Sinusoidal Alternating Voltage for a Coil

As observed during energising and de-energising of a coil, current starts to flow through an inductor after a certain delay. Connecting a pure inductance (i.e. a coil with an ohmic resistance of zero) to a sinusoidal alternating voltage of the form INDUCTORS IN AC CIRCUITS

results in a sinusoidal coil current i which lags behind the voltage by an angle INDUCTORS IN AC CIRCUITS

The current is thus described by the following equation: INDUCTORS IN AC CIRCUITS

The corresponding characteristic is shown in the diagram below. 


The coil current attains its maximum value when the voltage is zero, and vice versa.

The current flowing through a coil lags behind the applied voltage by an angle j = 90°.

Though able to conduct alternating voltage in a similar way to a capacitor, a pure inductor only consumes reactive power due to the phase shift of 90°, causing the coil's magnetic field to build up and decay periodically. In reality, however, coils also have a very small ohmic resistance resulting in a phase shift slightly less than 90° and a corresponding consumption of active power. 

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