# 4.1.1 Magnetism

### Magnetic Fields

• In the space around a magnet there is a magnetic field
• Magnetic field lines are a useful way of helping us to picture that field: Its strength and its direction

The magnetic field around a bar magnet

• Magnetic field lines obey a couple of rules:
• They always go from north to south ((indicated by an arrow midway along the line)
• They never touch or cross other field lines
• When two magnets are held close together, there will be a force between the magnets:

Opposite poles attract; like poles repel

• Magnetic materials (which are not magnets) will always be attracted to the magnet, regardless of which pole is held close to it
• A magnet can only repel another magnet
(This can be a useful test for a magnet)

Extended Only

### Magnetic Forces

• Magnetic forces are caused by the interaction of magnetic fields

### Plotting a Magnetic Field

• In your IGCSE examination you might be asked to describe a method of plotting the magnetic field around a bar magnet
• There are two principle ways of doing this:

Using Iron Filings

• Place a piece of paper on top of the magnet
• Gently sprinkle iron filings on top of the paper
• Now carefully tap the paper to allow the iron filings to settle on the field lines

Iron filings can be used to plot a magnetic field

• Place the magnet on top of a piece of paper
• Draw a dot at one end of the magnet (near its corner)
• Place a plotting compass next to the dot, so that one end of the needle of the compass points towards the dot
• Use a pencil to draw a new dot at the other side of the compass needle
• Now move the compass so that it points towards the new dot, and repeat the above process
• Keep repeating until you have a chain of dots going from one end of the magnet to the other. Then remove the compass, and link the dots using a smooth curve – the magnetic field line
• The direction of the field line is the same as the direction of the plotting compass
• You can now repeat the whole process several times to create several other magnetic field lines

Compasses can be used to plot the magnetic field around a bar magnet

### Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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