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Physics - AC circuits

AC circuits

Many circuits utilise AC electricity. AC [1] 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 [2](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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Links:

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[1] http://alison.com/#

[2] http://alison.com/#

AC voltage supply varies for some country. Here, in the Philippines, we refer it as "220 volt supply".