Topic: AC IN RESISTOR, INDUCTOR AND CAPACITOR
Alternating current
Direct current (DC) circuits involve current flowing in one direction. In alternating current (AC) circuits, instead of a constant voltage supplied by a battery, the voltage oscillates in a sine wave pattern, varying with time as V = Vo sin ωt.
In a household circuit, the frequency is 60 Hz. The angular frequency is related to the frequency, f, by ω = 2πfVo represents the maximum voltage, which in a household circuit in North America is about 170 volts. We talk of a household voltage of 120 volts, though; this number is a kind of average value of the voltage. The particular averaging method used is something called root mean square (square the voltage to make everything positive, find the average, take the square root), or rms. Voltages and currents for AC circuits are generally expressed as r.m.s. values. For a sine wave, the relationship between the peak and the r.m.s. average is:
r.m.s. value = 0.707 peak value
Resistance in an AC circuit
The relationship V = IR applies for resistors in an AC circuit, so I = V/R = (Vo/R) sin(ωt) = Io sin(ωt)
In AC circuits we’ll talk a lot about the phase of the current relative to the voltage. In a circuit which only involves resistors, the current and voltage are in phase with each other, which means that the peak voltage is reached at the same instant as peak current. In circuits which have capacitors and inductors (coils) the phase relationships will be quite different.
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