Mains electricity

Figure 8 shows an electric kettle used to boil water.

The kettle is connected to the mains electricity supply by a three-core cable and plug.

Figure 9 shows the inside of the plug.

What are the colours of the insulation around the earth, live and neutral wires inside a plug?

Choose answers from the box.

The plug in Figure 9 contains a fuse.

What is the circuit symbol for a fuse?

Tick () one box.

The electric kettle contains a heating element.

The power of the heating element is 2420 W.

The current in the heating element is 11.0 A.

Calculate the potential difference across the heating element.

Use the equation:

The power of the heating element is 2420 W.

The current in the heating element is 11.0 A.

Calculate the resistance of the heating element.

Use the equation:

Kettle A and kettle B have the same total power input.

Kettle A has a greater efficiency than kettle B.

Which two of the following describe heating water in kettle A compared with kettle B ?

Tick () two boxes.

An electrical appliance is connected to the mains electricity supply using a three-core cable.

Figure 13 shows a three-core cable.

What colour is the insulation covering the live wire inside the cable?

Tick () one box.

What colour is the insulation covering the neutral wire inside the cable?

Tick () one box.

The plug connected to the cable contains a fuse.

A fuse contains a wire that is designed to melt when the current is too great.

What is the circuit symbol for a fuse?

Tick () one box.

The wire in the fuse melts when there is a charge flow of 2.0 C in a time of 0.40 s.

Calculate the current in the wire when it melts.

Use the equation:

The mass of the wire is 0.016 g.

specific latent heat of fusion of the wire = 60 000 J/kg

Calculate the change in thermal energy needed to melt the wire.

Use the Physics Equations Sheet.

The fuse transfers some energy to the surroundings as it melts.

How does transferring energy to the surroundings affect the total energy needed to melt the fuse?

Tick () one box.

The live wire in a three-core cable is connected to a fuse inside a plug.

A fuse contains a wire that is designed to melt when the current gets too great.

Figure 17 shows a fuse.

What colour is the insulation covering the live wire in a three-core cable?

Draw the circuit symbol for a fuse in the box below.

The fuse wire melts when there is a charge flow of 2.0 C for 400 ms.

Calculate the current in the fuse wire.

Use the Physics Equations Sheet.

When the fuse wire is at its melting point, the additional energy needed to melt the wire is 1.02 J.

specific latent heat of fuse wire = 60 kJ/kg

Calculate the mass of the fuse wire.

Use the Physics Equations Sheet.

The calculation in Question 10.4 assumes there is no energy transferred to the surroundings.

How would the time taken for the wire to melt be affected if some energy was transferred to the surroundings?

Give a reason for your answer.

Tick (✓) one box.

Figure 5 shows part of a lighting circuit in a house.

What is the frequency of the ac mains electricity supply in the UK? Tick (✓) one box.

The mains electricity supply has an alternating potential difference. Which diagram shows an alternating potential difference? Tick (✓) one box.

The potential difference across the lamp is 230 V. The current in the lamp is 0.020 A. Calculate the power output of the lamp. Use the equation: power = potential difference current

The potential difference across the lamp is 230 V. Calculate the energy transferred by the lamp when 180 C of charge flows through the lamp. Use the equation: energy transferred = charge flow potential difference

An electrician needs to replace the light switch in Figure 5. Describe the possible hazard and the risk to the electrician of changing the light switch.

Figure 13 shows part of a mains electricity lighting circuit in a house.

A fault in the switch caused a householder to receive a mild electric shock before a safety device switched the circuit off. The mean power transfer to the person was 5.75 W. The potential difference across the person was 230 V. Calculate the resistance of the person.

An electrician replaced the switch. The electrician would have received an electric shock unless the circuit was disconnected from the mains supply. Explain why.

The current from an electric shock causes a person’s muscles to contract. The person cannot let go of the electrical circuit if the current is too high. Figure 14 shows how the maximum current at which a person can let go depends on the frequency of the electricity supply.

The UK mains frequency is 50 Hz. Explain why it would be safer if the UK mains frequency was not 50 Hz.

Figure 15 shows a coffee machine. The coffee machine uses an electric element to heat water.

Figure 15

The coffee machine has a metal case.

Why would it be dangerous for the live wire of the electric cable to touch the metal case?

The power output of the coffee machine is 2.53 kW.

The mains potential difference is 230 V.

Calculate the current in the coffee machine.

The coffee machine heats water from 20 °C to 90 °C.

The power output of the coffee machine is 2.53 kW.

The specific heat capacity of water is 4200 J/kg °C.

Calculate the mass of water that the coffee machine can heat in 14 seconds.