Direct and alternating potential difference

A hairdryer contains a heating element and a motor.

The motor is connected to a fan.

Figure 2 shows part of the electrical circuit inside the hairdryer.

How does the current in the motor compare with the current in the heating element?

Give a reason for your answer.

Tick (✓) one box.

The current is greater in the motor.

The current in both components is the same.

The current is less in the motor.

The potential difference across the motor is 20 V.

What is the potential difference across the heating element?

Give a reason for your answer.

Tick (✓) one box.

20 V 50 V
210 V
230 V

The hairdryer transfers 460 000 J of energy in 250 seconds.

Calculate the power of the hairdryer.

Use the equation:

The current in the hairdryer is 8.0 A.

Calculate the time taken for 1200 C of charge to flow past a point in the hairdryer circuit.

Use the equation:

The hairdryer is connected to the mains electricity supply.

What is the frequency of the UK mains electricity supply?

Tick (✓) one box.

50 Hz 60 Hz
100 Hz
230 Hz

A normal bicycle can be converted into an electric bicycle.

Figure 1 shows a converted bicycle.

Figure 2 shows the circuit diagram for the bicycle.

The circuit symbol for a motor is: M

The potential difference across the motor is 36 V.

The power output of the motor is 252 W.

Calculate the current in the motor.

Use the equation:

The bicycle battery can be recharged using the mains electricity supply.

A battery supplies direct current.

Mains electricity supplies alternating current.

Which graph shows an alternating current?

Tick (✓) one box.

A diode is used to change the alternating current to a direct current.

Which graph shows how the current in a diode varies with potential difference?

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The mean charging current from the mains is 5.0 A for 7200 seconds.

Calculate the charge flow to the battery.

Use the equation:

charge flow = current × time

Choose the unit from the box.

Calculate the work done in charging the battery when the power input is 1150 W for 7200 seconds.

Use the equation:

work done = power × time

A normal bicycle can be converted into an electric bicycle.

Figure 9 shows a converted bicycle.

Figure 10 shows the circuit diagram for the bicycle.

The circuit symbol for a motor is: M

The switch is used to turn the motor on or off.

Explain how the variable resistor is used to control the speed of the motor.

The battery can be recharged using the mains electricity supply.

The mains supply is alternating current.

Figure 11 shows a simplified version of the circuit that is used to recharge the battery.

Explain why charge only passes in one direction through the circuit.

The mains supply fully recharged the battery in 2 hours.

The mean charging current was 5.0 A.

The resistance of the battery was 0.18 Ω.

Calculate the energy dissipated due to the resistance of the battery in 2 hours.

Use the Physics Equations Sheet.

Figure 17 shows a desk lamp connected to the mains electricity supply.

The desk lamp is fitted with a high-efficiency LED bulb.

What does ‘high-efficiency’ mean?

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A large proportion of the total energy input is destroyed.

A large proportion of the total energy input is usefully transferred.

A large proportion of the total energy input is wasted.

The LED bulb wastes energy as thermal energy.

How does the thermal energy affect the temperature of its surroundings?

The output power of the lamp is 2.8 W.

Calculate the energy transferred by the lamp in 60 seconds.

Use the equation:

energy transferred = power × time

Mains electricity can be dangerous.

Table 2 shows information about the effects of different electrical supplies on the human body.

ac is alternating current.

Compare the effects on the human body of 50 Hz ac with 10 000 Hz ac.

Use data from Table 2.

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

Figure 3 shows the three-core cable.

The insulation covering the earth wire has green and yellow stripes.

Give the colours of the insulation covering the live wire and the neutral wire.

A thermistor is used as part of a temperature sensor in the oven.

What is the circuit symbol for a thermistor?

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Figure 4 shows how the resistance of the thermistor in the oven varies with temperature.

Which statement describes the relationship shown in Figure 4 ?

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As temperature increases, resistance decreases.

As temperature increases, resistance stays the same.

As temperature increases, resistance increases.

Which temperature range shows the greatest decrease in the resistance of the thermistor?

Tick (✓) one box.

Between 50 and 100 °C

Between 100 and 150 °C

Between 150 and 200 °C

Between 200 and 250 °C

The resistance of the heating element in the oven is 5.0 Ω.

The current in the heating element is 12 A.

Calculate the power output of the heating element.

Use the equation:

power = (current)² × resistance

Calculate the energy transferred by the oven when 8000 C of charge flows through the heating element.

