Electrical charge & current

A student built a circuit to investigate how the efficiency of a motor varies with the speed of the motor.

Figure 12 shows the circuit.

The student used the motor to lift a mass through a height of 1.0 m.

The motor took 12 seconds to lift the mass.

The charge flow through the circuit was 6.0 C.

Calculate the current in the circuit.

Use the equation:

The potential difference across the motor was 8.0 V.

The charge flow through the motor was 6.0 C.

Calculate the energy transferred to the motor.

Use the equation:

energy transferred = charge flow × potential difference

Use the Physics Equations Sheet to answer questions 07.3 and 07.4.

Which equation links efficiency, total input energy transfer and useful output energy transfer?

Tick () one box.

The efficiency of the motor was 0.70

The useful output energy transfer of the motor was 1.96 J.

Calculate the total input energy transfer to the motor.

The student adjusted the speed at which the mass was lifted.

For each speed, the student determined the efficiency of the motor.

Figure 13 shows the results.

Use Figure 13 to predict the efficiency of the motor when the speed is 0.6 m/s.

How does Figure 13 show that the efficiency of the motor is not directly proportional to the speed?

Figure 11 shows a garden water feature.

The water feature has an electric motor that pumps water from the bottom container to the top container.

The pump transfers 0.343 J of gravitational potential energy to the water each second.

gravitational field strength = 9.8 N/kg

Determine the mass of water passing through the pump each second.

Use the Physics Equations Sheet.

The power of the pump motor is 4.86 W.

The resistance of the pump motor is 6.0 Ω.

Calculate the charge flow through the motor in 30 minutes.

Use the Physics Equations Sheet.

The pump motor breaks and is replaced with a new motor.

The new pump motor has the same power input, but has a greater efficiency.

Explain how the new pump motor would affect the flow of water in the water feature.

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.

A student investigated how the current in a filament lamp varies with the potential difference across the lamp.

Figure 1 shows the circuit used.

What is component P ?

Complete the sentences.

Choose answers from the box.

The ammeter in the circuit measures.

The voltmeter in the circuit measures.

How will increasing the resistance of the variable resistor in Figure 1 affect each of the following quantities?

Tick (✓) one box in each row.

A charge flow of 15 coulombs passed through the filament lamp in a time of 60 seconds.

Calculate the current in the lamp.

Use the equation:

When the current in the filament lamp is 0.12 A, the potential difference across the lamp is 6.0 V.

Calculate the resistance of the filament lamp.

Use the equation:

The student repeated the investigation after replacing the lamp with a resistor at constant temperature and then a diode.

The student plotted a graph for each component.

Draw one line from each component to its graph.

Figure 2 shows an ammeter.

The ammeter is not connected to a circuit.

What type of error does the ammeter display?

Tick (✓) one box.

Figure 14 shows an electrical circuit used to heat the windscreen of a car.

Each resistor in the circuit represents a heating element.

The 12 V battery supplies direct potential difference.

What is meant by ‘direct potential difference’?

Use the Physics Equations Sheet to answer questions 10.2 and 10.3.

Which equation links charge flow (Q), energy (E) and potential difference (V)?

Tick (✓) one box.

Calculate the charge flow through the 12 V battery when the battery transfers 5010 J of energy.

Ice forms on the windscreen at a temperature of 0 °C.

The electrical circuit transfers 5010 J of energy to the ice.

A mass of 0.015 kg of ice melts.

Calculate the specific latent heat of fusion of water.

Use the Physics Equations Sheet.

The electrical circuit was left switched on while the ice changed from a solid to a liquid and increased in temperature to 5 °C.

Explain the changes in the arrangement and movement of the particles as the ice melted and the temperature increased to 5 °C.

Figure 4 shows an electrical circuit used to heat the windscreen of a car. Each resistor in the circuit represents a heating element.

The 12 V battery supplies direct potential difference. What is meant by ‘direct potential difference’?

Use the Physics Equations Sheet to answer questions 03.2 and 03.3.

Which equation links charge flow (Q), energy (E) and potential difference (V)? Tick (✓) one box.

