State Lenz's law.

Explanation

The direction of the induced current in a circuit is given by Lenz.s law. It states that the induced currect is always in such a direction as to oppose the cause which is producting it. Lenz.s law is just an illustration of the law of conservation of energy . If `dPhi` is positive, then induced emf will be negative and if `dPhi` is negative then the induced emf will be positive.