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A second experiment can be carried out to investigate how the strength of the electromagnet depends on the number of turns in the coil.

Method

1. Set up the equipment as in the previous experiment.
2. Wind the insulated wire tightly around the nail to make an electromagnet of 10 turns. Record the number of turns in a suitable table.
3. Adjust the power pack so that a current of 0.5 A flows.
4. Attach as many paper clips to the electromagnet as it will hold. Count the number of paper clips and record in the table. Switch off the electromagnet.
5. Switch on again. Check that the current is still 0.5 A and repeat the paper clip reading. Record in the table and calculate the average number of paper clips held.
6. Repeat this process increasing the number of turns by 10 up to 60 coils. Keep the current at 0.5 A.

As the number of turns increases the number of paper clips held increase.

This means that the strength of the magnet increases with increasing number of turns in the coil.

Once again, since the line of best fit is a straight line through the origin, we can be even more precise.

We can say that, the strength of the electromagnet is directly proportional to the number of turns in the coil.

If you double the number of turns, you double the strength of the electromagnet.

Material of the core

Repeat the experiment once more, keeping the current and number of coils the same.

Replace the iron core with similar lengths of different materials such as steel, wood, plastic, aluminium and air.

Count the number of paper clips held by each material.

Record in a table and compare results.

Iron and steel make strong electromagnets.

Key points

The strength of an electromagnet depends on:

1. The size of the current.
2. The number of turns in the coil.
3. The material of the core.

The strength of the magnetic field increases as:

1. The current increases.
2. The number of turns in the coil increases.
3. An iron core makes a strong electromagnet which can be easily magnetised and demagnetised.