# Using magnetic fields to keep things moving

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## Electromagnets

The force between magnets: like poles repel; unlike poles attract.

An electromagnet consists of a coil of insulated wire wrapped round an iron core. The electromagnet turns off when the current is turned off.

Electomagnet uses:

• Scrapyard crane
• Electric bell
• Circuit breaker
• Relay

Electromagnetic induction is the process of creating a potential difference using a magnetic field. When a conductor cuts the lines of a magnetic field, a potential difference is induces across the ends of the conductor. When an electromagnet is used, it needs to be switched on or off to induce a pd.

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## The motor effect

In the motor effect, the force:

• is incresed if the current or the strength of the magnetic field is increased.
• is at right angles to the direction of the magnetic field and to the wire
• is reversed if the direction of the current or the magnetic field is reversed.

An electric motor has a coil which turns when a current is passed through it and it changes direction when the current changes direction.

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## Transformers

• A transformer is used to change the size of the alternating pd from generator to homes.
• A transformer only works on ac because a changing magnetic field is necessary to induce ac in the secondary coil.
• A transformer has an iron core unless it's a switch mode transformer with an a ferrite core.
• A switch mode transformer is lighter and smaller than an ordinary transformer. It operates at high frequency

A step up transformer makes the pd across the secondary coil greater than the pd across the primary coil. It's secondary coil has more turns than it's primary coil.

A step-down transformer makes the pd across the secondary coil less than the pd across the primary coil. It's secondary coil has less turns than it's primary coil.

• In a transformer, an alternating current is passed through the primary coil
• This coil creates an alternating magnetic field that passes through the secondary coil
• As a result, an alternating pd is induced in the coil

Primary pd, Vp/Secondary pd, Vs = np/ns (np/ns, number of primary/secondary turns)

For a 100% transformer, Vp x Ip = Vs x Is (I=current, V=potential difference)

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