Lenz's Law:

Lenz's Law determines the polarity of the induced voltage. They have a polarity which will oppose the modify causing the induction.  Whenever current flows due to the induced voltage, a magnetic field is set up around in which conductor so that the conductor's magnetic field reacts along with the external magnetic field. This generates the induced voltage to oppose the change in the external magnetic field.  A negative sign in equation (1-20) is an indication in which the emf is in like a direction as to produce a current whose flux would reduce the magnitude of the emf, if added to the original flux.