Lenz' Law
(a) When this bar magnet is thrust into the coil, the strength of the magnetic field increases in the coil. The current induced in the coil creates another field, in the opposite direction of the bar magnet's to oppose the increase. This is one aspect of Lenz's law—induction opposes any change in flux. (b) and (c) are two other situations. Verify for yourself that the direction of the induced Bcoil shown indeed opposes the change in flux and that the current direction shown is consistent with the right hand rule.
Source
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"OpenStax College, Faraday’s Law of Induction: Lenz’s Law. February 7, 2013."
http://cnx.org/content/m42392/latest/
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