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Properties of magnets

Key idea: O Level properties of magnets: carry out the sure test, explain compass alignment and predict the poles formed when a magnet is cut.

  • SEC G3 Physics 2027
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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 magnet has a north pole and a south pole. Its important testable properties are that it can repel a like pole, aligns with Earth’s magnetic field when freely suspended, and still forms complete magnets when cut.

Review magnetic interactions and material classification first if attraction, repulsion or magnetic materials are unfamiliar.

2. Key Ideas

Repulsion is the sure test for a magnet. Attraction is not enough because an unmagnetised iron or steel bar is also attracted by a known magnet.

Use a complete procedure:

  1. Choose one pole of a known magnet, for example its north pole.
  2. Bring that same known pole near one end and then the other end of the unknown bar.
  3. If either end repels it, the unknown bar is a magnet.
  4. If both ends attract it, the unknown bar could be an unmagnetised magnetic material; this observation does not prove that it is a magnet.

Testing both ends matters. A magnet’s south pole would attract the known north pole, but its north pole would repel it.

The sure test and the effect of cutting a magnetThree comparisons show unlike poles attracting, like poles repelling, and a magnet attracting an unmagnetised iron bar. A final panel shows that cutting a bar magnet produces two smaller magnets, each with north and south poles.Read the facing ends firstUnlike polesNSattractForce is towards the gap.Like polesNNrepelForce is away from the gap.Magnet + ironNironalso attractsInduced magnetism makes thenear end an opposite pole.Cutting does not isolate one poleNScutN SN Seach piece has both polesSure-test conclusionUse the same known pole atboth ends of the unknown bar.Repulsion at either end:unknown is a magnetAttraction at both ends:could be unmagnetised iron
Scroll diagram horizontally to read all labels.
Unlike poles attract and like poles repel. Attraction is inconclusive because an unmagnetised iron bar is also attracted; observed repulsion with a known magnet proves the unknown is a magnet.
Exam method

Write “bring the same pole of a known magnet near both ends of the unknown bar”. Then state the decisive observation: repulsion at either end proves that the unknown is a magnet.

3. Detailed Explanations

Alignment in Earth’s magnetic field

A freely suspended bar magnet settles approximately along the north–south direction because it experiences a turning effect in Earth’s magnetic field. Its north-seeking pole points roughly towards geographic north.

A compass needle behaves in the same way because it is a small permanent magnet. The needle indicates the local magnetic field direction, so nearby magnets or magnetic objects can deflect it.

Cutting a magnet

Cutting a bar magnet does not separate its north and south poles. Each piece becomes a smaller magnet with both a north pole and a south pole. New poles appear at the cut faces.

For example, cutting one bar magnet into three pieces produces three smaller magnets: there are three north poles and three south poles in total.

4. Common Mistakes

  • Testing only one end of the unknown bar. An unlike pole can attract and hide the fact that the bar is a magnet.
  • Saying attraction proves magnetism. An unmagnetised magnetic material is also attracted.
  • Saying a suspended magnet points north because it is “pulled north”. It aligns with Earth’s magnetic field through a turning effect.
  • Drawing one isolated north piece and one isolated south piece after a magnet is cut.

5. Exam Tips

  • Bring the same known pole near both ends of the unknown. Testing one end cannot distinguish an unlike pole from an unmagnetised magnetic material.
  • Record attraction or repulsion at each end before naming the unknown; repulsion is the decisive evidence.
  • In a compass investigation, keep other magnets and magnetic objects away and distinguish magnetic north-seeking direction from exact geographic north.

6. Worked Examples

Modelled example 1

Identify the unknown bar

Core

Problem

The north pole of a known magnet attracts end A of an unknown bar and repels end B. What can you conclude?
Study the worked solution

  1. Use the decisive observation

    Method

    Conclude that the unknown bar is a magnet.

    Reason

    Only another magnet can repel a known magnetic pole; an unmagnetised magnetic material can attract but not repel.

    Working

    Repulsion at B → unknown is a magnet.

  2. Identify both poles

    Method

    Label B north and A south.

    Reason

    The known north repels a like north at B, and a magnet’s other end is south.

    Working

    B = N; A = S.

Common misconception 2

An inconclusive result

Find and correct the mistake

Learner response

A known north pole attracts both ends of an unknown bar. A learner concludes that the bar is definitely an unmagnetised iron bar. Diagnose the conclusion.

Separate observation from justified identity

View solution step by step

  1. State what attraction permits

    Method

    Say the unknown could be an unmagnetised magnetic material.

    Reason

    Induced magnetism makes such a material attract a known pole.

    Working

    Attraction alone is compatible with induced magnetism.

  2. Limit the conclusion

    Method

    State that this test has not proved the unknown is a magnet or definitely identified its material.

    Reason

    The sure test requires repulsion from at least one end when the same known pole tests both ends.

    Working

    No repulsion observed → no proof of permanent magnetism.

Challenge 3

Cut into two

Minimal support

Fragmentation transfer

A bar magnet is cut across its middle. State the poles on the two pieces and explain what appears at the cut faces.

Represent each fragment as a complete magnet

Hints

Hint 1: dipole rule
A magnet fragment does not become an isolated pole.
Hint 2: cut faces
The two new faces must be opposite poles.
View solution step by step

  1. Complete each fragment

    Method

    Give each piece one north and one south pole.

    Reason

    Cutting divides the magnet into smaller aligned magnetic regions; it does not isolate a single pole.

    Working

    Original N—S → N—S + N—S.

  2. Label the new faces

    Method

    Make the facing cut surfaces opposite poles.

    Reason

    Each fragment must retain a complete dipole orientation.

    Working

    No isolated north or south pole is produced.

7. Mind Stretchers

In the Bar magnet mode of the Magnetism and Induction Lab, predict the compass north-seeking end at several labelled probes before checking. Notice that each compass follows its local field rather than always pointing towards the top of the screen.

Suppose attraction occurs at both ends of an unknown bar. Explain why the result is inconclusive, propose the next observation, and state how that observation would change the conclusion.

8. Practice, Quiz and Next Step

Apply the sure test in the Magnetism Quiz, then learn how a plotting compass maps Magnetic Fields.

Continue with the next resource in this course.

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