decreases
- V across bulb increases
- bulb turns on

Using darkness to switch something on:
* connected in parallel
- light intensity decreases
- R of LDR increases
- V across LDR increases
- V across bulb increases
- bulb turns on

Question 24

Thermistors

Answer 24

as the temperature increases, the resistance of the thermistor decreases

using heat to turn something on:
* connected in series
- temp increases
- R of thermistors decreases
- V across thermistor decreases
- V across bulb increases
- bulb turns on

using cold to turn something on:
* connected in parallel
- temp decreases
- R of thermistor increases
- V across thermistor increases
- V across bulb increases
- bulb turns on

Question 25

Strain Gauges

Answer 25

is a thin metal wire
if it is stretched its cross-sectional area gets smaller and so its resistance increases

Question 26

Electrical Power Definition

Answer 26

“amount of energy used or supplied per unit time.”

Question 27

Kirchoff’s Law (conservation of charge)

Answer 27

at any junction in a circuit, the total current entering the junction equals the total current leaving

Question 28

Kirchoff’s Law (conservation of energy)

Answer 28

around any closed path of a circuit, the total of all the potential differences is zero

Question 29

Applying Voltage Rule (components)

Answer 29

when going around the loop in same direction as the current the potential difference is negative
when going around the loop in the opposite direction the potential difference is positive

Question 30

Applying Voltage Rule (voltage sources)

Answer 30

it you hit the small side first, V is positive
if you hit the big side first, V is negative

Question 31

Primary Cells

Answer 31

cannot be recharged
irreversible chemical reaction

Question 32

Secondary Cells

Answer 32

designed to be recharged
reversible chemical reaction
to recharge it, it must be connected to an external DC power source

Question 33

Graph of Discharging Electric Cells

Answer 33

initial terminal p.d. > emf but quickly drops to emf
remains at emf for most of the time (may be a slight decline)
rapid final drop of terminal p.d due to increase in internal resistance

Question 34

EMF (electromotive force)

Answer 34

defined as: “energy supplied per unit charge by the power supply to the circuit”
it is the potential difference measured across the terminals of the battery when no current is flowing. It is a measure of the energy the electrons gain in the battery.

Question 35

What do field lines represent for magnetic fields?

Answer 35

• show the direction of force on a N pole of a magnet
• called lines of flux
• the closer the field lines, the stronger the field

Question 35

Terminal Potential Difference (V)

Answer 35

this is the potential difference measured across the terminals of the battery when a current is flowing. It is a measure of the energy available to the external circuit.

Question 36

Rule for moving charges

Answer 36

RH Slap Rule (positive charges)
LH Slap Rule (negative charges)
thumb = velocity
fingers = magnetic field
palm = force
* force is always at right angles to the velocity of the charge so the resultant motion is circular

Question 37

Rule for Current Carrying Wires

Answer 37

RH Grip Rule
thumb = current
fingers = field
(conventional current)

Question 38

Two Wires in Same Direction

Answer 38

fields cancel out between wires
and the wires are pulled together

Question 39

Two Wires in Opposite Direction

Answer 39

strong field between wires as the field adds together
wires are pushed apart

Question 40

Rule for Solenoid

Answer 40

variation on RH Grip Rule
fingers = direction of current
thumb = north pole

Question 41

The Motor Effect

Answer 41

any current carrying wire will experience a force in a magnetic field
RH Slap Rule
thumb = current
fingers = magnetic field
palm = force