Alternating Current — Class 12 Physics

Alternating current reverses direction periodically, varying as . Because its average over a cycle is zero, we use the root-mean-square value , which gives the same heating as a steady current. In an AC circuit a resistor keeps current in phase with voltage, an inductor makes current lag by with reactance , and a capacitor makes current lead by with reactance . In a series LCR circuit the impedance is and the current lags or leads the voltage by a phase angle . Resonance occurs when , at , where impedance is minimum and current maximum—vital for tuning circuits. Average power is , the power factor determining useful power; a purely reactive (wattless) current delivers no average power. Transformers, working by mutual induction, step voltage up or down by the turns ratio, enabling efficient long-distance power transmission.