Lenz's law
As another example of the application of Lenz's law, consider a coil of wire to which a battery is suddenly connected. Suppose that the battery starts a current flowing clockwise, as viewed by the observer. This current will give rise to a magnetic field whose lines will thread the coil and circle back outside it. Thus, as the current due to the battery builds up, there is a changing magnetic flux through the coil and this must result in an induced emf in the coil. What is the direction of this induced emf? Lenz's law tells us immediately that it must be counterclockwise, so as to oppose the building up of the current. Similarly, when the current in a circuit is broken, the induced emf seeks to keep the current from dying out, and this accounts for the sparking observed when switches are opened slowly. The induced emf in a circuit whose current is changing is called the back emf, since it always opposes the alteration in the current. It arises from change in the current's own magnetic field, an effect referred to as self-inductance.