S-Cool Revision Summary

S-Cool Revision Summary

Alternating Currents

Cells produce currents that travel in the same direction all of the time, direct currents. However, this is not always useful - for instance, transformers will only work if the current is constantly changing.

An alternating current is constantly changing direction. It is normally sinusoidal.

The frequency of an alternating current supply, f, is the number of cycles completed per second.Measured in Hertz (Hz).

The period, T, of an alternating supply is the time taken to complete one cycle.

The peak values of current, Io, and voltage, Vo, are the maximum values at the crest or trough. They are equivalent to the amplitude of a wave. Sometimes we quote the peak-to-peak value, which is of course, double the peak value.

RMS Values

RMS values are the d.c. equivalent of an a.c. value. In other words, if you had two circuits, one d.c. and one a.c., and you wanted them to use exactly the same amount of power (energy each second) then you would choose the d.c. values of current and voltage to be the same as the rms values of current and voltage in the a.c. circuit.

Equations

Alternating currents

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Symbols

Alternating currents

Vo = peak voltage, V

Io = peak current, A

Vrms = rms voltage, V

Irms = rms current, A

ω = angular frequency, Hz

t = time, s

T = period, s (time for one cycle of the pd or current.)