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Energizing and De-energizing a Coil

The processes of energising and de-energising a coil are already familiar from the treatment of DC circuits. If a coil is connected to a direct voltage source as shown in the diagram below (switch closed), a voltage Uind is self-induced at the same time that a current starts to flow. According to Lenz's law, this voltage is in the opposite direction to the externally applied DC voltage U0. As a result, the current through the coil rises with a delay described by an exponential function, reaching its maximum value after a period which depends on the inductance L and ohmic resistance R (the range labelled (1) in the timing diagram). 

INDUCTORS IN AC CIRCUITS

Turning off the external voltage (opening the switch) cuts off the power generating the coil's magnetic field and gives rise to a self-induced voltage in the coil which is opposite to the cause of the change in magnetic field (i.e. disconnection of the external voltage) and initially allows the current to keep flowing (the range labelled (2) in the timing diagram). The induced voltage thus causes the decay of the coil current to take longer. 

The following animation illustrates these relationships. 

Energising and de-energising a coil:

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