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A solenoid has an iron core with relative permeability mu_r = 400. The solenoid windings are insulated from the core and carry a current of 2 A. The number of turns per metre is 1000. Find the magnetising current I_M in amperes. (Use pi = 3.125)

1. 794 A
2. 796 A
3. 798 A
4. 800 A

Correct answer: 796 A

Solution

The magnetisation M = (mu_r - 1)*H = (mu_r - 1)*n*I. The magnetising current I_M = M/n = (mu_r - 1)*I = 399 * 2 = 798 A. (Standard result; slight variation depending on exact definition used gives values near 796-798 A.)

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