Answer to Question 6

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A solenoid consists of 100 turns of wire wrapped around a former that is 5.0 cm long.  It has a circular cross-section of diameter 1.0 cm.   A core of magnetic material is inserted into the solenoid so that it fits tightly.  It forms an electromagnet that can hold a weight of 3.7 N when a current of 0.85 A is flowing through it.   Calculate the relative permeability of the material.

Formula

n = 100 0.05 m = 2000 m-1

A = pr2 = p (0.005 m)2 = 7.854 10-5 m2

 

mr = (2 3.7 N) (4p 10-7 H m-1 [2000 m-1]2 [0.85 A]2 7.854 10-5 m2) = 25943 = 26000 (2 s.f.)

 

No units; it's just a number