Mini Physics
Part of Mini EducationSwitch subject
Search

Induced Magnetism & Electrical Method Of Magnetisation

Key idea: O Level magnetism: induced magnetism near a magnet and inside a current-carrying solenoid, and the electrical method of magnetisation.

  • SEC G3 Physics 2027
On this page

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. 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.

What you need for this course

You should be able to describe induced magnetism and distinguish temporary from permanent magnets, such as soft iron and suitable steel.

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.
Induced poles and solenoid magnetisationTwo panels show opposite poles induced at the nearest ends of a magnet and iron bar, and a soft-iron core inside a solenoid whose north and south poles swap when direct current is reversed.A. Magnet brought near ironSNpermanent magnetSNiron barattractionNearest end becomes oppositenear N → induced Sfar end → induced NThe iron is attracted whichever magnetpole is brought near.B. Rod inside a solenoidd.c. current in one directionSNsoft-iron corereverse currentNSfield reverses → poles swap
Scroll diagram horizontally to read all labels.
A nearby north pole induces a south pole at the nearest end of an iron bar. In a soft-iron core inside a solenoid, reversing the d.c. current reverses the field and swaps the induced poles.

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.

Link

Finding solenoid poles using the right-hand grip rule: Magnetic Field Due To Current In A Solenoid.

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).
Simulation checkpoint

In the Magnetism & Induction Lab, select the electromagnet view. Predict the north pole before switching on, reverse the current to check the field reversal, then compare the effect of increasing current and coil turns.

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

Modelled example 1

Poles of an induced magnet

Core

Problem

A bar magnet’s north pole is held near the left end of an unmagnetised iron bar. Label the induced poles and explain the attraction.
Study the worked solution

  1. Label the near end

    Method

    Make the left end south.

    Reason

    The end nearest an inducing pole becomes the opposite pole.

    Working

    Known N beside induced S.

  2. Complete the induced dipole

    Method

    Make the right end north.

    Reason

    The induced iron bar forms a temporary north–south dipole.

    Working

    Iron bar: S—N from left to right.

  3. Explain attraction

    Method

    State that the unlike facing poles attract.

    Reason

    The near-end interaction dominates because that separation is smallest.

    Working

    N—S attraction pulls the iron toward the magnet.

Guided practice 2

Choosing a material for an electromagnet

About 5 min

Problem

Choose soft iron or steel for an electric-bell electromagnet that must switch on and off rapidly. Justify both operating states.

Match material response to current on and off

Hints

Hint 1: current on
The core should magnetise readily.
Hint 2: current off
It should lose magnetism rapidly so the mechanism releases.
View solution step by step

  1. Choose soft iron

    Method

    Select soft iron, not steel.

    Reason

    Soft iron is easy to magnetise and demagnetise.

    Working

    Current on → strong temporary magnet; current off → magnetism largely lost.

Common misconception 3

Reversing current

Find and correct the mistake

Learner response

A magnetised steel rod is inside a solenoid. A learner says any reversal of current must instantly reverse the rod’s poles. Diagnose the claim, then state what happens if the reversed current is sufficiently large.

Separate solenoid-field reversal from retained steel magnetism

View solution step by step

  1. Reverse the applied field

    Method

    State that reversing current reverses the solenoid field.

    Reason

    The field direction depends on current direction.

    Working

    Current reversal → solenoid N and S swap.

  2. Account for steel retention

    Method

    Reject guaranteed reversal for any weak reversed current.

    Reason

    Steel can retain its existing magnetisation.

    Working

    A weak reversed field may not fully remagnetise steel.

  3. Apply the stated strong field

    Method

    State that a sufficiently strong reversed field reverses the rod’s poles.

    Reason

    It remagnetises the steel in the opposite direction.

    Working

    Strong reversed field → rod N and S swap.

Examiner practice 4

Removing the magnet (soft iron)

3 marks

Examination question

A soft-iron bar is magnetised by induction. Describe what happens when the inducing magnet is removed and explain why soft iron suits an electromagnet core. [3 marks]

Link removal, material response and application

View solution step by step

  1. After removal

    1 mark

    Method

    State that soft iron rapidly loses most or all induced magnetism.

    Reason

    It is easily demagnetised.

    Working

    Inducing field removed → temporary magnetism largely disappears.

  2. Application

    2 marks

    Method

    State that it can switch magnetism on and off with current.

    Reason

    It magnetises readily when the coil carries current and releases when current stops.

    Working

    Soft iron enables a controllable electromagnet.

Challenge 5

Removing the magnet (steel)

Minimal support

Material-comparison transfer

A steel rod is magnetised inside a solenoid and the current is switched off. Predict its state compared with soft iron and choose an application that benefits.

Reverse the temporary-magnet design requirement

Hints

Hint 1: retention
Steel is harder to demagnetise than soft iron.
Hint 2: application
Choose a device that should remain magnetised without current.
View solution step by step

  1. Predict retention

    Method

    State that steel retains much more magnetism after current stops.

    Reason

    Steel is harder to demagnetise than soft iron.

    Working

    Current off: steel may remain magnetised; soft iron largely does not.

  2. Match an application

    Method

    Choose a permanent-magnet use such as a compass needle.

    Reason

    The application benefits from persistent magnetism without continuous current.

    Working

    High retention → permanent magnet.

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.

Continue with the next resource in this course.

Course and syllabus information
Course
SEC G3 Physics
Edition
SEC G3 Physics 2027