Physics Revision: Motors,Generators and Transformers

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The Motor Effect

When a current flows through a wire a magnetic field is produced around the wire, the magnetic field's field lines will have varying direction depending on the direction of the current.

When a wire carrying a current is placed in a magnetic field, a force is exerted. This is called the motor effect, however the conductor WILL NOT experience this force if it's parallel to the magnetic field.

The force increases as the strength of the magnetic field increases or the size of the current increases.

The direction of the force also depends on the direction of the current and magnetic field, as if these are reversed the direction of the force will also reverse

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The Generator Effect

If an electrical conductor 'cuts' into the magnetic field then a p.d. is induced across the ends of this conductor.

If a magnet is moved into a coil of wire, a p.d. is induced across the ends of the coil. THIS IS THE GENERATOR EFFECT, and it also occurs if the magnetic field is stationary and the coil moves.

If the coil is part of a complete circuit, then the current is induced in the wire.

If the direction of motion (or polarity of the magnet) is reversed, the direction of the p.d. or current is reversed.

The size of the p.d. increases if the speed of movement, strength of the field, number of turns on the coil, or area of the coil increases.

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Transformers

A transformer consists of a primary and secondary coil, wound on a soft iron core.

An alternating current in the primay coil produces an alternating magnetic field in the soft iron core and therefore the secondary coil. This induces an alternating p.d. across the ends of the secondary coil.

In a step-up transformer the p.d. is greater in the secondary coil than the primary coil, and subsequently this is the opposite in a step-down transformer.

The p.d. across the primary and secondary coils of a transformer A and B are related to the number of turns on the coil a and b by:

A/B=a/b

If transformers are assumed to be 100% efficient, the power output would equal the power input.

V X I=v x i (upper case=primary coil,lower case=secondary coil)

Switch mode transformers are ones that operate at a higher frequency, are lighter and smaller, and use very little power when switched on but no load is applied.

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