A current in a top wire of length 50.4 m is 36 A and is rigidly supported horizo

Question

A current in a top wire of length 50.4 m is 36 A and is rigidly supported horizontally. A bottom wire having the same length as the top wire has a current of 26 A in the same direction as the current in the top wire, is parallel to the top wire, is 0.3 cm away and is free to rise or drop. How much mass does the bottom wire support? A) 5.6484 grams B) 320.9 grams C) 128 grams D) 121.68 grams E) 10.485437 grams 20. You want to use a 8.5 ohm, 4 mA galvanometer coil as a 40 v voltmeter. What resistance should be placed in series with the coil? A) 1088 B) 11220 C) 3400 D) 850 E) 9992

Explanation / Answer

19)

Force due to magnetic field on the lower wire:

F = u*I1*I2*L/(2*pi*r)

= 1.26*10^-6*36*26*50.4/(2*3.14*0.003)

= 3.16 N

So, now this magnetic force must be balanced by the gravity

So, 3.16 = m*9.8

So, m = 0.322 kg = 322 g <------ option B

20)

Let the required resistance be R

So, total resistance of the circuit = 8.5+R

So, (8.5+R)*4*10^-3 = 40

So, R = 9992 ohm

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