Mains electricity

The mains supply is AC.

The uk mains supply is 230 volts.

It is an alternating current (AC) which means the current is constantly changing direction.

The frequency of the AC mains supply is 50 cycles per secong or 50 Hz (hertz)

The battery supply is DC.

The battery supply is a direct current (DC) this means the current keeps flowing in the same direction.

Electricity supplies can be shown on oscilloscope screens.

A cathode ray oscilloscope (CRO) is basically a voltmeter.

If you plug a 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. 

Sometimes the pattern is negative, sometimes positive.

If you plug a DC supply then the trace is just a straight line.

The vertical heigh 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, the voltage is the distance from the straight line trace to the centre line.

An oscilloscope screen.

A DC source is always at the same voltage so you get a straight line.

A AC source gives a regualarly repeating wave. From that you can work out the period and the frequency of the supply.

Work out frequency using: Frequency (hz)= 1/ time period (s)

Example question

The trace below comes form an oscilloscope with the timebase set to 5 ms/div. Find a) the time period and b) the frequency of the AC supply.

Answer:

a) to find the time period, measure the horizontal distance betwen two peaks. The time period of the signal is 6 divisons. Multiply by the time base: time period = 5 X 6= 0.03 s

b) using the frequency formula on the left: frequency = 1/0.03 = 33 Hz