Calculate the charge flow through the 12 V battery when the battery transfers 5010 J of energy.

Ice forms on the windscreen at a temperature of 0 °C. The electrical circuit transfers 5010 J of energy to the ice. A mass of 0.015 kg of ice melts. Calculate the specific latent heat of fusion of water. Use the Physics Equations Sheet.

The electrical circuit was left switched on while the ice changed from a solid to a liquid and increased in temperature to 5 °C. Explain the changes in the arrangement and movement of the particles as the ice melted and the temperature increased to 5 °C.

Figure 8 shows how electricity is supplied to consumers.

Electricity from the power station can be generated using renewable or non-renewable energy resources.

Complete Table 2 to show which energy resources are renewable and which are non-renewable.

Tick (✓) one box in each row.

Transformers are used to make power transmission an efficient process.

Complete the sentences.

Choose answers from the box.

Each answer may be used once, more than once or not at all.

decreases the.

transfer from the overhead transmission cables to the surroundings.

The step-down transformer decreases the

Use the Physics Equations Sheet to answer questions 07.3 and 07.4.

Write down the equation which links charge flow (Q), current (I) and time (t).

The town of Hornsdale in Australia has electricity supplied by a huge battery.

The battery supplies a current of 130 000 A.

Calculate the charge flow from the battery in 5 minutes.

Choose the unit from the box.

Figure 8 shows how electricity is supplied to consumers by the National Grid.

Explain why transformer X is used in the National Grid.

Explain why transformer Y is used in the National Grid.

The town of Hornsdale in Australia has electricity supplied by a huge battery. On one day the battery transferred 3.24 × 10¹¹ J of energy to the town. The potential difference of the town’s electricity supply is 230 V. Calculate the charge flow to the town on this day. Use the Physics Equations Sheet. Give your answer to 3 significant figures.

Charge flow (3 significant figures) = C

Figure 2 shows part of the National Grid linking a power station to consumers.

Name the parts of Figure 2 labelled A and B.

Electricity is transmitted through A at a very high potential difference.

What is the advantage of transmitting electricity at a very high potential difference?

Tick (✓) one box.

The power station generates electricity at a potential difference of 25 000 V.

The energy transferred by the power station in one second is 500 000 000 J.

Calculate the charge flow from the power station in one second.

Use the equation:

The electricity supply to a house has a potential difference of 230 V.

Table 1 shows the current in some appliances in the house.

Calculate the total power of all the appliances in Table 1.

Use the equation:

power = potential difference × current

Each appliance in Table 1 is switched on for 2 hours.

Which appliance will transfer the most energy?

Give a reason for your answer.

The average energy transferred from the National Grid every second for each person in the UK is 600 J.

There are 32 000 000 seconds in one year.

Calculate the average energy transferred each year from the National Grid for each person in the UK.

Figure 10 shows a house with a solar power system.

The solar cells generate electricity.

When the electricity generated by the solar cells is not needed, the energy is stored in a large battery.

The solar cells on the roof of the house always face in the same direction.

Explain one disadvantage caused by the solar cells only facing in one direction.

The mean current from the solar cells to the battery is 3.5 A.

Calculate the charge flow from the solar cells to the battery in 3600 seconds.

Use the equation: charge flow = current × time

Write down the equation which links efficiency, total power input and useful power output.

At one time in the day, the total power input to the solar cells was 7500 W.

The efficiency of the solar cells was 0.16

Calculate the useful power output of the solar cells.

The wasted energy that is not usefully transferred by the solar cells is dissipated.

What happens to energy that has been dissipated?

Tick (✓) one box.

Why is it unlikely that all the UK’s electricity needs could be met by solar power systems?

Tick (✓) one box.

During one year, 1.25 × 10¹⁸ J of energy was transferred from the National Grid. number of seconds in 1 year = 3.16 × 10⁷ Calculate the mean energy transferred from the National Grid each second. Give your answer to 3 significant figures.

Figure 5 shows a house with a solar power system. The solar cells generate electricity. When the electricity generated by the solar cells is not needed, the energy is stored in a large battery.

