Transformers

The battery inside an electric toothbrush is recharged when the toothbrush is placed on its charger.

Figure 9 shows part of the toothbrush and the charger.

Explain how the current is produced in the secondary coil of the transformer in Figure 9.

The potential difference across the primary coil is 230 V.

The potential difference across the secondary coil is 1.5 V.

The primary coil has 1840 turns of wire.

Calculate the number of turns of wire in the secondary coil.

number of turns =_........

In Figure 9, a diode is connected in series with the rechargeable battery.

Explain why a diode is needed in the toothbrush in this part of the circuit.

Figure 10 shows a simplified diagram of the split ring commutator in a d.c. electric motor.

Explain the function of the commutator in a d.c. electric motor.

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.

Figure 11 shows part of the UK National Grid system for the supply of electricity to homes.

Electricity supplied to homes has a frequency of

Explain why the National Grid uses high voltages with small currents to transfer electricity from power stations.

Figure 12 shows details of a transformer.

primary coil secondary coil 80 000 turns of wire 1600 turns of wire

primary voltage = 12 000 V secondary voltage = 240 V

primary current = 0.64 A secondary current = 32 A

Figure 12

Calculate the power in the primary coil.

Use the equation

power in the primary coil =_........

Calculate the following for the transformer in Figure 12.

number of turns in secondary coil number of turns in primary coil

For the transformer in Figure 12, evaluate, in its simplest form, the ratio

secondary voltage: primary voltage

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.

This question is about using the mains electricity supply.

An electric kettle is used to boil some water. The mains supply voltage is 230 V. The power supplied to the kettle is 1.9 kW.

Calculate the current in the kettle. Use the equation

current supplied to the kettle =_........

A coffee machine takes 120 s to heat some water.

Mains supply voltage = 230 V Current in this coffee machine = 7.4 A

Calculate the energy transferred to the coffee machine in 120 s.

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

energy transferred to coffee machine =_........

Figure 10 shows the inside of a mains plug. The neutral wire is labelled.

State the name of wire X and the name of wire Y.

State the name of component Z.

A transformer is used to connect a laptop computer to the mains electricity supply. The input voltage to the transformer is 230 V. The output current from the transformer is 2.37 A. The transformer has an output voltage of 19.0 V. The transformer used is 100% efficient.

Calculate the input current to the transformer.

Use the equation

input current × input voltage = output current × output voltage

input current =_........

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 4 shows the inside of a mains plug.

The mains plug has three safety features.

One of these safety features has been ticked in the table.

Put two more ticks in the table to show the other two safety features.

Figure 5 shows a charger for a car battery.

The meter on the battery charger shows the current supplied to a battery.

The meter on the battery charger is

The battery charger supplies a steady current of 2.5 A to the battery.

Calculate the charge flowing to the battery in 8 minutes.

Use the equation

charge = currenttime

The transformer in another battery charger has a primary coil and a secondary coil.

The voltage across the primary coil = 230 V.

The voltage across the secondary coil = 15 V.

The current in the secondary coil is 3.1 A.

Calculate the current in the primary coil.

Use the equation

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.