Mains electricity

The diagram shows a power station connected to a town by the national grid.

Power Station Transformer Pylons and Wires Transformer
Town

The national grid uses alternating voltage.

A simple circuit powered by a battery uses direct voltage.

Explain the difference between alternating voltage and direct voltage.

The table shows the current in one of the wires in the national grid at two different voltages.

The wire has a resistance of 2.0 .

Calculate how much power is dissipated from the wire at 11 000 V.

Use the equation: power = current² resistance

Give your answer to 2 significant figures and in MW.

The national grid transfers 8 000 kJ of energy from the power station every second.

When the voltage across the wires is 400 000 V, the transformers and wires dissipate 160 kJ of this energy every second.

Calculate the efficiency of the energy transfer at 400 000 V.

Explain how step-up transformers allow the national grid to transfer energy more efficiently.

A UK mains plug has 3 wires: live, neutral and earth.

Describe the function of the neutral wire.

The national grid uses alternating voltage.

A simple circuit powered by a battery uses direct voltage.

Explain the difference between alternating voltage and direct voltage.

The table shows the current in one of the wires in the national grid at two different voltages.

The wire has a resistance of 2.0 .

Calculate how much power is dissipated from the wire at 11 000 V.

Use the equation: power = current² resistance

Give your answer to 2 significant figures and in MW.

The national grid transfers 8 000 kJ of energy from the power station every second.

When the voltage across the wires is 400 000 V, the transformers and wires dissipate 160 kJ of this energy every second.

Calculate the efficiency of the energy transfer at 400 000 V.

Explain how step-up transformers allow the national grid to transfer energy more efficiently.

A UK mains plug has 3 wires: live, neutral and earth.

Describe the function of the neutral wire.

Fig. 16.1 shows a heater plugged into an electrical socket.

Fig. 16.1

The heater is operating normally.

What are the names of the two wires in the plug that carry a current?

Tick (✓) one box.

A live wire is accidentally connected to the earth wire in the house. This causes a large current to flow in the earth wire.

This can cause electrocution.

Suggest another reason why this is dangerous.

A 2.4 kW heater is used for 2.5 hours each day for 1 week.

Calculate the energy transferred in kW h by the heater in 1 week.

Use the equation: energy transferred = power time

Energy transferred in 1 week =........ kW h

A residual current device (RCD) is a safety feature found in homes.

The RCD turns off the electricity quickly if it detects a fault due to a difference between the current in the live wire and the current in the neutral wire.

Fig. 16.2 shows the effects that mains current has on a human body.

Fig. 16.2

Current passing through a human body (mA)

Describe one trend shown by Fig. 16.2.

A current of 10 mA is passing through a human body.

How long can the current pass through the body before the person has difficulty breathing?

Calculate the charge flowing when 150 mA is in the human body for 0.3 s.

Use the equation: charge flow = current time

Sensitive RCDs are designed to shut off the electricity supply within 40 ms.

Suggest why this is an important feature for currents below 200 mA. Use Fig. 16.2.

An electrician has a choice of two different RCDs, A and B, to use in a house.

Suggest which RCD the electrician should use in the house.

Give two reasons. Use Fig. 16.2.

Fig. 11.1 shows a heater plugged into an electrical socket.

Fig. 11.1

The heater is operating normally.

What are the names of the two wires in the plug that carry a current?

Tick (✓) one box.

A live wire is accidentally connected to the earth wire in the house. This causes a large current to flow in the earth wire.

This can cause electrocution.

Suggest another reason why this is dangerous.

A 2.4 kW heater is used for 2.5 hours each day for 1 week.

Calculate the energy transferred in kW h by the heater in 1 week.

Use the equation: energy transferred = power time

Energy transferred in 1 week =........ kW h

A residual current device (RCD) is a safety feature found in homes.

The RCD turns off the electricity quickly if it detects a fault due to a difference between the current in the live wire and the current in the neutral wire.

Fig. 11.2 shows the effects that mains current has on a human body.

Fig. 11.2

Current passing through a human body (mA)

Describe one trend shown by Fig. 11.2.

A current of 10 mA is passing through a human body.

How long can the current pass through the body before the person has difficulty breathing?

Calculate the charge flowing when 150 mA is in the human body for 0.3 s.

Use the equation: charge flow = current time

Sensitive RCDs are designed to shut off the electricity supply within 40 ms.

