Magnetism and magnetic fields

In a nutshell

All magnets have a north-seeking pole and a south-seeking pole which produces a magnetic field. The direction of magnetic field lines can be detected using a compass. A magnet can be permanent or induced.

Magnetic fields

All magnets have a north-seeking (north) pole and a south-seeking (south) pole. They also all produce a magnetic field. A magnetic field is the space around a magnet where other magnets, or magnetic materials, experience a force.

A magnetic field can be shown using magnetic field lines. These lines always go from a north pole to a south pole.

Physics; Magnetic fields; KS4 Year 10; Magnetism and magnetic fields

The direction of the lines represent the direction of the force acting on a north pole. A south pole would experience a force in the opposite direction.

How close together the lines are represents the strength of the field and the strength of the magnetic force. The closer the lines, the stronger the magnetic field and force acting on the magnet. The force is strongest when the lines are closest together, this occurs at the poles of the magnet.

Two poles placed near each other will exert a force on each other. Depending on the poles, the force will either be attractive or repulsive.

Two like poles		Repulsive force
Two unlike poles		Attractive force

Using a compass

A compass can be used to find the directions of magnetic field lines around a magnet.

A compass itself is made up of a small bar magnet. The north pole of the compass' magnet is attracted to the south pole of a nearby magnet and the needle points in this direction. This can be used to find field lines as a field line always shows the direction a north pole would experience a force in.

Plotting a magnetic field

Physics; Magnetic fields; KS4 Year 10; Magnetism and magnetic fields

Procedure

1.	Place a bar magnet on a piece of paper.
2.	Place the compass at a point around the magnet.
3.	Trace the direction the compass needle points.
4.	Repeat for multiple points around the magnet.
5.	Join up magnetic field lines and compare to the image of a bar magnet's magnetic field.

Permanent and induced magnets

A permanent magnet is a type of magnet that produces its own magnetic field.

An induced magnet is magnetic material that turns into a magnet if it is put in a magnetic field.

The magnetic force exerted between a permanent and induced magnet is always attractive. This means the force between a magnet and magnetic material is always attractive.

Induced magnets will quickly lose their magnetism when they are removed from a magnetic field.

Magnetic materials at room temperature

Iron (and steel- made mostly from iron)
Cobalt
Nickel

Uses of magnets

Magnets have various uses. These are listed below.

Permanent	induced
Compasses	Induction Cooking
Loudspeakers	Electromagnets
Fridge Magnets	Circuit Breakers
Motors and Generators	Electric Bells

The Earth's magnetic field

When a compass isn't near another magnet, the needle will point to a position near the Earth's North Pole. This shows that Earth has a magnetic field and it is similar to the magnetic field of a bar magnet.

The Earth's magnetic field is thought to be due to currents in its outer core. The outer core is a molten mixture of iron and nickel which are both magnetic materials.

Curiosity: The north pole of the compass points towards the North Pole of the Earth. This doesn't seem right - north poles are meant to repel each other! In fact, the Earth's geographical North Pole is not the same as its magnetic north pole. What we call the North Pole is actually its magnetic south pole. This is why the north pole of the compass it attracted to it.

Physics; Magnetic fields; KS4 Year 10; Magnetism and magnetic fields

1.	Axis of the Earth's rotation.