Induced Magnetism & Electrical Method Of Magnetisation

1. Definition

Induced magnetism is the magnetisation of a magnetic material when it is placed in a magnetic field, for example near a magnet or inside a current-carrying solenoid.

2. Key Ideas

• The end of a magnetic material nearest a magnet becomes the opposite pole, so it is attracted.
• Soft iron: easy to magnetise, easy to demagnetise → good for electromagnets (temporary magnets).
• Steel: harder to magnetise, keeps magnetism better → good for permanent magnets.
• A current-carrying solenoid behaves like a bar magnet; reversing the current reverses the poles.

Scroll diagram horizontally to read all labels.

3. Detailed Explanations

A. Induced magnetism near a bar magnet

If the north pole of a magnet is brought near an unmagnetised iron bar:

• the near end becomes a south pole
• the far end becomes a north pole

So the iron bar is attracted (unlike poles face each other).

This happens whichever pole of the magnet is brought near: the nearest end of the iron becomes the opposite pole. Therefore, attraction is not proof that an object is already a magnet. Repulsion is the decisive test because an unmagnetised magnetic material can be attracted by either pole.

B. Electrical method of magnetisation (solenoid method)

1. Place an iron or steel rod along the axis of a solenoid.
2. Pass a d.c. current through the solenoid so its field has a steady direction.
3. The rod becomes magnetised in the direction of the solenoid field.

Reversing the current reverses the solenoid field. A soft-iron rod follows this reversal readily. For a steel rod that already retains magnetism, the reversed field must be sufficiently strong to reverse its magnetisation; simply reversing a weak current may not fully reverse the retained poles.

C. Temporary vs permanent (iron vs steel)

• With a soft iron core, the magnetism usually disappears quickly when the current stops (useful for switching on/off).
• With a steel core, some magnetism can remain (permanent magnet).

4. Common Mistakes

• Saying the near end becomes the same pole as the magnet (it becomes the opposite pole).
• Claiming attraction proves that both objects are magnets. An unmagnetised iron object is attracted by either pole through induction; use repulsion as the test for a magnet.
• Saying induced magnetism is always permanent (soft iron is usually temporary).
• Assuming any reversed current must reverse a previously magnetised steel rod. The reversed field must be sufficiently strong.

5. Exam Tips

1. Use the key phrase: “nearest end becomes opposite pole, so it is attracted”.
2. If asked to choose a core: soft iron for electromagnets, steel for permanent magnets.
3. For the electrical method, state rod along the solenoid axis and d.c. current.

6. Worked Examples

Example 1: Poles of an induced magnetCore

The north pole of a bar magnet is held near the left end of an unmagnetised iron bar. Label the poles induced on the iron bar.

Show Answer

The left end (near the north pole) becomes south, and the right end becomes north.

Example 2: Choosing a material for an electromagnetCore

You want a magnet that can be switched on/off quickly (e.g. electric bell). Should you use soft iron or steel for the core? Explain.

Show Answer

Use soft iron. It magnetises easily when current flows and loses magnetism quickly when current stops, so it can switch on/off rapidly.

Example 3: Reversing currentCore

A steel rod is magnetised inside a solenoid. The current direction is reversed and made sufficiently large. What happens to the poles of the rod?

Show Answer

The solenoid field reverses. Because the reversed field is sufficiently strong, the steel rod is remagnetised in the opposite direction and its poles reverse.

Example 4: Removing the magnet (soft iron)Core

A piece of soft iron is attracted to a magnet. The magnet is taken away. What happens to the soft iron’s magnetism?

Show Answer

It usually loses most (or all) of its magnetism quickly because soft iron is a temporary magnet.

Example 5: Removing the magnet (steel)Core

A steel rod is magnetised in a solenoid and then the current is switched off. Compared to soft iron, what happens to the magnetism in steel?

Show Answer

Steel tends to retain magnetism better, so it may stay magnetised (more permanent) compared to soft iron.

7. Mind Stretchers

Mind stretcher 1: Chain of paper clipsExtension

A magnet can pick up a chain of paper clips. Why does each paper clip attract the next one?

Show Answer

Each paper clip becomes temporarily magnetised by induction. The end nearest the magnet becomes the opposite pole, so it can attract the next paper clip and induce magnetism in it too.

Mind stretcher 2: Stronger effect near the polesExtension

Why is induced magnetism usually stronger when the object is nearer to the poles of a bar magnet?

Show Answer

The magnetic field is stronger near the poles. A stronger field produces stronger induced magnetism in the magnetic material.

8. Practice and next step

Vary coil turns and current in the Magnetism & Induction Lab, then compare Temporary and Permanent Magnets.