P5: Electricity in the home

Question 1

What is a power rating?

Answer 1

The rate at which energy is transferred

Question 2

How do you calculate the total energy transferred by an electrical appliance?

Answer 2

E = Pt
Energy transferred (J) = Power (W) * time (s)

Question 3

What is the formula for power relating potential difference and current?

Answer 3

P = VI
Power (W) = Voltage (V) * Current (A)

Question 4

What is the formula for power relating current and resistance?

Answer 4

P = I²R
Power (W) = Current² (A) * Resistance (Ω)

Question 5

What is direct current (DC)?

Answer 5

Electric current only ever travels in one direction

Question 6

What is an alternating current (AC)?

Answer 6

Electric current is constantly changing direction

Question 7

What is the benefit of using alternating current?

Answer 7

It’s very easy to use a transformer to increase or decrease the voltage

Question 8

What is the frequency of the ac supply in the UK?

Answer 8

50Hz

Question 9

What is the voltage of the ac supply in the UK?

Answer 9

230 V

Question 10

What is an oscilloscope?

Answer 10

A device which shows the amplitude and frequency of electrical current

Question 11

How do you calculate the frequency in an oscilloscope?

Answer 11

Read the time interval between two of the maximum points on the graph
Number of cycles/ time = frequency
1 cycle = two peaks

Question 12

What is the live wire?

Answer 12

The brown wire, carries the potential difference from the supply - 230 V.
It is connected to a fuse in the plug

Question 13

What is the neutral wire?

Answer 13

The blue wire, completes the circuit with the live wire
- 0V

Question 14

What is the earth wire?

Answer 14

Green/ yellow, safety wire.
Attached to metal casing, connected to the ground with a metal rod.

Question 15

Why could the live wire be dangerous?

Answer 15

Can be fatal if touched, first part still has a p.d. of 230V, if touched, a current would flow through them and into the earth and they would be electrocuted

Question 16

Why can appliances with metal casing be dangerous?

Answer 16

If the live wire comes loose and touches the metal case, it can become live and give someone an electric case

Question 17

What happens when a metal case becomes live?

Answer 17

A huge current flows to the earth, the fuse melts and shuts off the current, preventing someone being electrocuted

Question 18

What is the national grid?

Answer 18

The system which transports energy from power stations to homes - a system of transformers and high voltage cables

Question 19

How can energy loss from the cables be reduced?

Answer 19

Build power stations closer to homes

Question 20

What is a step-up transformer?

Answer 20

Increases potential difference, so less energy wasted in the power cables when p.d. is high

Question 21

What is a step-down transformer?

Answer 21

Reduces p.d. to 230V, so it can be safely transported to homes

Question 22

Why are metals good conductors?

Answer 22

Electrons can easily flow through them

Question 23

Describe the electrical field of a positively charged object

Answer 23

Goes outwards

Question 24

Describe the electrical field of a negatively charged object

Answer 24

Goes inwards