Magnetism

Magnetism arises from the movement of electrically charged particles and it’s pretty useful - we can use it for brain scanning with MRI scanners, finding out way when we’re lost on a hike and sticking postcards on the fridge. Read this page to find out more about magnetic materials and drawing field lines.

Magnetic materials

Magnets repel and attract other magnets and attract magnetic materials (but won’t repel them). Like poles of magnets repel and opposite poles attract. Magnetic force is a non-contact force, which means magnets affect each other without touching. Placing the opposite poles of two bar magnets together will create a uniform magnetic field between the two magnets. This is drawn as a series of straight parallel lines from the north to south pole.

Only three elements are magnetic: iron, cobalt and nickel. Steel, which is made of iron mixed with carbon, is also magnetic.

Permanent vs temporary magnets

Permanent magnets are made of magnetic materials and will always exert a force on other magnets or magnetic materials. A bar magnet is an example of a permanent magnet since its magnetism cannot be switched on and off.

A temporary (or induced) magnet can only be magnetised temporarily. Temporary magnets only become magnetic when placed in a magnetic field and will quickly lose its magnetism when removed from the magnetic field. Soft iron is a temporary magnet.

Unlike permanent magnets, temporary magnets are unable to repel another magnet. This means that we can check to see whether something is a permanent magnet by checking whether it is able to repel another magnet.

Magnetic field lines

The magnetic field is the area around the magnet in which there is a magnetic force. We use magnetic field lines to represent the force exerted by a magnet, which can be visualised using a plotting compass or iron filings. A compass contains a small magnet which will align itself with a magnetic field. Iron filings become induced magnets (temporary magnets) when placed in a magnetic field.

Drawing magnetic field lines using a plotting compass: this is done by placing a bar magnet on a piece of paper and marking on the paper the direction the needle of the compass points. This is repeated several times, moving the compass to different places around the magnet. Once plenty of marks have been made, they can be connected by drawing lines between the points.

Using iron filings: place the bar magnet along with some iron filings on a sheet of paper. The iron filings will align with the magnetic field to form a clear pattern.

Remember the following rules when drawing magnetic field lines:

• Closer lines indicate a stronger magnetic field

• Lines always go from North to South

• Lines never overlap