2.6. The Magnetic Field

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General Theory of the Electromagnetic Field

The electric field strength contains besides the Coulombian component, that is a potential one, a curl (rotational) component, also called solenoidal component or induced component:

E Ec E

r . (2.35)

Therefore, in the case of the reference frames K o and K1 considered previously, the strength of the resulting electric field in the reference frame K o is Eo and can be written:

Eo E oc E

or , (2.36)

where the quantity Eoc represents the Coulombian component, and the quantity Eor represents the curl component (rotational, solenoidal or induced component) of the electric field strength.

The solenoidal component of the electric field strength and the magnetic induction at a point are produced by the electric charge in motion with respect to the considered reference frame.

2.6. THE MAGNETIC FIELD

Firstly, we shall recall some generalities about magnetism and magnets. Various bodies, among which the loadstone (iron ore), have certain properties, explained below, and for this reason they are called magnets. A body, which after having been subjected to certain treatments gets such properties, becomes a magnet and the treatment represents the process of magnetization. The material of which the body is made represents a magnetic material. The properties mentioned above consist in actions or forces of magnetic origin. For instance, a body suspended near any part of the earth surface, so as to turn freely about a vertical axis, could tend to set itself in a certain position. If it is disturbed from this position, it could tend to come back in the previous position, after several oscillations. In the case in which the body satisfies the mentioned tendencies, then it is a magnetized body, and in the contrary case, it is an unmagnetized one. In the first case the body is a magnet. The study referring to actions of magnetic origin is called magnetism.

Between certain bodies, like those of magnetite ( 32O Fe ), ponderomotive actions (forces and torques) are exerted. These actions are not of thermal, mechanical or electrical nature, but of magnetic nature.

Also, between conductors carrying macroscopic electric currents, ponderomotive actions are exerted. These actions are not of thermal, mechanical or electrical nature, but of magnetic nature. Therefore, upon bodies carrying electric currents, ponderomotive actions that are not of thermal, mechanical or electrical nature but of magnetic nature, can be exerted.

The ponderomotive actions of magnetic nature between magnetized bodies, or carrying electric currents, and other magnetized bodies, or carrying electric currents, are not directly exerted but by means of the magnetic field.