Physics · Alternating Current · NEET
This is the most common NEET confusion. Voltage and current are SCALAR quantities, not vectors. A phasor is only a picture, a rotating arrow, that we use to represent a sinusoidal scalar. NCERT states it directly: phasors are rotating vectors, but the quantities they represent (v and i) are not vectors themselves. We borrow the arrow-adding rule because out-of-phase sine quantities happen to combine the same way arrow projections do.
Let the phasor have length v_m (peak) and rotate at angle wt from the x-axis. Its vertical component is v_m sin(wt), which is exactly the AC equation v = v_m sin(wt). So as the arrow spins, its vertical shadow traces a sine wave. That shadow at any instant IS the instantaneous value. This is why the sine graph and the rotating arrow are two views of the same thing.
The length of a phasor is fixed and equals the amplitude (peak value), v_m or i_m. The arrow does not grow or shrink as it turns. Only its projection changes with time. So a longer phasor means a larger peak voltage or current, not a larger instantaneous value.
Draw both the voltage phasor and current phasor from the origin. The fixed ANGLE between them is the phase difference. In a resistor they point the same way (phase 0). In a pure inductor the current phasor is 90 degrees BEHIND the voltage phasor (current lags). In a pure capacitor the current phasor is 90 degrees AHEAD (current leads). The angle never changes as both rotate together.
When you connect R, L and C, the voltages across them peak at different times (they are out of phase). Adding v_m1 sin(wt) + v_m2 sin(wt+90) directly needs messy trigonometry. As phasors these become arrows at fixed angles, so you just add them like vectors and read the resultant with Pythagoras. For NEET this turns LCR and impedance problems into simple right-triangle geometry.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A phasor is an arrow that spins around the origin at the same angular speed w as the AC. Its length is the peak value, and its vertical shadow at any moment gives the instantaneous value of the voltage or current.
No. They are scalars. We only DRAW them as rotating vectors (phasors) because out-of-phase sine quantities add together the same way arrow projections do. NCERT clearly says v and i are not vectors.
It rotates with angular speed w = 2 pi f, the same as the AC source frequency. Both the voltage and current phasors spin at this w together, keeping the phase angle between them fixed.
No. The length stays fixed at the peak value. Only its projection on the vertical axis changes with time, tracing out the sine wave.
Because they turn out-of-phase voltage addition into simple vector addition. You draw V_R, V_L and V_C as arrows and combine them with a right triangle to get impedance and phase angle, avoiding heavy trigonometry.
