P2 Mains Electricity

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5.1 Alternating Current:

DC or direct current is when the current is going in only one direction. always either positive or negative.

AC or alternating current is when the current repeatedly reverses its direction. Positive one moment then negative the next.

Frequency of alternating current is the number of cycles it passes through each second. So,

Frequency = 1/time taken for each cycle

Or the number of waves that occur in a second so,

Frequency = (no. of waves)/1 second.

Mains supply provides alternating potential difference, which is why current through mains appliances (live wire/neutral wire) alternate.

The neutral wire is earthed at the local substation. It has 0 volts.

Voltage of the live


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