Properties of magnets

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.

Scroll diagram horizontally to read all labels.

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

Example 1: Identify the unknown barCore

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

Show answer

The unknown bar is a magnet because repulsion is observed. End B is a north pole and end A is a south pole.

Example 2: An inconclusive resultCore

The north pole of a known magnet attracts both ends of an unknown bar. Is the unknown definitely unmagnetised iron?

Show answer

It behaves like an unmagnetised magnetic bar in this test, but attraction alone is not a sure identification. The justified conclusion is that the test has not shown the unknown to be a magnet.

Example 3: Cut into twoCore

A bar magnet is cut across its middle. State the poles on the two pieces.

Show answer

Each piece has a north pole and a south pole. The newly cut faces become opposite poles, so no isolated 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.