Magnetism is a fascinating invisible force – it influences the environment around it. A magnet is a material that can pull certain types of metal towards itself. Unlike many other forces, magnetism doesn’t have to touch the objects it affects. Gravity is also an invisible force that acts at a distance.
Seeing an apple falling out of a tree, no-one says ‘Wow, that’s so exciting, see the apple fall’, or when a rugby ball goes over the post, that’s not exciting, but when you look at two magnets, it’s just as natural, but it is exciting and fascinating!Dr Simon Taylor
What causes magnetism?
All matter is made of atoms. In most materials, electrons in the atoms spin around in different directions. In magnets, the electrons all spin in the same direction. Only a few materials have the properties to allow the electrons to do this. Iron, cobalt, nickel and some rocks, minerals and alloys that contain a lot of these elements are able to be magnetised, for example, steel, magnetite and lodestone. The direction of the spin of the electrons determines the direction of the magnetic field.
The movement of the electrons causes the object to have two poles, called north and south. The north pole of a magnet is generally considered to be negative, and the south pole of a magnet is considered to be positive. If a bar magnet is broken into two pieces in an attempt to separate the poles, the result will be two smaller bar magnets, each of which has both a north and south pole. Magnetic forces flow from the north pole to the south pole, creating a magnetic field.
An object like an iron nail or steel needle can be magnetised by stroking the object repeatedly in one direction with a strong magnet. This will cause the electrons to spin in the same direction, and the object will become magnetised.
What happens in a magnetic field?
The area within which a magnetic force acts is called the magnetic field. Substances that are magnetic are attracted to the magnetic field. Iron is the most magnetic material and is most commonly attracted to a magnet. Steel is an alloy of iron, so steel can be used as well.
Magnets also influence each other. Two opposite poles will attract each other, and two poles that are the same (two north poles or two south poles) will push apart.
Although the magnetic field is invisible, it can be seen indirectly. Iron filings placed in the magnetic field will show where the lines of force lie. Small compasses placed in the magnetic field will show the direction of the force field. The activity Investigating magnetism explains how you can demonstrate this.
The iron filings and compasses provide indirect evidence of a magnetic field from which a diagram can be drawn to represent the force field.
The Earth as a magnet
The Earth is made up of an upper crust, mantle and an outer and inner core. It is this structure that causes the Earth to behave like a giant magnet. The Earth’s inner core is made largely of iron and nickel. The outer core is molten and spins with the rotation of the Earth on its axis. This spin creates a magnetic field around the Earth with north and south poles. The poles, like those of a magnet, create a force field that runs from south to north (the opposite of smaller magnets).
Magnetic objects react to the force field of the Earth, and this property allows several things to occur:
- Compasses will line up with the magnetic field, allowing us to determine where north and south are, which is very useful for navigation – try making your own simple compass.
- Tiny magnetic parcels in the ears of migrating birds and fish allow them to determine direction and thereby navigate on their seasonal migrations
- Solar winds are deflected by the magnetic field, reducing the impact of radiation on the Earth’s surface. The interplay of these particles with our atmosphere can be seen in the Aurora (Southern and Northern Lights).
Nature of science
The concept of non-contact forces such as magnetism requires students to begin to think in an abstract manner. This article supports the development of this skill – fundamental to the nature of science – by supporting them to interpret representations of magnetic forces in action.
There are several articles and a PLD session related to magnetism. They include Using magnetism, Mapping the Milky Way’s magnetic field, Fossil compasses, Magnetic resonance imaging (MRI) and Exploring magnetism.