The laws involved in the expression e = −dφ/dt are

• A. Coulomb’s and Ampere’s laws
• B. Coulomb’s and Faraday’s laws
• C. Faraday’s and Ampere’s laws
• D. Faraday’s and Lenz’s laws

Answer: (D)

Faraday’s 1st laws of electromagnetic induction tell us about the condition under which an e.m.f. is induced in a conductor or coil a when the magnetic flux linking a conductor or coil changes.
Faraday’s 2nd laws of electromagnetic induction give the magnitude of the induced e.m.f in a conductor or coil and may be stated as:
The magnitude of the e.mf induced in a conductor or coil is directly proportional to the rate of change of magnetic flux linkages.
Suppose a coil has N turns and magnetic flux linking the coil increases (i.e. changes) from φ1 Wb to φ2 Wb in t seconds. Now, magnetic flux linkages mean the product of magnetic flux and the number of turns of the coil.
N = e dφ/dt
Lenz Law:- Lenz’s law states: the direction of the induced e.m.f. is such as to oppose the change producing it. Therefore, the magnitude and direction of induced e.m.f. should be written as :
N = −e dφ/dt