magnetic_pole

# Magnetic pole

 Stan Zurek, Magnetic pole, Encyclopedia Magnetica, http://www.e-magnetica.pl/doku.php/magnetic_pole

Magnetic pole - an apparent surface or region in space which appears to emanate magnetic field.1)2)

Magnetic poles are always created if there is a loop with electric current. By convention, the magnetic polarity is such that when looking at the loop if the conventional current flows anti-clockwise, then it is the north magnetic pole (N); when the current flows clockwise then it is the south magnetic pole (S).3)4)5) This is related to the right-hand rule.

Opposite magnetic poles attract: N-S or S-N, like poles repel: N-N or S-S.

Magnetic poles N and S are created for any loop of electric current

Opposite magnetic poles attract (north-south N-S), similar poles repel (north-north N-N, or south-south)
Magnetic field lines in a solenoid (cross-section view)

## Magnetic pole strength

In the past magnetic poles were perceived as sources (or sinks) of magnetic field, and they were useful concepts (to some extent) when approaching the magnetostatic calculations as an analogy to electrostatic.11) By using the involved magnetic quantities it is possible to define magnetic pole strength, and the definitions differ depend on the employed system of units.

### Magnetic pole strength in SI

In SI system, magnetic pole strength p is expressed by the ratio of magnetic flux Φ emanating from that pole to the permeability of vacuum.12)

 $$p = \frac{Φ}{μ_0}$$ (A·m) where: Φ - magnetic flux (Wb), μ0 - permeability of vacuum

This equation can be related to magnetic dipole moment m:

 $$m = \frac{Φ·l}{μ_0} = p·l$$ (A·m2) where: Φ - magnetic flux (Wb), l - length of magnetic dipole or the distance between the poles (m), μ0 - permeability of vacuum, p - magnetic pole strength (A·m)

### Magnetic pole strength in CGS

In the CGS system, unit pole strength p is defined by the force and length. If two identical magnetic poles are placed 1 centimetre apart, and they act on each other with force of 1 dyne then then each pole has a strength of unity.13)

 $$F = \frac{p_1 · p_2}{r^2}$$ 1 (dyn) where: F - mechanical force equal to 1 dyne, p1 and p2 - two unity poles, r - distance between the poles equal to 1 cm