Magnetism is a well known effect that was observed in nature thousands of years ago. In fact, the word “magnetic” comes from the region of Greece where materials that could attract and repel each other were first discovered: this region was called Magnesia.
The knowledge you need to have about magnets is very straightforward but this topic then extends into electromagnetism, which is where things get much more interesting (and a bit more complicated).
So let’s start with a quick recap of magnetic fields, which are instantly recognisable as those curved lines that are often shown around bar magnets. The diagram below is a typical example.
The lines show the strength and the direction of the magnetic field. Notice that the lines point away from the North pole and towards the South pole of the magnet. If you placed a magnetic object inside the field, it would feel a force: in the case of a compass, its needle would turn to point along the field lines. The density of the lines (how close they are together) indicates the strength of the force and you should be able to deduce that the force is strongest at the magnet’s poles, because that is where the lines are closest together.
It is worth pointing out that although we draw these diagrams in two dimensions, magnetic fields are three-dimensional structures. JavaLab hosts a nice rotating animation that shows this: you can view the animation here.
By definition, the direction of magnetic field lines show the direction of force that would be experienced by a north magnetic monopole (an imaginary particle that has a north pole only). You should recognise this as being very similar to the definition of electric field lines, which shown the direction of force on a positively-charged particle. There is more about electric fields in a previous physbang post, which you can read here.
Interestingly, there are some organisms that can sense magnetic fields. This isn’t part of the GCSE course but it’s an area of current research – and it can lead to some fun videos, such as this one showing bacteria doing line dancing!
It is important that you can recall and recognise the magnetic fields that are created when two magnets are brought close to each other.
- If the two magnets have opposite poles facing each other then the magnets will be attracted (pull towards each other).
- If the two magnets have the same poles facing each other then the magnets will be repelled (push each other away).
We explain these non-contact forces by considering the ways in which magnetic fields interact with each other.
When there are different poles facing each other, the lines of magnetic field that leave one magnet’s north pole can “blend” with the lines of magnetic field that are leading to the other magnet’s south pole. A simple way of visualising this is shown in the illustration below, which is a simple duplication of the single-magnet image at the start of this article.
In reality, the joins between the two sets of field lines are gradual (rounded) rather than sudden (pointed) and the final combined magnetic field between two opposite poles is as shown below.
You should notice that there is a set of almost-parallel magnetic field lines within the two opposite poles, with the lines curving outwards towards corners of pole faces. Moving further away from the facing poles, the magnetic field lines break away from the second magnet and turn backwards to link to each magnet’s own opposite pole in the usual way.
Rotating one of the magnets, so that there are identical poles facing each other, the two sets of magnetic field lines show forces acting in opposite directions. These forces act not only on an imaginary north monopole but also on the magnetic field lines themselves and result in a region between the two identical poles where the magnetic field lines have completely repelled each other. The result is a small region in which there is no magnetic field, as shown in the centre of the image below.
That is all you need to know for your GCSE syllabus but if you want to learn more about magnetism and magnets then I strongly recommend browsing the Magnet Academy website, which has been created by the US National High Magnetic Field Laboratory, the largest magnet laboratory in the world. Although some of the content goes beyond what you need to know, there is a lot that I’m sure you will find interesting – not least the various interactive tutorials (located under the Watch & Play tab).