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AC Circuit

An alternating current - AC - is generated by a sinusoidal voltage source

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An alternating current - AC - is generated by a sinusoidal voltage source.

Ohm's Law

Uz = Iz Z                    (1)


Uz = voltage drop over the load (volts, V)

Iz = current through the load (ampere, A)

Z = impedance of the load (ohms, Ω)


The momentary voltage in an AC circuit as function of time can be expressed as

u(t) = Umax sin(ω t + θ)                (2)


u(t) = voltage in the circuit at time t (V)

Umax = maximal voltage at the amplitude of sine (V)

t = time (s)

ω = 2 π f

    = angular frequency of sine wave (rad/s)

f = frequency (Hz, 1/s)

θ = phase of sine wave (rad)

The momentary current as a function of time can be expressed as

i(t) = Imax sin(ω t + θ)                 (2b)


i(t) = current at time t (A)

Imax = maximal current at the amplitude of sine (A)

RMS or Effective Voltage

RMS - Root Mean Square - or effective voltage of an AC sine wave can be expressed as

URMS = Ueff

        = Umax / (2)1/2

        = 0.707 Umax                     (3)


URMS = RMS voltage (V)

Umax = maximum voltage (amplitude) of sinusoidal voltage source (V)

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