Magnetic Fields

Magnetic fields are produced by permanent magnets such as the Earth's magnetic field or bar magnets and by current carrying wires.

Magnetic fields are represented by magnetic field lines or magnetic flux density, with the lines closer together representing a stronger magnetic field. The magnetic flux density can be increased by increasing the current, increasing the number of coils or by adding an iron core. The lines always point from north to south.

Remember that opposite poles attract and like poles repel!

To determine the direction of the magnetic field lines the Corkscrew Rule can be applied.

Long wire
A long wire has a magnetic field, which can be determined using the corkscrew rule.



A solenoid is a coil of wire which creates a magnetic field similar to that of a bar magnet. It is possible to determine the poles of this 'magnet' using the following principal:

Imagine you are looking down the coil of wire, the direction in which the coils turn form a clockwise circular path when looking down it from one end and a anti-clockwise path when looked down it from the other.



Force on a current carrying wire

Fleming’s left hand rule

First finger = Field
seCond finger = Current
thuMb = Movement

The force can be calculated using the following equation:


Where B = Magnetic flux density or magnetic field strength (T or NA-1m-1)
L = Length perpendicular to the field (m)
F = Force (N)
I = Current (A)

Note that the wire must be perpendicular to the field in order to experience a force.

Defining an ampere
The amp is defined as the force (equal to 2×10–7 Newton per meter of length) experienced between two straight parallel conductors of infinite length, of negligible circular cross section, placed 1 meter apart in a vacuum.

This definition is based on the force experienced between two conductors.