Alternet current and Mains electricity

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Alternating current
Cell and batteries supply current that passes round the circuit in one
direction.
This is called direct current, or dc.
Mains Electricity
The UK mains supply is approximately 230 volts.
It is an AC supply (alternate current), which means the current is constantly
changing directions
The frequency of the AC mains supply is 50 cycles per second or 50 Hz (hertz)
By contrast, cells and batteries supply direct current (DC). This just means
that the current always keeps flowing in the same direction.
Oscilloscope screens
A cathode ray Oscilloscope (CRO) is basically a snazzy voltmeter.
If you plug an AC supply into an Oscilloscope, you get a trace on the screen
that shows how the voltage of the supply changes with time. The trace goes up
and down in a regular pattern ­ some of the time it's positive and some of the
time it is negative.
If you plug in a DC supply, the trace you get is just a straight line.
The vertical height of the AC trace at any point shows the input voltage at that
point. By measuring the height of the trace you can find the potential
difference of the AC supply.
For DC it's a lot simpler- the voltage is just the distance from the straight line
to the centre line.
How to read an Oscilloscope screen

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