Direct and alternating potential difference

Figure 5 shows a plastic kettle.

The kettle is connected to the mains electricity supply.

What is the frequency of the UK mains electricity supply?

Tick () one box.

The cable that connects the kettle to the plug contains two insulated wires.

Draw one line from each wire to the colour of the insulation on the wire.

Use the Physics Equations Sheet to answer questions 04.3 and 04.4.

Write down the equation that links current (I), potential difference (V) and power (P).

The power of the heating element in the kettle is 2100 W.

The potential difference across the heating element is 230 V.

Calculate the current in the heating element.

Give your answer to 2 significant figures.

The kettle is designed to switch off when the water reaches boiling point.

Before the kettle switches off, 0.0080 kg of water changes state.

The energy transferred to change the state of the water is 18 000 J.

Calculate the specific latent heat of vaporisation of the water.

Use the Physics Equations Sheet.

Specific latent heat of vaporisation = J/kg

A different kettle is used to heat some water.

The useful output energy transferred to the water is 75 600 J.

The total input energy transferred by the kettle is 84 000 J.

Calculate the efficiency of the kettle.

Use the equation:

Explain one advantage of electrical appliances having a high efficiency.

Figure 5 shows a laptop computer, and a charger plugged into the mains supply.

Give the potential difference and frequency of the UK mains electricity supply.

The cable that connects the charger to the plug contains two wires that have coloured insulation.

The cable does not contain an earth wire.

Name each wire in the cable.

Give the colour of the insulation covering each wire.

Name of wire Colour of insulation

The input to the charger is an alternating potential difference.

The output from the charger is a direct potential difference.

Describe the difference between an alternating potential difference and a direct potential difference.

A student is using the laptop computer.

The student notices that the battery has no stored energy.

The student quickly plugs the charger into the laptop and continues to use the laptop.

The laptop charger provides a constant power to the laptop.

The laptop takes longer to charge when the student uses the laptop than when the laptop is not used.

Explain why.

The potential difference across the laptop circuit is 15 V.

The average current in the laptop circuit when it is being used is 4.8 A.

The laptop charger provides a constant power of 97 W.

The maximum energy that the battery can store is 225 kJ.

Calculate the time taken to fully charge the battery from zero when the laptop is used continuously.

Use the Physics Equations Sheet.

Give your answer in hours.

Figure 10 shows an electric whisk that plugs into the mains electricity supply.

The whisk can mix food by spinning a beater.

Give two energy stores that increase when the whisk is switched on.

Use the Physics Equations Sheet to answer questions 06.2 and 06.3.

Work is done by the whisk when it is used to mix food.

Write down the equation that links power (P), time (t) and work done (W).

The power output of the whisk is 92 W.

Calculate the time for the whisk to do 23 000 J of work.

Use the Physics Equations Sheet to answer questions 06.4 and 06.5.

Which equation links current (I), power (P) and resistance (R)?

Tick (✓) one box.

The current in the whisk is 500 mA.

The resistance of the whisk is 640 Ω.

Calculate the power of the whisk.

The whisk has several settings that allow the beater to spin at different speeds.

A faster beater speed needs a greater power input from the mains electricity supply.

What is the potential difference between the live wire and neutral wire in the mains electricity supply?

Changing the beater speed does not change the potential difference between the live wire and neutral wire.

The power input to the whisk changes because the current in the whisk changes.

Complete the sentence.

When the beater speed increases, the current in the whisk increases because the

Figure 1 shows an electric whisk that plugs into the mains electricity supply.

The whisk can mix food by spinning a beater.

Give two energy stores that increase when the whisk is switched on.

Use the Physics Equations Sheet to answer questions 01.2 and 01.3.

Work is done by the whisk when it is used to mix food.

Write down the equation that links power (P), time (t) and work done (W).

The power output of the whisk is 92 W.

Calculate the time for the whisk to do 23 000 J of work.

Use the Physics Equations Sheet to answer questions 01.4 and 01.5.

Which equation links current (I), power (P) and resistance (R)?

Tick () one box.

The current in the whisk is 500 mA.

The resistance of the whisk is 640 Ω.

Calculate the power of the whisk.

The whisk has several settings that allow the beater to spin at different speeds.

A faster beater speed needs a greater power input from the mains electricity supply.

What is the potential difference between the live wire and neutral wire in the mains electricity supply?

Changing the beater speed does not change the potential difference between the live wire and neutral wire.

The power input to the whisk changes because the current in the whisk changes.

Complete the sentence.

When the beater speed increases, the current in the whisk increases because the

A TENS machine uses an electrical current to relieve pain. Figure 8 shows the electrodes of a TENS machine connected across an ankle.

The maximum power of the TENS machine is 240 mW. The potential difference across the battery in the TENS machine is 2.5 V. Calculate the maximum current from the battery.

Figure 9 is a sketch graph showing how the potential difference across the electrodes varies with time.

A student concluded that there was an alternating potential difference across the electrodes. How does Figure 9 show that the student was not correct?

Figure 10 shows a circuit the student built using the battery from the TENS machine.

The student recorded how the current in the resistor varied with the potential difference across the resistor. Figure 11 shows a sketch graph of the results.

What relationship does Figure 11 show?

Figure 12 shows two more circuits that the student built using the battery from the TENS machine.

The resistors all have the same resistance. Compare the readings on the voltmeter and ammeter in circuit A and circuit B.

Figure 5 shows the inside of a plug.

The plug is not wired correctly. What should be done to connect the wires in the plug correctly?

The correctly wired plug and cable connects a washing machine to the mains electricity supply.

Give the potential difference and frequency of the mains electricity supply in the UK.

The washing machine is switched on. What is the potential difference between the neutral wire and the earth wire?

The plug has a fuse. Draw the circuit symbol for a fuse in the space below.

The washing machine has a metal case. A fault causes the live wire to make an electrical connection with the metal case of the washing machine.

The earth wire is not connected to the metal case of the washing machine. Explain why it would not be safe for a person to touch the metal case.

The earth wire is now connected to the metal case of the washing machine. Explain why it would now be safe for a person to touch the metal case, even if the live wire touches the metal case.

Figure 1 shows the inside of a plug.

The plug is not wired correctly. What should be done to connect the wires in the plug correctly?

The correctly wired plug and cable connects a washing machine to the mains electricity supply.

Give the potential difference and frequency of the mains electricity supply in the UK.

The washing machine is switched on. What is the potential difference between the neutral wire and the earth wire?

The plug has a fuse. Draw the circuit symbol for a fuse in the space below.

The washing machine has a metal case. A fault causes the live wire to make an electrical connection with the metal case of the washing machine.

The earth wire is not connected to the metal case of the washing machine. Explain why it would not be safe for a person to touch the metal case.

The earth wire is now connected to the metal case of the washing machine. Explain why it would now be safe for a person to touch the metal case, even if the live wire touches the metal case.

A hybrid car has an electric motor and a petrol engine.

Petrol is a non-renewable energy resource. What is meant by a non-renewable energy resource?

The electric motor in the car is powered by a battery. To charge the battery, the car is plugged into the mains supply at 230 V The power used to charge the battery is 6.9 kW Calculate the current used to charge the battery.

Mains electricity is an ac supply. Explain the difference between direct and alternating potential difference.

The cable used to connect the car to the mains electricity supply has a low resistance. Explain why it is better to use a cable with a low resistance than to use a cable with a high resistance.