Electromagnetism

Electromagnetism is the physical interaction among electric charges, magnetic moments and the electromagnetic field.

For a long time electricity and magnetism were thought to be separate forces. In the 19th century Maxwell demonstrated that they were interrelated phenomena then Einstein proved with the Special Theory of Relativity that they are aspects of one unified phenomenon.

The core of the relationship is that a changing magnetic field produces an electric field and conversely, a changing electric field produces a magnetic field.

Electric charge

We know that charge is an innate property of all charged fundamental particles. If a particle is charged it has a positive or negative charge. The most common charged particles in the universe are negatively charged electrons or positively charged protons. When charged particles are moving, they are known as electric currents.

Magnetism

Magnetism is a physical property produced by the motion of electric charge, which of course, is the same thing as electric current

A magnet is a material or object that produces a magnetic field. This field is invisible but visible by its effects: pulling on other magnetic materials such as iron, steel, nickel, cobalt etc and attracting or repelling other magnets.

All magnets have two ends where their magnetic effects are strongest. These regions are called the poles of the magnet. Materials can be magnetic but they are not magnetized until another magnetic material has entered into their field. At this point, the attraction and repulsion behaviour can be observed.

This behaviour is a function of the magnetic force which is transmitted via the magnetic field.

Relation to electrons

Magnetism, understood as the effect of a magnetic field, arises from the properties of the electrons in an atom. We know that atoms ‘orbit’ the nucleus of the atom but as they circle the nucleus they also spin or rotate on their own axis.

As they spin they produce a magnetic dipole: the two poles noted above. We call this propensity of electrons the intrinsic magnetic moment of the electron. It is aggregates of these miniature magnetic behaviours that produce the overall magnetic property of the material.

In most materials, equal numbers of electrons spin in opposite directions. As a result, their magentic effects are cancelled out. However in strongly magnetic materials an overall majority of electrons spin in one particular direction. This breaks the equilibrium and produces the magnetic field.

If you have material A where the electrons all spin in one direction and material B where the electrons all spin in a direction opposite to A, then B will be attracted to A and you can observe the effects of the magnetic force in action.

Fields and forces

What is a force in physics?

At its most abstract, divorced from the specific types of force, a force is an influence that can change the motion of an object. It is an external agent capable of changing the velocity of a body with mass. A force has both a magnitude and a direction.

What is a field

A field is a property of space. It means that a physical quantity is assigned to every point in space. This quantity has a numerical value and may vary over time.

The electric field

There are different types of field. The electric field is an instance of a vector field. With vector fields there is more than one number for each point in space:

• a magnitude: a size value, i.e. being larger or greater than something else
• a direction

The value of the electric field at a point in space equals the force that would be exerted on a unit of charge at that position in space.

Every charged object sets up an electric field in the surrounding space. A second charge “feels” the presence of this field. The second charge is either attracted toward the initial charge or repelled from it, depending on the signs of the charges. Of course, since the second charge also has an electric field, the first charge feels its presence and is either attracted or repelled by the second charge too. The electric field from a charge is directed away from the charge when the charge is positive and toward the charge when it is negative.

The magnetic field

As already noted, the magnetic field is the field created by a magnetic material: a material where the spin of the electrons is in disequillibrium. As with the electric field, a magnetic field exerts an attraction or repulsion on other spinning electrons. This is the magnetic force; the force is transmitted by the field. Attraction occurs when the two sets of electrons spin in opposite directions to each other. Repulsion occurs when the two sets of electrons spin in the same direction.

Crucially the magnetic force influences only those charges that are already in motion.

The magnetic field and force is more complex than the electric field/force. Whereas the electric field and force point either towards or away from the charge, the magnetic field is different:

• The magnetic field points perpendicular to its source
• The magentic force points perpendicular to the magnentic field

This is illustrated below which shows the magnetic field operating at right angles to the flow of charge within a wire.

The electromagnetic field

Although we have described the electric and magnetic fields separately, they are in fact a single unified and inseparable field created by charged particles and particles with a magentic moment. In describing electricity and magnetism we have focusing on different attributes of the same phenomenon.

The electromagnetic field does not carry charge or magnetic moment, it carries energy and momentum. This energy and momentum can be transferred to charged particles and particles with magnetic moment.

Maxwell’s Equations

The equations express four laws that offer a complete account of the electromagnetic field. The equations integrate separate discoveries and laws identified by other scientists with a common mathematical representation. This is why they are not called ‘Maxwell’s Laws’ since he didn’t originate the laws, he unified them into a common expression. These collective laws capture everything we have discussed so far in relation to electric and magnetic fields.

1. An electrically charged particle creates an electric field. (Gauss’ Law for Electricity)
2. Magnetically charged particles do not exist. (Gauss’ Law for Magentism)
3. A changing magnetic field creates an electric field. (Faraday’s Law of Induction)
4. A changing electric field creates a magnetic field. (Ampere’s Law)

The electromagnetic force

Maxwell’s Equations describe how electromagnetic fields are generated and behave but they don’t specify how the fields interact with charged particles. An account of this is provided by Lorentz’s force law which specifies what we have already described above:

An electric field exerts a forward or backward force on a charged particle and a magnetic field exerts a sideways/perpendicular force on a moving charged particle.

Electromagnetic radiation / waves

Electromagnetic radiation consists in waves of the electromagnetic field propagating through space carrying electromagnetic radiant energy.

Electromagnetic waves consist of rapidly changing electric and magnetic fields. They are emitted any time an electrically charged particle accelerates. These waves are generally referred to as light. However, more accurately, ‘light’ refers to the types of electromagnetic wave that we can see. Electromagnetic waves form a spectrum based on their frequency and wavelength. For example, ‘radio waves’ are low-frequency / long wavelength electromagnetic waves and gamma rays are high-frequency / short wavelength waves:

The image below shows the propagation of an electromagnetic wave through space. We can identify the core components as follows

• The vector of the magnetic field \(B\) propagates outwards along the \(z\) axis
• The magenetic field is perpendicular to the vector of the electric field \(E\) which propagates upward along the \(y\) axis
• The directionality of both waves is forward along the \(x\) axis

Using magnetism to generate electricity

Understand better:

• How do EM waves relate to simple electrical circuits.
• Link discussion here to discussion of Hertz

https://www.britannica.com/science/electromagnetism/Coulombs-law