Faraday's Law -- from Eric Weisstein's World of Physics

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Faraday's Law

The law giving the voltage (a.k.a. electromotive force) produced in a current ring by a time changing magnetic flux through the ring. In cgs,

and in MKS,
Here B is the magnetic field, and is a surface integral. Eric Weisstein's World of Math

Electromotive Force, Magnetic Flux