Lesson 3 of 10 · 5 min
Phasors
NCERT §7.3
Riya replaces the lamp with her 200 Ω resistor and 15.0 μF capacitor in series. Now the current's wave runs ahead of the voltage's. How can she picture two sine waves that are out of step?
The lesson in notes
In short
In an inductor or a capacitor the current and voltage are not in phase. Phasors make these phase relations easy to see.
A phasor is an arrow that rotates anticlockwise about the origin with angular speed ω. Its length is the peak value (vm or im) of the quantity it stands for.
The projection of the phasor on the vertical axis gives the value at that instant, vm sin ωt or im sin ωt. As the phasors turn, the projections trace out the sine curves.
For a resistor, the V and I phasors point the same way at every instant: the phase angle between them is zero.
Voltages and currents are scalars. Phasors are only a device: the amplitudes and phases of sinusoidal quantities add in the same way as rotating arrows do.