Amp Flux

a coil of 1000 turns enclosing a magnetic circuit 20 cm sq. in section.with 4Amp the flux density is 1 wb/m sq

and with 9 aMP it is 1.4 WB/M sq.
find the mean value of inductance between this current limits and induced emf if the current falls uniformly from 9 to 4 amp in .05 seconds…

By Faraday’s law emf = – N*dF/dt, where N = 1000 and dF/dt is derivative of magnetic flux through one winding. In your case initial flux is (1.4/100)*20 = 0.28 Wb and final flux is (1/100)*20 = 0.2 Wb. The flux change in 0.05 sec is (0.2 – 0.28)/0.05 = -1.6 Wb/s, and induced emf = -1000* (-1.6) = 1600 V.
emf can be defined through inductance L as: emf = L*di/dt, where i is the induction current. In your case di/dt = (4 – 9)/0.05 = – 100 A/s. Therefore (the mean value of) L = 1600/100 = 16 H (sign neglected).

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