Suggest why this is an important feature for a current of 200 mA. Use Fig. 11.2.

An electrician has a choice of two different RCDs, A and B, to use in a house.

Suggest which RCD the electrician should use in the house.

Give two reasons. Use Fig. 11.2.

Which part of the plug is connected to the metal case of an appliance?

An electric plug contains a live wire, a neutral wire and an earth wire.

Which statement is correct?

A The neutral wire completes the circuit.

B The neutral wire only carries a current if there is a fault.

C The potential difference between the earth wire and the neutral wire is 230 V.

D The potential difference between the live wire and the neutral wire is 400 000 V.

Which sentence describes a feature of the live wire in a plug?

A It carries current to earth if there is a fault.

B It completes the circuit.

C It is at a potential difference of 230 V compared to earth.

D It melts if the current is too small.

This question is about domestic electricity.

Draw lines to connect each term with the correct description.

Fig. 14.1 is a diagram of the wiring in a metal kettle.

The diagram in Fig. 14.1 is incomplete.

Use these words to label the 3 wires on the diagram.

You may use each word once, more than once, or not at all.

Earth Live Neutral

The table shows power losses for different voltages in a power line.

Using the data in the table, plot a graph of power loss against voltage and draw a line of best fit.

Describe the relationship between power loss and voltage.

You may use values from the graph or table in your answer.

Explain why the national grid is an efficient way to transfer energy.

This question is about the way electricity is used in the home.

The national grid transfers electrical power from power stations.

The table shows information for different types of power line.

The resistance of each power line is 3 .

Calculate the current in power line 3.

Use the equation: power = current² × resistance

Explain how power loss is reduced in the national grid.

Use the values for power line 1 and power line 2 to help you.

In Fig. 14.1, an electrician wires an electrical socket next to a sink.

The switch on the socket is open. Explain why this situation is dangerous if water enters the socket.

Write about the live wire and earth wire in your answer.

This question is about electrical devices.

The diagram shows two different electrical devices.

Write down the source of electrical energy for each device.

torch........

Describe how energy is dissipated in the electric fan.

The electric fan is rated 230 V, 0.3 A.

Calculate the power of the electric fan.

Use the equation: power = potential difference × current

*(b) Here is a diagram of a tumble dryer and an electric drill. Each appliance is wired differently.

Explain whether the tumble dryer and electric drill are both safe to use.

Include the function of the earth, live and neutral wires in your answer.

A hairdryer is connected to the mains with only two wires.

Why will this hairdryer operate and be safe to use?

A The wires are earth and neutral and the hairdryer is insulated.

B The wires are earth and neutral and the plug has a fuse.

C The wires are live and earth and the plug has a fuse.

D The wires are live and neutral and the hairdryer is insulated.

An electric kettle is plugged into a socket. The kettle is switched on. A qualified electrician uses three voltmeters, V₁, V₂ and V₃.

Use the relationship: V₃ = V₁ + V₂

Which row in the table, A, B, C or D, shows the correct readings for the three voltmeters?

In the UK, three wires are used in the domestic electricity supply.

They are the earth wire, the live wire and the neutral wire.

Which sentence is correct?

A The alternating p.d. between the neutral wire and the earth wire is 230 V.

B The alternating p.d. between the neutral wire and the live wire is 230 V.

C The earth wire contains a fuse for safety.

D The live wire is connected to the case of an appliance for safety.

The table shows some information about electrical appliances in the home.

Appliance	Power (W)	Current (A)	Resistance ()
Electric drill	800	3.48	66.1
Iron	2000	8.69	26.5
Kettle	2500	10.86	21.1
Security light	500	2.17	105.8
Toaster	1650	7.17	32.1

Use the table to describe the relationship between power and resistance.

Explain this relationship. Use ideas about resistance in your answer.

The security light is switched on for 45 minutes every day for 7 days.

Use the equation: Energy transferred = Power × Time

Calculate the energy transferred in kWh.

Give your answer to 2 significant figures.

Explain the difference between direct voltage and alternating voltage.

The electric drill does not need an earth wire.

Explain why.

Mains electricity can be produced in a power station that burns coal.

An electric iron is plugged into the mains and switched on. The temperature of the iron increases.

Describe this process.

Use ideas about energy stores in your answer.