NUST NET Physics · Physics · Magnetism and alternating current. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
An alternating current changes size and direction all the time, so we quote its rms value: the steady current that would give the same heating. Mains voltages and meter readings are rms values.
Inductors and capacitors oppose AC without wasting energy. This opposition is called reactance and it depends on frequency: an inductor opposes high frequencies more, a capacitor opposes low frequencies more. In an inductor the current lags the voltage by 90°; in a capacitor it leads by 90°.
Resistance and reactance are out of phase, so they are combined like the sides of a right-angled triangle to give the impedance. At resonance the two reactances cancel and the current is greatest.
Adding resistance and reactance directly. A 30 Ω resistor in series with a 40 Ω reactance gives 50 Ω, not 70 Ω, because they combine as √(R2 + X2).
Answer: 5.0 A
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