Direct and alternating potential difference

Diagram 1 shows a generator inside a small wind turbine. The generator is connected to a lamp and the windmill blades.

When the coil rotates in the direction of the arrow, the ammeter displays a small current.

Explain how the generator produces a current.

The generator in the wind turbine acts as an alternating current (a.c.) power supply.

Diagram 2 shows an electric circuit containing the generator being used to charge a mobile phone battery.

Direct current (d.c.) is needed to charge the battery.

Explain why there is a diode in the circuit.

Explain how the current in the battery will change if the wind speed increases.

The mean voltage across the battery is 7.2 V.

The battery gains 14 kJ of energy in 8400 seconds.

Calculate the mean current in the battery.

The diagram shows four graphs, P, Q, R and S. Each graph shows a different relationship between current and time.

The table gives descriptions of the relationships between current and time shown by graphs P, Q, R and S.

Complete the table by giving the correct graph for each description.

Diagram 1 shows some of the stages of electricity generation in a nuclear power station.

Nuclear fission takes place inside the reactor of the nuclear power station.

Give the name of a fuel that could be used in the reactor.

Energy is released from the fuel in the reactor by nuclear fission.

Describe the process of nuclear fission that takes place inside the reactor.

High pressure steam transfers energy from the reactor to a turbine.

The turbine spins as the steam passes through it.

Diagram 2 shows a simplified generator connected to the turbine.

Explain how electricity is generated by the generator.

At night the power station does not need to generate as much electricity.

Suggest how the output of this generator could be reduced.

The generator produces alternating current.

Describe the differences between alternating current (a.c.) and direct current (d.c.).

A device called a metal detector can be used to find metal buried underground.

The metal detector has two circuits, each containing a coil of copper wire.

Diagram 1 shows the circuit for the transmitter coil.

Suggest why there is a magnetic field around the transmitter coil.

The cell supplies direct current (d.c.). The electronics in diagram 1 change the direct current into alternating current (a.c.) in the coil.

Describe the difference between direct current (d.c.) and alternating current (a.c.).

Diagram 3 shows the circuit for the receiver coil.

As a result of the alternating current in the gold ring, there is an alternating current in the receiving coil.

Explain how an alternating current in the receiving coil causes a sound to be emitted from the loudspeaker.

The National Grid is used in the United Kingdom for the large-scale transmission of electricity. The photograph shows solar panels connected to the National Grid.

Solar panels provide direct current to a device that outputs an alternating current so that the energy from the solar panels can be supplied to the National Grid.

Explain why a step-up transformer is used to supply the National Grid.

This is the label on the step-up transformer.

State the formula linking input voltage, output voltage and turns ratio for a transformer.

Calculate the number of turns on the secondary coil of the step‑up transformer.

Solar panels produce constant direct current (d.c.).

Explain why a transformer will not work when connected to constant direct current.

A student uses the circuit shown in diagram 1 to investigate how the current changes with voltage for a filament lamp.

Give the name of component Y.

Give a reason why component Y is included in the circuit.

The graph shows some of the student’s results.

State the formula linking charge, current and time.

Determine the current in the lamp when the voltage across the lamp is 10 V.

Calculate the charge transferred through the lamp in 30 s when the voltage across the lamp is 10 V.

Give the unit.

Calculate the time for the lamp to transfer 250 J of energy when the voltage across the lamp is 10 V.

The student disconnects the cell and reconnects it with its terminals reversed.

Complete the graph to show how the current in the lamp varies with voltage across the lamp when the cell is connected with its terminals reversed.

The student replaces the filament lamp with a light emitting diode (LED) and replaces the cell with an alternating current (a.c.) power supply, as shown in diagram 2.

The student also removes the ammeter and voltmeter from the circuit.

Explain why the LED flashes on and off in this circuit.

Diagram 1 shows a simple loudspeaker.

The coil is connected to an alternating current (a.c.) supply.

Describe how the loudspeaker produces sound.

Diagram 2 shows two loudspeakers connected in series with a variable resistor.

The variable resistor is set to 5.0 Ω.

The total voltage across the two loudspeakers is 0.75 V and the current in the circuit is 0.15 A.

Show that the total power of the two loudspeakers is about 0.1 W.

[power = current × voltage]

A student varies the resistance of the variable resistor.

The table shows the power of the loudspeakers for different resistance values of the variable resistor.

Plot the student’s results on the grid.

Draw a curve of best fit.

Diagram 3 shows the loudspeakers connected in series to a cell.

Diagram 4 shows the loudspeakers connected in parallel to the same cell.

Comment on how the total resistance of the loudspeakers in diagram 3 compares with the total resistance of the loudspeakers in diagram 4.

A racing car uses an engine and an electric motor to power its wheels.

The electric motor can only operate at maximum power for part of each lap of the race.

The electric motor has a maximum power output of 1.210⁵W.

The electric motor transfers 4.010⁶J of energy when it operates at maximum power.

Calculate the amount of time the electric motor can operate at maximum power.

The diagram shows a simplified view of a direct current (d.c.) electric motor.

State what is meant by the term direct current.

Explain the purpose of the brushes and the commutator in the d.c. motor.

The electric motor in the racing car can also be used as a generator to charge a battery pack. It is known as a motor generator unit (MGU).

The rotating wheels of the car cause the magnets in the MGU to spin near a coil of wire.

This induces a current in the coil when the coil is connected to the battery pack.

Name the energy store of the battery pack that increases when the battery pack is being charged.

Explain why the kinetic energy store of the car must decrease when the MGU charges the battery pack.

Magnetic Resonance Imaging (MRI) machines can take scans of the inside of a patient.

There is an electromagnet inside the MRI machine.

Describe the construction of an electromagnet.

You may draw a diagram to help your answer.

During the scan, music can be played through a loudspeaker to the patient.

The oscilloscope trace in the diagram represents a sound played to the patient.

Determine the period of the sound wave.

Calculate the frequency of the sound wave.

Deduce whether humans can hear the sound wave.

The current in the loudspeaker is alternating current (a.c.).

Give the evidence from the oscilloscope trace to support this statement.

Draw the circuit symbol for an a.c. power supply.

The photograph shows a solar power farm.

Discuss the advantages and disadvantages of using solar power rather than fossil fuels to generate electricity.

Solar panels produce direct current (d.c.).

The National Grid in many countries operates on alternating current (a.c.).

Describe the difference between direct current (d.c.) and alternating current (a.c.).

This question is about magnetic fields.

Diagram 1 shows a positively charged proton moving downwards in a uniform magnetic field.

magnetic field

Diagram 1

The proton experiences a force to the right.

What is the direction of the magnetic field?

When a current passes through a flat circular coil, a magnetic field is produced.

Complete diagram 2 by drawing the magnetic field of the flat circular coil.

Diagram 2

A wireless charging base uses a magnetic field to charge the battery of a mobile phone.

There is an alternating current in a coil of wire in the charging base.

There is another coil of wire connected to the battery in the mobile phone.

Explain how the wireless charging base charges the battery of the mobile phone.

Discuss the advantages and disadvantages of using a high current in the wireless charging base.