Magnetic field

:For other senses of this term, see magnetic field (disambiguation).

Properties

Maxwell did much to unify static electricity and magnetism, producing a set of four equations relating the two fields. However, under Maxwell's formulation, there were still two distinct fields describing different phenomena. It was Albert Einstein who showed, using special relativity, that electric and magnetic fields are two aspects of the same thing (a rank-2 tensor), and that one observer may perceive a magnetic force where a moving observer perceives only an electrostatic force. Thus, using special relativity, magnetic forces are a manifestation of electrostatic forces of charges in motion and may be predicted from knowledge of the electrostatic forces and the movement (relative to some observer) of the charges.

Related Topics:
Albert Einstein - Special relativity - Tensor - Electrostatic

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

A thought experiment one can do to show this is with two identical infinite and parallel lines of charge having no motion relative to each other but moving together relative to an observer. Another observer is moving alongside the two lines of charge (at the same velocity) and observes only electrostatic repulsive force and acceleration. The first or "stationary" observer seeing the two lines (and second observer) moving past with some known velocity also observes that the "moving" observer's clock is ticking more slowly (due to time dilation) and thus observes the repulsive acceleration of the lines more slowly than that which the "moving" observer sees. The reduction of repulsive acceleration can be thought of as an attractive force, in a classical physics context, that reduces the electrostatic repulsive force and also that is increasing with increasing velocity. This pseudo-force is precisely the same as the electromagnetic force in a classical context.

Related Topics:
Thought experiment - Time dilation - Classical physics

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Changing magnetic fields, according to Faraday's law of induction, can induce an electric field and thus an electric current; similar currents can be induced by conductors moving in a fixed magnetic fields. These phenomena are the basis for many electric generators and electric motors.

Related Topics:
Faraday's law of induction - Electric field - Electric current - Electric generator - Electric motor

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Magnetic field lines

Technically, the magnetic field isn't a vector according to the formal definition, it is a pseudovector: it gains an extra sign flip under improper rotations of the coordinate system. (The distinction is important when using symmetry to analyze magnetic-field problems.) This is a consequence of the fact that B is related to two true vectors by a cross product (e.g. in the Lorentz force law). To simplify the study of magnets an arbitrary (but valid) description of magnetic field

Related Topics:
Pseudovector - Improper rotation

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

lines was created. 1 magnetic field line = 1 gauss line. 10,000 gauss

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

lines per square meter is equal to 1 tesla. The total number of lines emanating from a magnet pole is the magnetic flux. Count only north or only south pole lines, i.e. monopole or one sided value.

Related Topics:
Tesla - Magnetic flux

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Although the field line orientation is typically indicated in diagrams with an arrow, the arrow should not be interpreted to indicate any actual movement or flow of the field line.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Pole labeling confusions

It is necessary to note that the labeling of north and south on a compass is in opposition to the labeling of the north and south pole of the Earth.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

If you have two labeled magnets, it is clear that like poles repel, while opposing poles attract. However, this is clearly wrong when using a compass to find the North Pole of the Earth, because the "north" end of the compass points to the "North" Pole.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

By convention, the pole of a magnet is labelled according to the direction it points, hence when we speak of the "north pole" of a magnet, we really mean the "north-seeking pole". Magnetic field lines point from north to south of a magnet, and hence the natural magnetic field lines run from south to north along the Earth's surface. This choice, along with the choice of sign convention in the Biot-Savart law, is equivalent to choosing a sign convention for charge.

Related Topics:
Biot-Savart law - Charge

~ ~ ~ ~ ~ ~ ~ ~ ~ ~