4.7.1.1Poles of a magnet
The poles of a magnet are the places where the magnetic forces are strongest.
When two magnets are brought close together they exert a force on each other. Two like poles repel each other. Two unlike poles attract each other. Attraction and repulsion between two magnetic poles are examples of non-contact force.
A permanent magnet produces its own magnetic field.
An induced magnet is a material that becomes a magnet when it is placed in a magnetic field. Induced magnetism always causes a force of attraction. When removed from the magnetic field an induced magnet loses most or all of its magnetism quickly.
You should be able to describe the attraction and repulsion between unlike and like poles for permanent magnets, and the difference between permanent and induced magnets.
That difference gives a neat test used in exam questions: repulsion is the only sure proof that something is a permanent magnet. Attraction proves nothing on its own, because an induced magnet or a piece of magnetic material would also be attracted.
Common mistake
An induced magnet can never repel. If a question describes two objects repelling each other, both must be permanent magnets — that is the whole point of the question.
- Permanent magnet
- a magnet that produces its own magnetic field all the time
- Induced magnet
- a material that becomes a magnet when placed in a magnetic field and loses most or all of its magnetism when removed from it
4.7.1.2Magnetic fields
The region around a magnet where a force acts on another magnet or on a magnetic material — iron, steel, cobalt and nickel — is called the magnetic field.
The force between a magnet and a magnetic material is always one of attraction.
The strength of the magnetic field depends on the distance from the magnet. The field is strongest at the poles of the magnet.
The direction of the magnetic field at any point is given by the direction of the force that would act on another north pole placed at that point. The direction of a magnetic field line is from the north (seeking) pole of a magnet to the south (seeking) pole of the magnet.
You should be able to draw the magnetic field pattern of a bar magnet, showing how strength and direction change from one point to another. Three conventions carry the marks:
• every line has an arrow, pointing from the north pole to the south pole outside the magnet • lines are closest together at the poles, where the field is strongest, and further apart away from the magnet, where it is weaker • lines never cross, and they start and end on the magnet.
A magnetic compass contains a small bar magnet. The Earth has a magnetic field. The compass needle points in the direction of the Earth's magnetic field.
You should be able to describe how to plot the magnetic field pattern of a magnet using a compass.
The method: place the bar magnet on a sheet of paper and draw round it. Put a plotting compass near one pole and mark two dots, one at each end of the needle. Move the compass so the tail of the needle sits on the dot just made, and mark the new position of the tip. Repeat until you reach the other pole or the edge of the paper, then join the dots into a smooth line and add an arrow. Start at a different point and repeat to build up the pattern.
You should be able to explain how the behaviour of a magnetic compass is related to evidence that the core of the Earth must be magnetic.
The argument: a compass needle is a small bar magnet, and it turns to line up with a magnetic field. Anywhere on Earth, away from other magnets, it settles pointing roughly north. Something must therefore be producing a magnetic field over the whole planet. Since the field is there in the absence of any nearby magnet, its source must be the Earth itself — and the core, which contains a great deal of iron, is what produces it.
Common mistake
Field lines run from north to south outside the magnet. A pattern drawn without arrows, or with arrows pointing into the north pole, loses the mark even if the shape is perfect.
- Magnetic field
- the region around a magnet where a force acts on another magnet or on a magnetic material such as iron, steel, cobalt or nickel
Before you practise
- The poles are where the magnetic forces are strongest; like poles repel and unlike poles attract, both non-contact forces.
- A permanent magnet produces its own field; an induced magnet becomes magnetic only in another field and loses it quickly.
- Induced magnetism always causes attraction — so repulsion proves an object is a permanent magnet.
- The magnetic materials are iron, steel, cobalt and nickel, and a magnet always attracts them.
- The field is strongest at the poles and weakens with distance.
- Field lines point from north to south outside the magnet, are closest together at the poles, and never cross.
- Plot a field with a plotting compass, marking the needle's ends step by step.
- A compass points north wherever it is, which is evidence that the Earth itself — its core — produces a magnetic field.