Electricity

Question 1

What is electric current?

Answer 1

Electric current is a flow of electrical charge. The size of the electric current is the rate of flow of electrical charge. Charge flow, current and time are linked by the equation:
charge f low = current × time
A current has the same value at any point in a single closed loop.

Question 2

Why do current , resistance and voltage all depend on each other?

Answer 2

The current (I) through a component depends on both the resistance (R) of the component and the potential difference (V) across the component. The greater the resistance of the component the smaller the current for a given potential difference (pd) across the component

Question 3

What are the factors affecting the resistance of electrical circuits?

Answer 3

• the length of a wire at constant temperature
• combinations of resistors in series and parallel.

Question 4

How is the current through an ohmic conductor?

Answer 4

(at a constant temperature) is directly proportional to the potential difference across the resistor. This means that the resistance remains constant as the current changes.

Question 5

How is the resistance of components such as lamps ?

Answer 5

is not constant; it changes with the current through the
component.

Question 6

Describe the resistance of the filament lamp:

Answer 6

The resistance of a filament lamp increases as the temperature of the filament increases.

Question 7

Describe the current through a diode:

Answer 7

The current through a diode flows in one direction only. The diode has a very high resistance in the reverse direction.

Question 8

Describe the resistance of a thermistor

Answer 8

The resistance of a thermistor decreases as the temperature increases.

Question 9

Describe the resistance of an LDR:

Answer 9

The resistance of an LDR decreases as light intensity increases.

Question 10

Explain the ways of joining electrical components in series:

Answer 10

For components connected in series:
* there is the same current through each component
* the total potential difference of the power supply is shared
between the components
* the total resistance of two components is the sum of the
resistance of each component

Question 11

Explain the ways of joining electrical components in parallel:

Answer 11

• the potential difference across each component is the same
• the total current through the whole circuit is the sum of the currents through the separate components
• the total resistance of two resistors is less than the resistance of the smallest individual resistor.

Question 12

In the United Kingdom what is the domestic electricity supply frequency?

Answer 12

has a frequency of 50 Hz and is about
230 V.

Question 13

What is a live wire?

Answer 13

brown. The live wire carries the alternating potential difference from the supply.

Question 14

What is the neutral wire?

Answer 14

Blue. The neutral wire completes the circuit.

Question 15

What is the earth wire?

Answer 15

green and yellow stripes. The earth wire is a
safety wire to stop the appliance becoming live.

Question 16

What is the potential difference between the live wire and earth?

Answer 16

The potential difference between the live wire and earth (0 V) is about 230 V. The neutral wire is at, or close to, earth potential (0 V). The earth wire is at 0 V, it only carries a current if there is a fault.

Question 17

What doe the amount of energy an appliance transfers depend on?

Answer 17

The amount of energy an appliance transfers depends on how long the appliance is switched on for and the power of the appliance.

Question 18

When is work done in a circuit?

Answer 18

Work is done when charge flows in a circuit.

Question 19

What is the National grid?

Answer 19

The National Grid is a system of cables and transformers linking power stations to consumers. Electrical power is transferred from power stations to consumers using the National Grid.

Question 20

What are step-up transformers used for?

Answer 20

Step-up transformers are used to increase the potential difference from the power station to the transmission cables then step-down transformers are used to decrease, to a much lower value, the potential difference for domestic use.