Answer to Question 4

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A moving coil meter  has a coil of dimensions 1.5 cm 2.0 cm.  The coil has 600 turns and is placed in a magnetic field that has a flux density of 0.25 T.  The coil is mounted by two springs of torsion constant 1.2 mN m rad-1 and is deflected by 40o when a current passes through it.

 

Calculate the current flowing in the coil.

Formula:

Remember that there are two springs.

Remember to convert the angle to radians:

q = (40 2 p) 360 = 0.698 rad

 

I = (2 1.2 10-6 N m rad-1 0.698 rad) (0.25 T 3.0 10-4 m2 600) = 3.7 10-5 A (= 37 mA)

 

Answer is to 2 significant figures as the data are to 2 s.f.