Induced potential and the generator effect

Figure 23 shows a magnet being dropped through a coil.

The coil is connected to a sensitive voltmeter.

Explain why the voltmeter shows a reading as the magnet passes through the coil.

A student develops this investigation to find out how the reading on the voltmeter depends on the height the magnet is dropped from.

Describe how the student could use the arrangement in Figure 23 to do this investigation.

* (b) A transformer converts a voltage of 11 000 V to 230 V.

Explain how the design of this transformer enables the voltage to be converted from 11 000 V to 230 V.

Your answer should include

• details of the structure of a transformer

• how a transformer works, using ideas of electromagnetic induction

• how the design of this transformer enables this exact voltage of 230 V to be obtained.

You may draw a diagram to help your answer.

An electric car is travelling at a speed of 16.0 m/s.

The total mass of the car is 1200 kg.

Calculate the kinetic energy, in kJ, of the car.

kinetic energy =_........

On a journey, the car transfers energy from the battery at an average rate of 17.5 kW.

The battery in the car transfers a total of 126 MJ of energy before it becomes discharged.

Calculate the time taken for the battery to become discharged on this journey.

Give your answer in hours.

time taken =_........ hours

Figure 20 shows an electrical device connected to the wheels of an electric car.

dynamo when the car decelerates.

Explain how using the device can help to increase the time that the car can be driven before the battery becomes discharged.

The battery can be recharged at a charging point.

The charging point provides an average current of 15.0 A to the battery, at a potential difference (voltage) of 400 V.

It is claimed that 126 MJ of energy can be transferred to the battery in less than 6 hours.

Comment on this claim.

Use this equation to support your answer

Calculate the total charge that moves into the battery while it is being recharged.

Use the equation

Figure 23 shows a model dynamo.

The dynamo contains a coil of wire that can spin inside a permanent magnet.

The dynamo produces a D.C. output.

A teacher connects a voltmeter to the terminals of the dynamo.

The teacher rotates the handle to make the coil spin inside the magnet.

Figure 24(a) shows the reading on the voltmeter.

The teacher then rotates the handle differently.

Figure 24(b) shows the new reading on the voltmeter.

Which row of the table shows how the rotation of the handle has changed between (a) and (b)?

The teacher connects the dynamo to a lamp.

It is now more difficult for the teacher to rotate the handle.

Explain why it is more difficult to turn the dynamo when it is connected to a lamp.

Figure 25 shows a transformer.

The number of turns on the primary coil, N_p = 800

The potential difference across the primary coil, V_p = 230 V

The number of turns on the secondary coil, N_s = 18

Calculate the potential difference across the secondary coil.

Use an equation selected from the list of equations at the end of the paper.

potential difference across the secondary coil =_........

*(c) Figure 26 shows a picture of an electrical device and a simplified drawing of the important parts.

The device can be used as a loudspeaker or it can be used as a microphone.

Compare how the device operates when used as a loudspeaker with how the device operates when used as a microphone.

Figure 20 shows a magnet and a coil.

The coil is connected to a sensitive centre-zero ammeter.

Explain what will be observed on the meter when the magnet is pushed in and pulled out of the coil, repeatedly.

This question is about a transformer.

iron core

primary coil secondary coil 700 turns of wire 400 turns of wire

primary voltage = 230 V secondary current = 1.75 A

Figure 21

Calculate the current in the primary coil.

Use the information given in Figure 21 and equations selected from the list of equations at the end of this paper.

The transformer is 100% efficient.

current in the primary coil =_........

*(c) Figure 22 shows how electricity is delivered efficiently from a power station (P) to homes (T).

Using Figure 22, explain the stages in the process of delivering electricity efficiently from P to T.

Your answer should include details of the effects that Q, R and S have on efficiency.

There is an alternating current of 3 A in the primary coil of a transformer.

There is an alternating current of 6 A in the secondary coil of the transformer.

The transformer is 100% efficient.

The size of the potential difference (voltage) across the secondary coil is

Explain how an alternating current in the primary coil causes an alternating current in the secondary coil of the transformer.

The primary coil of a different transformer is connected to the 230 V mains supply.

The voltage across the secondary coil is 15 V.

The primary coil has 600 turns.

Calculate the number of turns on the secondary coil.

Use an equation selected from the list of equations at the end of the paper.

*(c) Figure 19 shows a coil of wire that is being rotated between the poles of a magnet.

Figure 20 shows how the current in the coil changes during one complete rotation of the coil.

Explain why the current changes in the way shown by the graph in Figure 20.

Your answer should include details of the position of the coil relative to the magnet at each of the times labelled P, Q, R, S and T.

You may use diagrams to help your answer.

Figure 15 shows three stages of a magnet moving into and then out of a coil of wire.

The coil is connected to a milliammeter.

Which row of the table shows the deflection on the milliammeter for the three stages in Figure 15?

Give two ways of increasing the deflections on the milliammeter shown in Figure 16.

Figure 17 is a diagram representing a loudspeaker.

Explain how sound is produced when an alternating current is supplied to the coil of the loudspeaker.

State the purpose of the transformer shown in Figure 18.

Calculate the output voltage of the secondary coil.

Use an equation selected from the list of equations at the end of this paper.

A teacher is demonstrating electromagnetic induction. The teacher has a bar magnet, a coil of wire and a sensitive voltmeter.

Draw a diagram to show how the teacher should arrange the apparatus.

Explain how the teacher could use this apparatus to demonstrate the factors affecting the size and direction of the induced potential difference.

There is a changing magnetic field in the core of a transformer.

Describe the cause of the changing magnetic field in the core of the transformer.

A potential difference of 230 V is applied across the primary coil of a transformer. There is a potential difference of 15 V across the secondary coil. The primary coil has 2000 turns.

Calculate the number of turns in the secondary coil.

Use an equation selected from the list of equations at the end of this paper.

Which of these could be the output for a dynamo?

Figure 17 shows the output from a battery.

Explain why a transformer will not work with the input current as shown in Figure 17.

A transformer has 30 turns on the primary coil and 150 turns on the secondary coil.

A potential difference of 25 V is applied across the primary coil.

Calculate the potential difference across the secondary coil.

Use an equation selected from the list of equations at the end of this paper.

*(c) High voltage transmission cables and transformers are used in the national grid.

Explain how using high voltage transmission cables and transformers allows the distribution of electrical power around the United Kingdom to be as efficient as possible.

Refer to the following equations in your answer.