The charge flow through the cable between the solar cells and the battery in 24 hours was 27 000 coulombs. Calculate the mean current in the cable.

At one time, the total power input to the solar cells was 7.8 kW. The efficiency of the solar cells was 0.15 Calculate the useful power output of the solar cells.

It is unlikely that all of the electricity that the UK needs can be generated by solar power systems. Explain why.

Figure 13 shows an LED torch.

The torch contains one LED, one switch and three cells. Which diagram shows the correct circuit for the torch? Tick (✓) one box.

Write down the equation which links charge flow (Q), current (I) and time (t).

The torch worked for 14 400 seconds before the cells needed replacing. The current in the LED was 50 mA. Calculate the total charge flow through the cells.

When replaced, the cells were put into the torch the wrong way around. Explain why the torch did not work.

Write down the equation which links efficiency, total power input and useful power output.

The total power input to the LED was 0.24 W. The efficiency of the LED was 0.75 Calculate the useful power output of the LED.

Figure 3 shows an LED torch.

The torch contains one LED, one switch and three cells. Which diagram shows the correct circuit for the torch? Tick (✓) one box.

Write down the equation which links charge flow (Q), current (I) and time (t).

The torch worked for 14 400 seconds before the cells needed replacing. The current in the LED was 50 mA. Calculate the total charge flow through the cells.

When replaced, the cells were put into the torch the wrong way around. Explain why the torch did not work.

Write down the equation which links efficiency, total power input and useful power output.

The total power input to the LED was 0.24 W. The efficiency of the LED was 0.75 Calculate the useful power output of the LED.

Figure 9 shows a circuit diagram.

In which position could a switch be placed so that both lamps can be switched on or off at the same time?

Tick (✓) one box.

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

In 30 seconds, 24 coulombs of charge flow through the battery. Calculate the current in the battery. Use the equation:

There is a potential difference of 3.6 V across the battery. Calculate the energy transferred by the battery when 60 coulombs of charge flows through the battery. Use the equation: energy transferred = charge flow × potential difference

A student built Circuit X and Circuit Y shown in Figure 10. The components used in each circuit were identical.

Figure 10

Circuit X Circuit Y

How would the reading on the ammeter in Circuit Y compare to the reading on the ammeter in Circuit X ?

Tick (✓) one box.

How does the total resistance of Circuit Y compare with the total resistance of Circuit X ?

Tick (✓) one box.

The student built another circuit which is shown in Figure 11.

Write down the equation which links current, potential difference and resistance.

There is a potential difference of 3.6 V across the lamp in Figure 11. The current through the lamp is 0.80 A

Calculate the resistance of the lamp.

Figure 12 shows a student after rubbing a balloon on his hair.

The balloon and hair have become charged.

Describe the force that acts on the student’s hair in Figure 12.

An earthed conductor was brought near the charged student. A spark jumped between the conductor and the student.

The potential difference between the conductor and the student was 2.5 kV The energy transferred by the spark was 0.0050 J

Calculate the charge transferred by the spark.

A defibrillator can transfer a charge to regulate a person’s heartbeat.

Figure 13 shows a defibrillator.

When the defibrillator is in use, a potential difference of 4800 V is applied across the person’s chest.

A charge of 0.16 coulombs passes through the person’s chest in 4.0 ms

Calculate the resistance of the person’s chest.

The plug of an electrical appliance contains a fuse.

What is the correct circuit symbol for a fuse?

Tick one box.

The appliance is connected to the mains electrical supply. The mains potential difference is 230 V.

Calculate the energy transferred when 13 C of charge flows through the appliance.

Use the equation:

energy transferred = charge flow × potential difference

Figure 9 shows the structure of a fuse.

Write down the equation that links charge flow, current and time.

The fuse wire melts when 1.52 coulombs of charge flows through the fuse in 0.40 seconds.

Calculate the current at which the fuse wire melts.

The mass of the fuse wire is 0.00175 kg. The specific latent heat of fusion of the fuse wire is 205 000 J/kg.

Calculate the energy needed to melt the fuse wire.

Use the Physics Equations Sheet.