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Lenz's law is expressed by the following formula, where $e$ is induced e.m.f., $\phi$ is magnetic flux in one turn, and $N$ is the number of turns.

1. $e=-N\,\dfrac{d\phi}{dt}$
2. $e=-\dfrac{dN\phi}{dt}$
3. $e=-\dfrac{d\phi}{N\,dt}$
4. $e=N\,\dfrac{d\phi}{dt}$

Correct answer: $e=-N\,\dfrac{d\phi}{dt}$

Solution

Lenz's law gives the direction of induced emf, and Faraday's law gives its magnitude. Together, they are written as $e=-N\,d\phi/dt$. The minus sign represents opposition to the change in magnetic flux.

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