Temporary and permanent magnets
Key idea: O Level comparison of temporary and permanent magnets: magnetic retention, soft-iron and steel choices, and device applications.
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The core idea
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Learning objectives
- State the properties of magnets
- Describe induced magnetism
- Distinguish temporary and permanent magnets
- Determine magnetic-field direction with a compass or bar magnet
- Interpret bar-magnet field patterns
- Draw the magnetic field pattern around a bar magnet and between the poles of two bar magnets
- Interpret the field pattern around a straight current-carrying wire
- Draw the magnetic field pattern around a straight current-carrying wire
- Interpret the field pattern around a current-carrying solenoid
- Draw the magnetic field pattern around a current-carrying solenoid
- Relate current magnitude and direction to magnetic field
- Describe electromagnet applications
- Describe experiments showing the force on a current-carrying conductor in a magnetic field
- Describe magnetic force on a charged-particle beam
- Predict force reversal when current or field reverses
- Use Fleming’s left-hand rule
- Explain the turning effect on a current-carrying coil
- Explain how current and turns increase the turning effect
- Describe split-ring commutator action
- Describe the effect of winding a motor coil on a soft-iron cylinder
- Deduce that a changing magnetic field can induce an e.m.f.
- Deduce that induced e.m.f. opposes the change producing it
- Deduce factors affecting induced e.m.f. magnitude
- Describe a simple a.c. generator and slip rings
- Sketch a simple a.c. generator voltage–time graph
- Describe a simple iron-cored transformer
- Apply ideal-transformer equations
- Explain cable loss and high-voltage transmission
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.
2. Key Ideas
| Magnetic property | Soft iron | Suitable magnetically hard steel |
|---|---|---|
| Ease of magnetisation | Easy | Harder |
| Retention after field is removed | Low; loses most magnetism readily | High; retains magnetism well |
| Best role | Switchable electromagnet core | Permanent magnet |
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.
If the device must follow current on and off, choose soft iron. If it must remain magnetised, choose magnetically hard steel. Always link the named property to the device requirement.
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
Modelled example 1
Lifting electromagnet
Problem
Study the worked solution
Translate the operating requirement
Method
Require strong magnetism while current flows and little retained magnetism afterwards.Reason
The scrap must be lifted and then released under electrical control.Working
The core must behave as a temporary magnet.Choose and justify the material
Method
Select soft iron.Reason
It magnetises readily in the coil’s field and loses most of its magnetism when that field is removed.Working
Soft iron allows lift while on and release when off.
Guided practice 2
Compass needle
Problem
Choose, then justify
Hints
Hint 1: consider the field after manufacture
View solution step by step
State the retention requirement
Method
The needle must remain magnetised without an external magnetising field.Reason
Its persistent poles allow it to align with Earth’s magnetic field.Working
A compass needle is a permanent-magnet application.Connect the material property
Method
Choose magnetically hard steel.Reason
It retains magnetism better than soft iron.Working
Steel is preferred because the needle must remain magnetised.
Common misconception 3
Wrong core material
Fault diagnosis
Diagnose the material fault
View solution step by step
Correct the causal claim
Method
Do not infer current solely from continued attraction.Reason
The steel core can retain magnetism after the current stops.Working
Residual magnetism can keep attracting the nails.Change the core material
Method
Replace steel with soft iron.Reason
Soft iron loses most of its magnetism readily when the coil field is removed.Working
The nails can then be released reliably when switched off.
Challenge 4
Material-selection answer
Requirement-to-material transfer
Write the full reasoning chain
Hints
Hint 1: begin with the off-field state
View solution step by step
State the operating requirement
Method
The bar must remain magnetised after the magnetising field is removed.Reason
That persistence defines its permanent-magnet role.Working
Required property: high magnetic retention.Select the material
Method
Use a suitable magnetically hard steel.Reason
It is difficult to demagnetise and retains the required magnetism.Working
Permanent bar magnet → high retention → magnetically hard steel.
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.
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Course and syllabus information
- Course
- SEC G3 Physics
- Edition
- SEC G3 Physics 2027