Faraday’s experiments shows that
The emf induced by a change in magnetic flux depends on only a few factors. First, emf is directly proportional to the change in flux ΔΦ.
Secondly, emf is greatest when the change in time Δt is smallest—that is, emf is inversely proportional to Δt. Finally, if a coil has N turns, an emf will be produced that is N times greater than for a single coil, so that emf is directly proportional to N.
The equation for the emf induced by a change in magnetic flux is
emf = − N•(ΔΦ/Δt)
This relationship is known as Faraday’s Law of Induction. The units for emf are volts, as is usual. The minus sign in Faraday’s law of induction is very important. The minus means that the emf creates a current I and magnetic field B that oppose the change in flux ΔΦ—this is known as Lenz’s law. The direction (given by the minus sign) of the emfis so important that it is called Lenz’s law.
The Russian Heinrich Lenz (1804–1865), who, like Faraday and Henry,independently investigated aspects of induction. Faraday was aware of the direction, but Lenz stated it so clearly that he is credited for its discovery...
From the above figure
(A) 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 Right Hand Rule.
