Mains Electricity: Answers

A drill is labelled 230 V, 30 W. What does the 30 W tell you about the energy transfer in the drill?

The number measured in Watts is the power of the device. Power is the rate of energy transfer, or in other words the number of Joules of energy transferred to the drill every second.

The drill has an earth connection. Explain how an earth connection protects the user.

If a wire becomes loose and touches the metal casing, the current passes down a path of low resistance inside the earth wire, discharging the electricity to the earth rather than to the user and preventing an electric shock.

A washing machine has an electric motor and an electric heater. It is fitted with a 13 A fuse. Explain why the washing machine is fitted with a fuse.

If there is a fault in the circuit causing an excess of current in the wires, the fuse melts to stop the current. This prevents overheating and a fire from starting.

When the motor in the washing machine is working, the current in it is 1.74 A. Explain why it would not be a good idea to replace the 13 A fuse with a 2 A fuse.

Within the washing machine, current is running through both the electric motor and the electric heater. The total current in the washing machine from both components will probably exceed 2 A so if a 2 A fuse is fitted, it will immediately melt and need to be replaced.

An electric heater has a power supply of 2000 W and is connected to a 230 V mains supply.

State the equation linking power, current and voltage.

Power (in W) = current (in A) x voltage (in V)

P = IV

Calculate the current in the heater.

We need to rearrange the equation to give I = P/V

I = 2000/230 = 8.7 A

What would be an appropriate fuse to use with the heater: 1 A, 5 A, 7 A or 15 A?

We need to use a fuse with a rating closest to but higher than the amount of current in the device. In this case, the rating slightly higher than 8.7 A is the 15 A fuse.