The potential difference across the heating element is 230 V.

Use the equation:

energy transferred = charge flow × potential difference

An LED on the oven is connected to an alternating current supply.

When the supply is switched on, the LED flashes on and off continuously.

Explain why.

Figure 6 shows a wind turbine.

An alternating potential difference is produced when the wind turbine spins.

What is meant by ‘alternating potential difference’?

Figure 7 shows how the power output of a wind turbine varies with the length of the wind turbine blades when the wind speed is 20 m/s.

It is more economical to construct and use one wind turbine with 40-metre blades than two wind turbines with 20-metre blades.

Explain why.

Use information from Figure 7.

Predict the power output of a wind turbine with a blade length of 50 metres.

Tick (✓) one box.

10 000 kW

12 000 kW

14 000 kW

15 000 kW

The kinetic energy of the air passing the wind turbine blades each second is 73.9 MJ.

The mass of air passing the wind turbine blades each second is 236 000 kg.

Calculate the speed of the air passing the wind turbine.

Some street lights automatically switch on when it gets dark.

Figure 13 shows a street light.

The electrical circuit for the street light includes a light-dependent resistor (LDR).

The power supply for the street light uses alternating current.

What is alternating current?

Tick (✓) one box.

Figure 14 shows how the resistance of the LDR varies with light intensity.

Light intensity is measured in lux

How does light intensity affect the resistance of the LDR?

Write down the equation which links current (I), potential difference (V) and resistance (R).

The potential difference across the LDR is 30 V

The current in the LDR is 0.05 A

Calculate the resistance of the LDR.

Determine the light intensity incident on the LDR.

Use your answer to Question 07.4 and Figure 14.

This street light may stay switched on when it is not dark.

What is the most likely cause of this problem?

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This street light is replaced with one which is more efficient.

What does more efficient mean?

Tick (✓) one box.

A student investigated the electrolysis of potassium chloride solution.

Figure 1 shows the apparatus used.

The student used the piece of equipment labelled A to measure the volume of gas collected.

What is the piece of equipment labelled A ?

Tick (✓) one box.

What does the circuit symbol B represent?

Complete the sentence.

Choose the answer from the box.

Direct current (dc) is supplied to the circuit.

What is direct current?

Tick (✓) one box.

Potassium chloride solution contains potassium (K⁺) ions and chloride (Cl^–) ions.

Why are chloride ions attracted to the positive electrode?

Hydrogen gas is produced at the negative electrode.

Which test should the student use to identify hydrogen gas?

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Hydrogen ions and potassium ions move to the negative electrode.

Hydrogen gas is produced at the negative electrode.

Why is hydrogen gas produced at the negative electrode?

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The student measured the volume of gas collected at each electrode every minute for 25 minutes.

Figure 2 shows the results.

Describe one similarity and one difference in the volume of hydrogen and the volume of chlorine collected during the 25 minutes.

Use Figure 2.

Some new cars have an electric motor that is powered by a battery.

A battery supplies direct current.

What is direct current?

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There are different types of battery available.

Table 2 shows the maximum distance a car can travel before the battery needs recharging.

Complete Figure 5.

You should:

• label the x-axis

• label the y-axis

• plot the data from Table 2.

A car with a lead-acid battery travels 80% of the maximum distance it can travel before the battery needs recharging.

Determine the distance the car travels.

A lithium-ion battery is put on charge for 1800 s

The current is 40 A

Calculate the total charge flow during this time.

Use the equation:

charge flow = current × time

The driver of a car saw an obstacle in the road. He applied the brakes until the car stopped.

The thinking distance was 9.0 m

The braking distance was 13.5 m

Calculate the stopping distance of the car.

The driver had been drinking alcohol. The car had worn brakes.

Explain why these factors would increase the stopping distance of the car.

A student investigated the motor effect.

Figure 9 shows the apparatus used.

The student closed the switch and the wire moved.

Explain why.

Give two ways of reversing the direction of the movement of the wire.

Electric motors use the motor effect.

Energy is supplied to the electric motor by a battery.

The battery is charged using a charger.

When the charger is connected to the battery, the potential difference across the battery is 15.0 V

The total energy stored when the battery is fully charged is 0.81 MJ

The average current used to charge the battery is 3.00 A

Calculate the time taken to fully charge the battery.

The charger is plugged into the mains electricity supply.

This is an alternating supply.

Give two other properties of the UK mains electricity supply.