Magnetism and Electromagnetism

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Magnets

HARD magnetic materials are used for magnets that we want to stay magnetised

SOFT magnetic materials are used when we need something to magnetise and demagnetise easily

Like poles repel and unlike poles attract

Magnetic field lines always go from north to south

Magnetic field lines do not cross

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Electromagnetism

When current flows through a wire a magnetic field is produced

The direction of the field line arrows can be remembered by the RIGHT-HAND GRIP RULE

The thumb points in the direction the current is flowing

A current in a coil of wire also produces a magnetic field

The field can be made stronger by increasing the current, increasing the number of turns, or by adding a soft iron core (soft so it is magnetised when there is current but demagnetises when there isn't)

Flemming's left hand rule (motor rule) can be used to work out the direction of the force that acts on the wire

The motor effect is also used in loudspeakers: the current produced by an amplifier is alternating and by passing it though a coil in a magnetic field the current results in alternating forces on the coil. The coil is attached to a paper cone and this transfers the vibrations to the air.

This diagram shows a simple DC motor which consists of a single turn of wire that is free to rotate in a magnetic field about an axle.

Carbon brushes make contact with the ends of the coil which are connected to a commutator so that a current can be passed through the coil

You can increase the speed a DC motor spins by increasing the strength of the magnetic field, increasing the strength of the current or put more turns in the coil

Electromagnetic induction and the generator

The magnetic field can change if:

The conductor is moving into or out of a magnetic field

The magnet is moving towards or away from the conductor

The magnetic field is being varied

If the conductor is part of a closed circuit then the induced voltage will cause a current to flow

Generators

Bicycle Dynamo

Small strong bar magnet

Spun by wheels of bicycle

Spins between the faces of a U-shaped iron core on which a wire is wound

The changing magnetic field induces an alternating voltage in the coil

Larger Generators

Spinning coils of wire in the magnetic field of strong permanent magnets (or in large generators, a magnetic field produced by electromagnets)

Since the coil in which the voltage is being induced is spinning, the electrical connections are made by brushes which slide over slip rings.