Magnetism is the class of physical attributes that occur through a magnetic field, which allows objects to attract or repel each other.

Iron, steel, nickel and cobalt are magnetic materials. They are affected by magnets and are attracted to either pole of a magnet.

Permanent magnets

A permanent magnet. Its magnetism cannot be turned on or off, unlike an electromagnet. is often made from a magnetic material such as iron. 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.

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.

Detecting and drawing magnetic fields

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:

1. place the plotting compass near the magnet on a piece of paper

2. mark the direction the compass needle points

3. move the plotting compass to many different positions in the magnetic field, marking the needle direction each time

4. 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.

Induced potential and the generator effect

A potential difference or voltage is needed to make an electric current flow in a circuit.

• a coil of wire is moved in a magnetic field.

• a magnet is moved into a coil of wire.

  
  

This is called electromagnetic induction and is often referred to as the generator effect.