Domestic electricity - Paper 1

Question 1

State the typical voltage for mains electricity?

Answer 1

230 V

Question 2

State the frequency of mains electricity

Answer 2

50 Hz

Question 3

Define a.c.

Answer 3

Alternating current, the electrons vibrate backwards and forwards. Mains electricity use a.c.

Question 4

Define d.c.

Answer 4

Direct current. Electrons only flow in one direction. This is what we get from batteries

Question 5

What would a.c. look like on an oscilloscope?

Answer 5

A wave that alternates positive and negative

Question 6

What does d.c. look like on an oscilloscope.

Answer 6

A horizontal line

Question 7

See image

Answer 7

Line A is live wire which alternates positive and negative at 230 V

Line B is the neutra lwire which alternates close to 0 V

Time period - each wave is 5 squares horizontally. Each square = 0.005 so T = 5 x 0.005 = 0.025

Frequency = 1 / Time period = 1 / 0.025 = 40 Hz

Question 8

State the names and colours of the 3 wires found inside a cable.

Answer 8

Live wire - Brown

Neutral wire - Blue

Earth wire - Green and yellow stripes

Question 9

Which wire can be missing from some appliances? Why?

Answer 9

Earth wire, if the appliance is plastic and you cannot get an electric shock from it.

Question 10

State the role of the live wire

Answer 10

To carry the alternating potential difference from the supply to the component. the potential difference between the live and the earth is 230 V

Question 11

State the role of the neutral wire

Answer 11

The neutral wire will complete the circuit from the component back to the supply. The neutral wire is at or close to 0V

Question 12

State the role of the Earth wire.

Answer 12

It is a safety wire to stop an appliance becoming live. the earth wire is at 0V as it only carries a current if there is a fault.

Question 13

State the relationship between power, potential difference and current.

Answer 13

Power (W) = potential difference (V) x current (A)

P = VI

Question 14

State the units of power

Answer 14

Watts (W)

Question 15

State the units of potential difference

Answer 15

Volts (V)

Question 16

State the units of current

Answer 16

Amps (A)

Question 17

State the relationship between power, resistance and current.

Answer 17

Power = Current² x resistance

Question 18

State the units for resistance

Answer 18

Ohms

Question 19

When using an electrical device what factors affect how much electrical energy it uses?

Answer 19

1. Time it is used for
2. The power of the appliance

Question 20

State the equation between energy transferred, power and time

Answer 20

Energy transferred (J) = Power (W) x time (s)

Question 21

State the equation between energy transferred, charge and potential difference

Answer 21

Energy transferred (J) = Charge (C) x potential difference (V)

E = QV

Question 22

State the parts that make up the national grid

Answer 22

Cables, pylons, step up transformers, step down transformers

Question 23

What does a step up transformer do? Why?

Answer 23

Increases the potential difference, which decreases the current. this reduced the amount of energy lost os heat so makes them more efficient.

Question 24

What does a step down transformer do? Why?

Answer 24

Decreases the potential difference which increases the current. this makes them safer to use within homes

Question 25

What is the casing of a plug made from? Why?

Answer 25

Plastic , it is a good insulator and strong

Question 26

What are electrical wires made from? Why?

Answer 26

Copper, ductile and a good conductor

Question 27

What are the pins in a plug made from? Why?

Answer 27

Brass, its a good conductor but also strong

Question 28

Where is the fuse in a plug?

Answer 28

Between the live pin and the live wire

Question 29

What is the role of the fuse?

Answer 29

If current gets too high the fuse melts and breaks the circuit, cutting off the live wire

Question 30

What happens when the live wire touches the metal case of an appliance, in terms of the earth wire?

Answer 30

The large current surges down the earth wire and the fuse blows