����ý

A magnet can exert a force on another nearby magnet. Magnets have two poles:

• a north pole
• a south pole

The magnetic force is strongest near the magnet's poles.

The rules of magnetism

Two magnets will either attract or repel each other in the following way:

• like poles (N-N or S-S) repel
• unlike poles (N-S or S-N) attract

Magnetic forces are non-contact forces - this means that magnets affect each other without touching.

A magnetic field is the region around a magnet where a force acts on another magnet or on a magnetic material. Iron, steel, cobalt and nickel are all magnetic materials and would feel force from a nearby magnet.

Iron, steel, nickel and cobalt are magnetic materials and would feel the force from a nearby magnet.

All other materials that do not contain one of these metals are non-magnetic. They are affected by magnets and are attracted to either pole of a magnet.

Magnetic materials are materials that feel a force from a magnetic field. If you bring a magnet and a magnetic material together they will pull towards each other, or attract.

The magnet and the magnetic material don’t have to touch to attract. The magnetic force is a non-contact force. This can be useful for holding pictures on fridges, for example, or keeping phone cases closed when a magnet is sewn into the case.

Permanent magnets

A permanent magnet is often made from a magnetic material such as steel. A permanent magnet always causes a force on other magnets, or on magnetic materials. Key features of a permanent magnet:

• it produces its own magnetic field
• the magnetic field cannot be turned on and off - it is there all the time

Bar magnets and horseshoe magnets are examples of permanent magnets.

Induced magnets

Unlike a permanent magnet, an induced magnet only becomes a magnet when it is placed in a magnetic field. The induced magnetism is quickly lost when the magnet is removed from the magnetic field.

The iron filings in the image become induced magnets when they are near the bar magnet. Like all induced magnets:

• they are only attracted by other magnets, they are not repelled
• they lose most or all of their magnetism when they are removed from the magnetic field

Testing for magnetism

A permanent magnet can:

• attract or repel another permanent magnet
• attract a magnetic material (but not repel it)

This means that you can only show that an object is a permanent magnet by checking if it repels another magnet.

A magnetic field is the region around a magnet where a force acts on another magnet or on a magnetic material.

Detecting magnetic fields

A magnetic field is invisible, but it can be detected using a magnetic compass. A compass contains a small bar magnet on a pivot so that it can rotate. The compass needle points in the direction of the Earth's magnetic field, or the magnetic field of a magnet.

Magnetic fields can be mapped out using small plotting compasses:

• place the plotting compass near the magnet on a piece of paper
• mark the direction the compass needle points
• move the plotting compass to many different positions in the magnetic field, marking the needle direction each time
• join the points to show the field lines

The needle of a plotting compass points to the south pole of the magnet.

The behaviour of a compass shows that the Earth has a magnetic field. The Earth's core, which is made from iron and nickel, produces this magnetic field.

Drawing a magnetic field

The diagram shows the magnetic field around a bar magnet.

The diagram shows these key features:

• the magnetic field lines never cross each other
• the lines have arrowheads to show the direction of the force exerted by a magnetic north pole
• the arrowheads point from the north pole of the magnet to its south pole

So the direction of a magnetic field at any point is the direction of the force on the north pole of the magnet at that point.

Did you know?

It is possible to differentiate coins by testing them with a magnet.

For example, 5p and 10p coins made by the UK Royal Mint before January 2012 are not magnetic. 5p and 10p coins in the UK made after January 2012 are magnetic. This is because the coins used to be made out of cupro-nickel, but this was changed to nickel-plated steel. Steel contains iron, making these coins magnetic.

When a current flows in a wire, it creates a circular magnetic field around the wire. This magnetic field can detect the needle of a magnetic compass. The strength of the magnetic field is greater:

• closer to the wire
• if the current is increased

Electromagnets

A solenoid with an iron core is called an electromagnet.The iron core increases the solenoid’s magnetic field strength. A simple electromagnet is made by coiling wire around an iron nail.

Electromagnets are used in devices such as electric bells, and door locks that can be controlled remotely.

Magnets are used in a variety of ways:

• A magnet that is able to spin freely will align itself with the Earth’s magnetic field and so point north to south. This is a compass.

• They are also used to hold fridge and freezer doors closed, as well as some mobile phone cases.

• The strip on the back of credit and debit cards is magnetic. It contains the data required for the card to work.

• Speakers and microphones also have magnets within them.

An electromagnet is one that is generated when a current flows through a wire. Electromagnets can therefore be switched on and off.

Some cranes use electromagnets to pick up scrap cars. They can let go of the car by turning off the current.

Electric bells use electromagnets. When the electromagnet has pulled the striker towards the gong, the current stops to let the striker move back away from the gong again. The current restarts and the movement of the striker continues.

The circuit is constantly made and broken.

Test your knowledge of the simple phenomena of magnetism with this quiz.