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(c) Choice of magnetising force ( H ) = 518 A/m :
( d) because the coil is wound onto a cast steel ring, so we must choose a value that is within the given B-H range for cast steel ; drawing a horizontal line through B = 0.6 T gives a plausible range H ε [ 350 , 700 ] A/m , within the specified region (although there is no reason to assume that one could not choose a value greater than 700 A/m since it is obvious that cast steel's magnetic properties will extend well beyond the Hmax value shown on the chart) Assignment guidance indicates a preference for "odd values" different to others in peer group so 518 A/m seems a reasonable pot-luck choice within the allowable range.
(b)
H = I n / L (1)
where :
I = current flowing in coil in Ampere ;
n = number of turns on coil ;
L = mean length of magnetic flux path ;
such that L = 2 π r = 2 π (70 x 10^-3) = 140π x 10^-3 m
implying :
I = H L / n [ transposing for current using (1) above ] ; thus
I = 518 x 140π x 10^-3 / 8000 = 28.5 mA
(a) mmf = I n = 28.5 x 10^-3 x 8000 = 28.5 x 8 = 227.8 A ≈ 228 A
[ or 228 At ; however, "turns" is not an official SI unit despite H being shown in At/m on the chart ! ]
--
Pepe Oldfag Wizard
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