Electromagnetic induction
Section: Magnetic Fields | Syllabus: Cambridge AS Level Physics 9702
Magnetic Flux and Flux Linkage Magnetic Flux (): product of magnetic flux density and the area perpendicular to the field. = BA Unit: Weber (Wb). 1 Wb = 1 T m^2. Magnetic Flux Linkage (N): For a coil with N turns: Flux Linkage = N = BAN (if field is perpendicular to area) If the field makes an angle θ with the normal to the area: = BA θ.
Laws of Induction Electromagnetic induction occurs when there is a change in magnetic flux linkage . Faraday's Law The magnitude of the induced e.m.f. is directly proportional to the rate of change of magnetic flux linkage.
E = Δ(N)/Δ t Flux Cutting (Straight Wire) For a straight conductor of length L moving at speed v perpendicular to a field B: E = B L v Lenz's Law The direction of the induced e.m.f. (or current) is such that it produces effects to oppose the change causing it.
(This is a statement of conservation of energy). E = -Δ(N)/Δ t The minus sign represents Lenz's Law. Eddy Currents and Damping If a metal plate (e.g. aluminium disc) moves through a magnetic field, eddy currents are induced effectively "inside" the metal.
By Lenz's Law, these currents create magnetic fields that oppose the motion of the disc, causing a braking force. This is electromagnetic damping . Factors Affecting Induced E.M.F. To increase the induced e.m.f (and current): Move faster: Increases rate of change of flux.
Stronger magnet: Increases B and thus flux . More turns (N): Increases flux linkage. Common Error A large flux does not mean a large e.m.f. It is the rate of change that matters. A constant high flux produces zero induced e.m.f.
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