Forces between magnetic poles

A magnet has two poles: a north pole, labelled N, and a south pole, labelled S. When two magnets are brought near each other, their poles exert forces on one another without touching.

The direction of the force depends on which poles face each other:

• Like poles repel: N and N, or S and S, push apart.
• Unlike poles attract: N and S pull towards each other.

Induced magnetism in magnetic materials

A magnetic material can be magnetised: it can acquire a north pole and a south pole. A non-magnetic material cannot be magnetised in this way. An unmagnetised piece of magnetic material does not have an overall north pole and south pole, even though it can become magnetised.

How a magnet induces magnetism

When a magnet is brought near a magnetic material, it can cause the material to become magnetised. This is called induced magnetism. The end of the material nearer the magnet becomes the opposite pole to the nearby pole of the magnet.

For example, if a magnet’s north pole is brought near a piece of unmagnetised magnetic material, the nearer end becomes a south pole and the farther end becomes a north pole. The opposite poles nearest each other attract, so the magnet attracts the material. If the magnet’s south pole is brought near instead, the nearer end becomes a north pole, so attraction still occurs.

Induced magnetism may be lost when the magnet that caused it is removed.

Temporary and permanent magnets

A magnetic material can become magnetised when it is exposed to a magnetising influence, such as a nearby magnet. What happens when that influence is removed distinguishes a temporary magnet from a permanent magnet.

Temporary magnets

A temporary magnet is readily magnetised but loses most of its magnetism when the magnetising influence is removed. Soft iron is used for temporary magnets because it gains and loses magnetism easily.

Permanent magnets

A permanent magnet retains its magnetism after the magnetising influence is removed. Steel is used to make permanent magnets: it is harder to magnetise than soft iron, but it retains its magnetism.

Permanent magnets are useful when a lasting magnetic effect is needed. For example, a compass needle remains magnetised, and a magnetic catch can continue to hold a door closed.

Magnetic fields and bar-magnet field lines

A magnetic field is a region in which a magnetic pole experiences a force. At any point in the field, the field direction is the direction of the force on a north pole placed at that point.

Magnetic field lines represent the pattern and direction of a magnetic field. Arrows on the lines show the field direction: a north pole placed at a point on a line would experience a force along the line, in the direction of the arrow.

Field lines around a bar magnet

Outside a bar magnet, field lines curve from the magnet’s north pole towards its south pole. A field-line diagram shows this curved pattern around both sides of the magnet, with arrows pointing from north to south.

Plotting magnetic fields

A magnetic field has a direction at each point around a magnet. A plotting compass shows the direction at the point where it is placed: the north-seeking end of its needle points in the direction of the field.

Plotting a field line with a compass

1. Place the compass near the magnet and mark the direction in which the needle points.
2. Move the compass a short distance in that direction and mark the needle’s new direction.
3. Repeat, following the direction indicated by the needle at each position.
4. Join the marks with a smooth line. Add arrows along the line in the direction indicated by the north-seeking end of the needle.

Each compass position gives the field direction at that location. Following these directions traces a magnetic field line.

Revealing a field pattern with iron filings

Sprinkle iron filings on a surface around a magnet and gently tap the surface. The filings align with the magnetic field, revealing its overall pattern.

The filings show the pattern but do not by themselves show the field direction. A plotting compass can be used to determine the direction and add arrows to the field lines.

Electromagnets

An electromagnet produces a magnetic field when an electric current flows through a coil of wire. The magnetic effect can be controlled by switching the current on or off.

A soft-iron core placed inside the coil strengthens the magnetic effect. The coil’s magnetic field magnetises the core, so the core produces a magnetic field too. When the current is switched off, the soft iron loses most of its magnetism.

Uses of electromagnets

An electromagnetic crane uses this control to move magnetic material. With the current switched on, the electromagnet attracts and lifts the material. Switching the current off allows the material to be released.

In an electric bell, an electromagnet attracts and moves part of the bell’s mechanism. The controlled movement allows the bell to operate.

Magnetic field interactions

A magnet produces a magnetic field in the region around it. When another magnet is placed in that region, it experiences a magnetic force. Each magnet is in the field of the other, so each experiences a force.

The direction of the force depends on how the magnets are oriented. Each magnet has a north pole and a south pole:

• Unlike poles facing each other attract: the magnets pull towards each other.
• Like poles facing each other repel: the magnets push apart.

For example, if the north pole of one magnet faces the south pole of another, they attract. Turning one magnet around so that two north poles face each other changes the interaction to repulsion.

Magnetic field strength and field-line spacing

Magnetic field lines are used to show how the strength of a magnetic field varies from place to place. Their spacing indicates the relative field strength:

• Where the lines are closer together, the field is stronger.
• Where the lines are farther apart, the field is weaker.

Around a bar magnet, the field lines are closest together near the poles. This shows that the field is strongest near the poles and weaker where the lines spread farther apart.