Physics · Alternating Current · NEET
They are perfectly in phase. If the applied voltage is v = vm sin(wt), the current is i = im sin(wt) with the SAME sin(wt) term. There is no extra angle added or subtracted, so the phase difference is exactly 0. Both cross zero together and both reach their peak at the same instant.
A resistor obeys Ohm's law instantly: i = v/R at every moment, so current copies the shape of voltage with no delay. An inductor opposes any change in current (self-induced emf), so current is delayed and lags by 90 degrees. A capacitor must charge first, so current flows ahead and leads by 90 degrees. Only the resistor has no energy-storing delay, so its phase shift is 0.
Neither. Lagging and leading only happen when an inductor or capacitor stores energy and creates a time delay. A pure resistor stores no energy, so current neither lags nor leads — it stays exactly together with the voltage.
The phase angle is 0 (zero). In the phasor diagram both the voltage phasor and current phasor point in the same direction along one line. This also means the power factor cos(phi) = cos(0) = 1, so a resistor uses maximum power.
Yes. Because they are in phase, at the instant voltage is at its peak vm, the current is also at its peak im = vm/R. When voltage is zero, current is zero at the same moment. This is exactly why the graphs of v and i for a resistor peak together.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Zero. The voltage v = vm sin(wt) and current i = im sin(wt) share the same phase, so the phase difference is 0 degrees (0 radians).
Applying Kirchhoff's loop rule gives v = iR, so i = (vm/R) sin(wt) = im sin(wt). The current term is the same sin(wt) as the voltage, with no added angle, so the phase difference is 0.
The power factor is cos(phi) = cos(0) = 1. A resistor dissipates the maximum possible average power for a given voltage and current, which is why it is not a 'wattless' element.
Both are sine curves that rise, peak, fall and cross zero at exactly the same instants. They overlap in timing — this is the visual meaning of 'in phase'.
Yes. Because phase is 0, you can directly use im = vm/R for peaks and Irms = Vrms/R for rms values, and average power P = Vrms x Irms = Irms^2 R with no phase correction.
