Magnetic fields

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A magnetic field is a region in which a particle with magnetic properties experiences a force, and in which a moving particle experiences a force.

Magnetic flux is the flow of field lines. (units Wb)

There are two types of magnets: Permanent, and electro. 

Permanent magnets:


The fields lines ALWAYS go from north to south.

The arrows represent the direction of the force that a free north pole would experience.

The closer the lines are, the stronger the magnetic field.



The cross in the centre of the wire shows the wire is going away from us.

Direction of field: into page- clockwise, out of page- anticlockwise

Where to magnets coincide and cancel each other out, it is called a neutral point.

Magnetic Field Strength (B)

Aka: mangetic flux density. (units T)

Flux density is dependent on the amount of flux and the angle of the area (often a coil) to the field.

1 T = 1 Wb/m²

1 T is VERY BIG. a strong permanent magnet is about 0.1 T

A magnetic field has a strength of 1T if a wire of length 1 metre experiences a force of 1N when a current of 1A flows in the wire.

Force on a wire generating a current = BIL

The direction of the force is given by Fleming's left hand rule:


A couple on a coil.


Torque (C) on the coil = Fa sinθ = BNIba sin θ   or:

Torque (C) on the coil = BANI sinθ

Treat this as a moments question.

Moving Charges 

Diriving the equation for moving charges.

F = BIL I =…


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