Alternating CurrentNEET Physics · Class 12 · NCERT Chapter 7

You can expect 1 to 2 questions in NEET 2027. The chapter has high PYQ frequency. RMS values, AC through inductor or capacitor (phase relations), series LCR impedance and resonance, and transformers are favourites.

For a sinusoidal current i(t) equals i_0 sin(omega t), the RMS (root mean square) value is i_rms equals i_0 over square root 2 ≈ 0.707 i_0. The mean over a full cycle is zero, but the heating effect (proportional to i squared) is non-zero. RMS is the equivalent DC current that would dissipate the same power.

In an inductor, voltage leads current by pi over 2 (90 degrees). Reactance X_L equals omega L equals 2 pi f L. Higher frequency means higher reactance. At very low f, an inductor acts like a wire; at very high f, like an open circuit.

In a capacitor, current leads voltage by pi over 2. Reactance X_C equals 1 over (omega C) equals 1 over (2 pi f C). Higher frequency means LOWER reactance. At very low f, a capacitor acts like an open circuit; at very high f, like a wire (DC blocks, AC passes).

Total opposition to AC: Z equals square root of R squared plus (X_L minus X_C) squared. Phase angle phi between voltage and current: tan phi equals (X_L minus X_C) over R. If X_L > X_C, voltage leads current (inductive). If X_C > X_L, voltage lags current (capacitive).

When omega_0 equals 1 over square root LC (so X_L equals X_C), impedance Z reduces to its minimum value R, and current is maximum equal to V over R. Phase angle phi equals 0; voltage and current are in phase. This is series resonance.

A transformer uses two coils linked by a common iron core. Changing AC current in the primary coil produces a changing flux which induces an EMF in the secondary. Voltage ratio equals turns ratio: V_p over V_s equals N_p over N_s. For an ideal (lossless) transformer, V_p I_p equals V_s I_s, so I_s over I_p equals N_p over N_s. Step-up: more turns on secondary, voltage rises, current falls.