MDCAT Physics · Physics · 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.
Resistors, inductors and capacitors each oppose AC differently. The opposition of an inductor or a capacitor is called reactance, measured in ohms. Inductive reactance rises with frequency and capacitive reactance falls with frequency. So an inductor passes DC freely and a capacitor blocks it.
In a resistor, current and voltage are in phase. In a pure inductor the current lags the voltage by 90° (π/2). In a pure capacitor the current leads by 90°.
The total opposition of a series circuit is its impedance Z. The voltages are out of step, so they add like the sides of a right-angled triangle, not as plain numbers. At resonance XL = XC, so Z = R (its smallest value) and the current is largest.
Adding resistance and reactance (or the voltages across R, L and C) as plain numbers is wrong; they must be combined with the square-root rule. Also remember the order: in a capacitor current leads, in an inductor current lags.
Answer: About 160 Hz (159 Hz).
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