Action-at-a-distance and magnetic fields

Magnetic fields

A fascinating feature of the working of magnets is that one magnet can attract or repel another without being in physical contact with it. Because magnets are familiar objects we may take this for granted, but just play with a couple of strong magnets and feel them pushing away at each other through thin air. It really is amazing! The magnetic force is one which acts-at-a-distance.

The space around a magnet where the magnetic force acts is called the magnetic field (just as the space around a mass where the gravitational force acts is called a gravitational field). In simple terms a magnetic field exists in a space if a magnet experiences a force when placed in that space. A crude way to make a magnetic field detector would be to suspend a bar magnet on a length of thread. If the suspended magnet is held in a space where there is a magnetic field, the bar magnet will feel a force and perhaps make a small movement.

It is important to recognise that a magnetic field is not a tangible object (like an iron nail) but a theoretical concept, which we use to describe the physical world. We can point to the space around a magnet and claim (quite correctly) that there is a magnetic field in that space, but of course there is nothing there to actually see!

Other action-at-a-distance forces are those associated with gravity and with electric charge. Since magnetic forces can be attractive or repulsive, they are quite different from gravitational forces which are only attractive, but do have parallels with electric forces which similarly can be attractive or repulsive.