Electromagnetic Induction and Transformers

Using the axes in Fig. 7.1, sketch a graph to show how the voltage of the supply varies in a time of $0.05\ \text{s}$.

The core of the transformer is made from iron.

Explain why iron is a suitable material for the core of the transformer.

Explain why there is a voltage across the secondary coil.

There are 60 turns on the secondary coil.

Calculate the maximum value of the voltage across the secondary coil.

maximum voltage = ______ $\text{V}$

State the material used for the core of the transformer.

A current in the primary coil produces a magnetic field in the core.

Explain how an alternating voltage is produced in the secondary coil.

The transformer has 4600 turns on the primary coil which is connected to a mains supply of $230\text{ V}$.

An output of $5.0\text{ V}$ is used to operate the bell.

Calculate the number of turns needed on the secondary coil.

number of turns = ______

State one change that can be made to the transformer shown in Fig. 8.1 so that it can be used as a step-up transformer.

Explain why there is a reading on the ammeter.

Complete Table 6.1 by stating what the ammeter shows when the metal bar moves in the direction shown by the arrow in each diagram.

Table 6.1

The student finds that using a stronger magnet increases the reading on the ammeter.

State one other way in which the student can produce a larger reading on the ammeter, using the same rod.

Describe how Lenz's law applies when the bar is moved upwards.

Sketch a labelled diagram to show the structure of a transformer that is used to increase voltage.

Describe the principle of operation of a transformer.

Explain one advantage of transmitting electricity at a high voltage.

State whether coil A or coil B in the transformer has the larger number of turns.

Give a reason for your answer.

One purpose of using an iron core in the transformer is to increase the magnetic field inside coil A.

State one other purpose of using the iron core.

Describe briefly how an output voltage is induced in coil B.

Suggest one advantage and one disadvantage of burying a power line underground rather than having an overhead power line.

advantage ______

disadvantage ______

Explain why there is a reading on the voltmeter.

The reading on the voltmeter is increased by using a stronger magnetic field.

State one other way of producing a larger reading on the voltmeter.

the core of a transformer.

______

Describe one use for a transformer.

In the space below, sketch a graph of the output voltage against time for the a.c. generator.

The transformer has $1700\text{ turns}$ on the primary coil and $85\text{ turns}$ on the secondary coil.

Calculate the output voltage of the transformer.

output voltage = ______

Explain why there is a current in the ammeter.

Explain how the current in the ammeter opposes the change producing it.

The magnet stops when the S-pole reaches the middle of the solenoid.

The reading on the ammeter is observed when the magnet is moved to the left at a constant speed that is less than its speed in (c)(i).

State two ways in which the reading on the ammeter differs from the reading observed in (c)(i) as the magnet moves to the left.

1. ______
2. ______

Explain why there is an induced e.m.f. in the coil when the magnet moves.

Explain why one LED lights up when the magnet moves into the coil and the other LED lights up when the magnet moves out of the coil.

The LEDs are brighter when the magnet moves faster.

Explain why.

why an e.m.f. is induced,

why the e.m.f. is sometimes positive and sometimes negative.

Changes are made to the a.c. generator, one at a time:

• stronger magnets are used
• more turns are wound on the coil
• the coil is turned faster.

Complete the table in Fig. 6.3 to show what happens to the maximum value of the e.m.f. and to the frequency of the alternating e.m.f.

Fig. 6.3

Explain why the needle on the ammeter deflects.

The wire is moved downwards very quickly between the two magnetic poles.

State what happens to the needle on the ammeter.

State what happens to the needle on the ammeter when the copper wire is moved horizontally between the two poles.