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If you have used bar magnets before, you probably noticed that the strength of the magnetic force caused by the magnet was not constant.
You might have noticed that the magnetic force caused by the magnet was greater closer to the north and south poles. The force is also weaker the further you are from the magnet.
This is all due to the specific shape of the magnetic field around the magnet. Let's learn about these fields and the techniques we can use to visualise them!
A magnetic field is an area around a magnet where a magnetic material, or another magnet, would experience a force.
This diagram shows the magnetic field around a bar magnet. The lines are called magnetic field lines and they help us to understand the shape of the field.
Any magnet or magnetic material (like iron) would experience a magnetic force if they were inside this field.
There are some important things to remember about magnetic field diagrams.
1. We use arrows to show the direction of the field lines. The direction is always away from the north pole, and towards the south pole.
2. The strength of the field is indicated by how close the lines are together. At the poles, the field lines are very close together. This means a strong magnetic field and force.
Further from the poles, the lines are more spaced out. This means that the field is weaker.
The image above is a diagram, but we can use experimental techniques to show the shape of a magnetic field too.
Here is an example:
Here, the bar magnet is placed on a white sheet of paper. Tiny pieces of iron called iron filings are slowly sprinkled on top.
If the paper is gently tapped, the filings will move into a position that shows the shape of the magnetic field.
As you can see, the shape matches the diagram from earlier. The filings are closer together at the poles and you can see the circular shape.
Another method uses small plotting compasses.
Compasses are small magnetised devices that always point towards the north pole. Tiny plotting compasses can be placed around a bar magnet, and the shape and direction of the field is indicated.
What about electromagnets? What do their magnetic fields look like?
When electricity runs through a wire, the wire becomes a magnet. We increase the strength of the magnet by wrapping the wire many times, around an iron core, like the nail in the image above.
The diagram shows the magnetic field for a solenoid (a coil of wire which is an electromagnet without the iron core).
The magnetic field shape is very similar to the shape of the field of a bar magnet. The field is strongest at the poles and the field lines are curved at the sides.
There is one difference - there is a field inside the coil. Here, the field is strong and uniform (constant).
Now let's try some questions!
