Physics - AC circuits
Many circuits utilise AC electricity. AC  stands for alternating
current. It means that the current flows one way through a circuit for half
a cycle, then in the opposite direction through the circuit during the
other half of the cycle. The other difference to note is that in DC
circuits the supplied voltage (and therefore usually current) values remain
steady when the circuit is in operation. In AC circuits, the size of the
applied voltage and current varies in the form of a sine function as shown
in the graph below.
The electricity transmitted to our homes is in the AC form. We commonly
refer to this as a 240volt supply. The value quoted is misleading as it
implies that the voltage has a steady voltage, which it does not. In fact
the voltage varies between + 339 volts and -339 volts as shown above. The
direction of the current changes twice per cycle. Since the supply has a
frequency of 50 hertz, the period for one cycle is one-fiftieth of a second
or 0.02 seconds.
When we refer to this as a "240volt supply", we are actually quoting the
DC voltage that would deliver the same amount of energy. The 240 volt value
is called the 'DC equivalent' to our AC signal (which actually varies
between +339 volts and -339 volts). To calculate this DC equivalent
involves dividing the peak voltage (Vpeak) by the square root of two. Hence
the name RMS (root mean square) is given to this label.
The Vrms is a most useful approach to use when describing the electricity
transmitted to your home. If you work with the RMS values of AC currents
and voltages then all of the formulae that apply to DC circuits can be
used. This is because you are actually working with 'DC equivalent' values.
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