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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 
results in a sinusoidal coil current i which lags behind the voltage by an angle 
The current is thus described by the following equation: 
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.