Electricity

Question 1

What is electric current?

Answer 1

The flow of electrical charge.

Question 2

State the equation linking charge

Answer 2

Q = I t
Charge (Coulombs), Current (Amperes),
Time (Seconds)

Question 3

What can be said about the value of current at any
point in a single closed loop?

Answer 3

Current is the same at all points in a
closed loop.

Question 4

What equation should be used to calculate potential
difference if current and resistance are known?

Answer 4

V = I R
Potential Difference (V), Current (A),
Resistance (Ω)

Question 5

What is an ‘Ohmic Conductor’? State the
condition required.

Answer 5

● A conductor for which current and potential
difference are directly proportional
● Resistance remains constant as current
changes
● Temperature must be constant

Question 6

List four components for which resistance is not
constant as current changes.

Answer 6

1. Lamps
2. Diodes
3. Thermistors
4. Light Dependant Resistors (LDRs)

Question 7

What happens to the resistance of a filament lamp
as the temperature increases? Why?

Answer 7

● Resistance increases
● Ions in metal have more energy, so vibrate
more, causing more collisions with
electrons as they flow through the metal,
creating greater resistance to current flow

Question 8

What is different about current flow
through a diode?

Answer 8

● The current only flows in one direction
● Resistance is very high in the other
direction, preventing current flow

Question 9

State what happens to the resistance of a thermistor
as temperature increases.

Answer 9

The thermistor’s resistance decreases.

Question 10

Give two examples of when a thermistor
may be used.

Answer 10

1. In a thermostat to turn a heater on
   below a certain temperature
2. In a freezer to turn on a cooler when
   the temperature becomes too high

Question 11

State what happens to the resistance of a LDR as
light intensity decreases.

Answer 11

The LDR’s resistance increases.

Question 12

Give an application for a LDR.

Answer 12

● Street lights often use LDRs
● When light levels become too low, the
light gains sufficient current to turn on

Question 13

What are the two ways that a component can be
connected in a circuit?

Answer 13

1. Series (same loop)
2. Parallel (adjacent loop)

Question 14

If two resistors are connected in parallel, what can
be said about their combined total resistance?

Answer 14

Their total resistance is less than the
smallest of the two individual
resistances.

Question 15

If two resistors are connected in series, what can be
said about their total resistance?

Answer 15

Their total combined resistance is equal
to the sum of the two individual
resistances

Question 16

Describe the current in a series circuit.

Answer 16

In a series circuit, the current is the same
at all positions since the charge only has
one path to flow through.

Question 17

Describe the current in a parallel circuit.

Answer 17

In a parallel circuit, the current is shared
between the different branches. When
the charge reaches a junction it splits.

Question 18

How should you connect an ammeter in a circuit to
measure current?

Answer 18

Ammeters should be connected in series
with the component that they are
measuring current through.

Question 19

How should you connect a voltmeter in a circuit to
measure potential difference?

Answer 19

Voltmeters should be connected in
parallel to the component that they are
measuring the potential difference of.

Question 20

Why is it advantageous to connect lamps in parallel?

Answer 20

If one lamp blows, the rest will be
unaffected and can still receive current
(i.e. the circuit is still complete).

Question 21

If one lamp blows, the rest will be
unaffected and can still receive current
(i.e. the circuit is still complete)

Answer 21

● Alternating current: Current that
continuously changes direction at a
specific frequency
● Direct current: One directional current
flow

Question 22

What is the frequency and voltage of the UK mains
electricity supply?

Answer 22

● Frequency: 50 Hz
● Voltage: 230V

Question 23

Explain when the Earth wire does and doesn’t carry
a current.

Answer 23

● Under normal circumstances, no current
flows through the Earth wire
● If a fault occurs in the appliance (such as a
surge or the casing becoming live), current
will flow to the ground

Question 24

What is the purpose of the neutral wire?

Answer 24

To complete the circuit by connecting the
appliance back to the mains supply.

Question 25

For metal appliances, where is the Earth wire
connected to? Why?

Answer 25

● Earth wire is connected to the metal casing
of the appliance
● If live wire becomes loose and touches the
casing, the current will flow through the
Earth wire, preventing electrocution

Question 26

State two equations for the power of a circuit. Give
appropriate units.

Answer 26

P = I V P = I² R
Power (Watts), Current (Amperes)
Potential Difference (Volts), Resistance
(Ohms)

Question 27

State an equation linking energy transferred, power
and time. Give appropriate units.

Answer 27

E = P t
Energy (Joules), Power (Watts), Time
(Seconds)

Question 28

State an equation linking energy transferred, power
and time. Give appropriate units.

Answer 28

E= Q V
Energy (Joules), Charge (Coulombs),
Potential Difference (Volts)

Question 29

What two main factors does the amount of energy
transferred by an appliance depend on?

Answer 29

1. How long the appliance is being used
   for
2. The power of the appliance

Question 30

Describe the energy transfers in a battery powered
torch.

Answer 30

● Battery converts chemical energy into
electrical energy
● Bulb converts electrical energy into light
as well as waste energy in the form of
heating

Question 31

What three things determine the power of a circuit
device?

Answer 31

1. The potential difference across the
   circuit
2. The current through the circuit
3. The amount of energy transferred in a
   given time

Question 32

What is the purpose of the National Grid?

Answer 32

To link power stations to consumers so
that they have access to a source of
electricity.

Question 33

What are the two types of transformers used in the
National Grid?

Answer 33

1. Step-Up Transformers
2. Step-Down Transformers

Question 34

Where are step-up transformers found in the
National Grid? What do they do?

Answer 34

● Step-Up Transformers are used when
connecting power stations to
transmission cables
● They increase the potential difference

Question 35

Where are step-down transformers found in the
National Grid? What do they do?

Answer 35

● Step-Down Transformers are used in
connecting transmission cables to
domestic buildings (like houses)
● They decrease the potential difference

Question 36

Why do transmission lines transfer electricity at high
potentials?

Answer 36

● A high potential, results in a low current
● The lower the current, the less energy
that is wasted as heat
● Therefore it is more efficient

Question 37

Why does the potential need to be decreased
between transmission lines and houses?

Answer 37

● Lower potentials are safer for domestic
use and reduces the likelihood of
severe electrocution
● Appliances are designed for 230V