Physics · Alternating Current · NEET
It really reverses. In one AC cycle the electrons drift one way, slow to zero, then drift the opposite way. In Indian mains this full back-and-forth happens 50 times per second (f = 50 Hz). The current value follows i = i0 sin(wt): it rises to a peak i0, drops to zero, goes negative to -i0, and returns. DC never does this; it stays on one side of zero.
Your wall socket gives AC. NCERT states the electric mains supply in homes and offices is a voltage that varies like a sine function with time. A phone charger or laptop adapter then converts that AC into the DC that the device needs internally. So the supply is AC, but many gadgets run on DC inside.
A battery is DC. Its two terminals stay fixed (one + and one -), so current always flows the same direction with a nearly steady value. That is why a battery gives a flat line on a current-time graph, while an AC source gives a sine wave crossing zero.
NCERT gives the reason clearly: AC voltage can be easily and efficiently stepped up or stepped down using a transformer. High voltage means low current in the wires, so less heat loss (P = I^2 R) over long distances. A transformer works only with changing (AC) current, so DC cannot be transformed this simply. The detailed reasoning is covered in the next concept, why AC is used for power transmission.
w is the angular frequency in rad/s, and w = 2*pi*f, where f is the frequency in hertz (cycles per second). For Indian mains f = 50 Hz, so w = 2*pi*50 = 100*pi rad/s (about 314 rad/s). A bigger w means the current swings back and forth faster.
A circuit when connected to an AC source of 12 V gives a current of 0.2 A. The same circuit when connected to a DC source of 12 V gives a current of 0.4 A. The circuit is
Try the real previous-year questions from this chapter — each with the answer and a full solution.
AC is still measured in amperes (A), the same as any current. The lowercase i (or i(t)) is used for the instantaneous value, i0 for the peak value, and I_rms for the effective (root-mean-square) value used in most calculations.
50 Hz. This means the current completes 50 full cycles every second, so its time period T = 1/f = 1/50 = 0.02 s and angular frequency w = 2*pi*f = 100*pi rad/s (about 314 rad/s).
Yes. AC is converted to DC by a rectifier (used inside chargers). DC is converted to AC by an inverter (used in home inverters and UPS). But AC to AC voltage change (step up or step down) is done most simply by a transformer, which is why supply is AC.
Over one full cycle the current spends equal time positive and negative, so the plain average is zero. But heating depends on i^2 (always positive), so the effective heating value is the RMS value I_rms = i0/sqrt(2), which is not zero. This is why we rate AC by its RMS value.
For NEET you only need the physics: AC continuously reverses direction while DC does not. The 220 V figure quoted for mains is the RMS value, and its peak voltage is actually higher, V0 = sqrt(2) * 220 = about 311 V.
