Lenz's law is a law in electromagnetism which states that a current induced by a changing magnetic field will oppose the change that produced it. Because of this, generated emfs still follow the conservation of energy, as it takes work to produce them. Lenz's law is usually shown by the negative sign in front of Faraday's law:

$ \nabla \times \mathbf{E} = -\frac{\partial \mathbf{B}}{\partial t} \, $
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