Temporary and permanent magnets

1. Definitions

A temporary magnet is magnetised easily but loses most of its magnetism readily when the magnetising field is removed. Soft iron is the standard O-Level example.

A permanent magnet retains its magnetism. A suitable magnetically hard steel is the standard O-Level material choice: it is harder to magnetise, but also harder to demagnetise.

Here, hard and soft describe magnetic behaviour, not resistance to scratching, bending or cutting. Magnetic hardness and mechanical hardness are different properties.

Scroll diagram horizontally to read all labels.

2. Key Ideas

The word suitable matters: steel is a family of alloys with different properties. In the simplified O-Level comparison, “steel” refers to a magnetically hard steel selected for making permanent magnets.

3. Detailed Explanations

Choose from the required behaviour, not from a memorised device list.

The magnetic effect must switch off

Use soft iron for the core of an electromagnet. It magnetises when current in the coil produces a field, then loses most of that magnetism when the current stops. This behaviour is needed in devices such as an electric bell and a magnetic circuit breaker.

The object must stay magnetised

Use a suitable magnetically hard steel for a permanent magnet. A compass needle and a permanent bar magnet must retain their magnetism when no external magnetising field is present.

4. Common Mistakes

• “Temporary” means weak. It describes poor retention, not the largest strength the magnet can reach while a field is applied.
• “Permanent” means it can never lose magnetism. It means the material retains magnetism well under normal use, not that demagnetisation is impossible.
• “Hard steel” only means mechanically hard. In this comparison the required property is magnetic hardness.
• Choosing steel for a switchable electromagnet core. Retained magnetism can prevent a device from releasing cleanly.
• Giving a material without a reason. State whether easy loss or strong retention of magnetism is required.

5. Exam Tips

• Start from the device requirement: must its magnetic effect switch off, or must it remain magnetised?
• Name the material, then link its magnetic retention to that requirement. A material name without the property does not complete the explanation.
• Do not use a simulation’s idealised field strength as evidence for residual magnetism; retention requires a before-and-after material test.

6. Worked Examples

Example 1: Lifting electromagnetCore

An electromagnet lifts steel scrap and must release it when the current is switched off. Choose a core material and explain.

Show answer

Use soft iron. It magnetises readily while current flows and loses most of its magnetism readily when the current is switched off, allowing the scrap to be released.

Example 2: Compass needleCore

Why is a suitable steel preferred to soft iron for a compass needle?

Show answer

A compass needle must remain magnetised without an external magnetising field. Magnetically hard steel retains magnetism better than soft iron, so the needle continues to align with Earth’s magnetic field.

Example 3: Wrong core materialCore

Nails remain attached to an electromagnet after its current is switched off. The core is steel. Explain the observation and suggest a change.

Show answer

Steel retains magnetism, so the core remains magnetised and continues to attract the nails. Replace it with a soft-iron core, which loses most of its magnetism readily when the current stops.

Example 4: Material-selection answerCore

A manufacturer needs a permanent bar magnet. Complete the reasoning from requirement to material.

Show answer

The bar must remain magnetised after the magnetising field is removed. Therefore it needs high magnetic retention, so a suitable magnetically hard steel should be used.

7. Mind Stretchers

In the Electromagnet view of the Magnetism and Induction Lab, vary the current and turns, then reverse the current to swap the poles. Apply the material model from this lesson: a soft-iron core is chosen when its magnetic effect must follow the coil current. Do not treat the idealised screen as a measurement of residual magnetism.

Design a fair comparison of two candidate core materials. State what you would keep constant, what you would measure while the coil is on and after it is switched off, and how the observations identify the better temporary-magnet material.

8. Practice, Quiz and Next Step

Choose the correct material in the Magnetism Quiz, then continue to the field around a Current-Carrying Wire.