Magnetic Field: UNIZOR.COM - Physics4Teens - Electromagnetism
Channel: Zor Shekhtman on UNIZOR Education
Date: 2020-05-03

Unizor - Physics through a Looking Glass of Mathematician


Magnetism - Magnetic Field

Magnetic forces act on a distance, so there must be a field that exists around each magnet - magnetic field.
Studying magnetic fields presents more difficulties than studying gravitational or electrostatic fields. We cannot have a point-object because each magnet has two poles and each of them act in some way. Even more, the magnetic force exhorted by a magnet is changing as we move from North pole to South, first diminishing to zero in the middle and rising again at the other pole.

Probably, the simplest magnet we can deal with, as a source of magnetic field and a probe object, is a thin rod with poles at its ends. But even in this case we have to take into account all the different forces, attracting and repelling, of different magnitude and directions that act on probe magnet.

Let's experimentally visualize the magnetic field in this simplest case.
For this experiment we need a bar magnet and iron shavings. Each shaving is a little temporary magnet that forms its poles based on the forces of the magnetic field. Then different shavings will attach to each other by opposite poles and form lines. The picture obtained will represent the vectors of forces in the magnetic field of a bar magnet. Along each line on a picture below lie iron shavings, each with North and South magnetic poles, linked by opposite poles and directed with their South pole closer to North pole of a bar magnet.

The picture below schematically represents these field forces.

Direction of forces from North magnetic pole to South was traditionally chosen, similarly to a direction of the electric current was chosen from a positive terminal to a negative one, regardless of the flow of electrons, unknown at the time of early experiments with electricity.

Let's discuss a concept of a magnetic field.
As usually, the following explanation is the model, which to some degree corresponds to experimental and theoretical data, but we do not claim that in reality the things are arranged in exactly this way.
However, we offer it as an aid to understanding the concept of a magnetic field.

Imagine a small particle rotating within a plane with certain speed on a certain radius around an axis that is perpendicular to this plane. On the same axis in a plane parallel to the first one another particle is rotating. Generally speaking, it might rotate on a different radius, in the same or opposite direction and with a different speed.

Consider a distance between these two parallel planes of rotation of these two particles. When it's large, particles don't really have any interaction. But, when we make this distance small enough, the particles will "feel" each other.

If the particles rotate in the same direction, there will be some attracting force between them and the planes of rotation tend to get closer to each other.
If the particles rotate in the opposite directions, there will be some repelling force between them and the planes of rotation tend to increase the distance between them.

The rotating particles in this model behaves like a bar magnets positioned along the axis of rotation with poles determined by a direction of rotation. We can assume that the North pole is defined by a rule, that, looking from it towards the rotating particle, this particle rotates counterclockwise.
If two particles rotate around the same axis in the same direction, each behaving like a bar magnet, the magnets will attract to each other because they will be facing by opposite poles.
If the particles rotate in opposite direction, the corresponding magnets will be facing each other by the same pole and will repel each other.

Now let's assume that these particles are electrons rotating around a nucleus in the atoms inside some object. According to this model, if all planes of rotation of electrons inside all atoms are parallel and the rotation is such that all North poles of all atoms are directed in the same way, we have a perfect permanent magnet.

If planes of electron rotation in some object are randomly directed and are insensitive to outside forces exhorted by magnets, we have a diamagnetic object - the one that cannot be magnetized.

If planes of electron rotation in some object are randomly directed, but outside magnetic forces, interacting with atoms of this object, align all planes of rotation in a parallel fashion with the same direction of the poles, we have an object that can be a temporary magnet.

In all those cases our model of rotating electrons, producing a force that in some way acts like a bar magnet positioned along the axis of rotation, seems to explain all the magnetic properties.
It also prompts to a connection between the electricity and magnetism because both are caused by position and movement of electrons.