# Mains Electricity - National Grid Ideas in Context OCR Additional Science

Topics that are likely to come up in the Additional Science Ideas in Context Exam 2012

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## Mains and Battery Supply

Mains supply is approximately 230 volts.

Produced by generators using electromagnetic induction.

Is an AC Supply meaning it changes direction.

Batteries supply a DC current meaning the current flows in the same direction

AC is easy to generate and can be distributed easily.

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## Coil of Wire

A voltage and possibly current can be created by moving a magnet in or near a coil of wire. This is electromagnetic induction.

As the magnet is moved the magnetic field changes which creates a voltage and a current flows if its part of a complete circuit.

The direction of voltage depend son how you move the magnet.

If you move it into the coil the voltage is opposite to when you move it out of the coil.

If you reverse the polarity as it enters the coil it is induced in the opposite direction.

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## AC Generators

Generators generate electricity.

In a generator the magnet rotates around a coil of wire, as the magnet turns the magnetic field changes which induces a voltage.

When it is turned through a half turn the direction of the magnetic field reverses, this makes the voltage reverse and the current flows in the opposite direction.

If the magnet keeps turning you get an AC current.

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## Factors that Affect Voltage

The strength of the magnet

The speed of movement

The number of turns of the coil.

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## Transformers

Step up transformers step the voltage up. They have more turns on the secondary coil than the primary.

Step Down Transformers step the voltage down.They have more turns on the primary coil.

The primary coil produces a magnetic field which stays within the magnetic core.

The AC current means the magnetic field in the core is constantly changing, which induces an AC current in the secondary coil.

The number of turns decides whether the voltage induced in the secondary is greater or less than that in the primary coil.

Using DC would mean no current in the secondary coil as there would be no changing field to induce the current.

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