Let us illustrate the meaning of the negative sign in the e.m.f. equation.
Consider a closed circuit made of a single-turn loop as in figure. If the field is normal to the plane of the loop and is increasing, an e.m.f. is induced in the conductor and a current flows in the closed circuit. We say a current is “induced” by the magnetic field. However, this flow will cease if stationary. What is the direction of current in the above case? This is stated conveniently in a general way be Lenz law: “The induced current in the loop is always so directed as to produce a flux opposing the change in B”. (In other words, in case of a change in a magnetic system, that thing happens which tends to oppose the change). In our particular case, the current is directed as shown, as dictated by right hand rule; the flux produced opposes the increase in flux. If, on the other hand, B is decreasing in the same direction (downward), the direction of current will get reversed.
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