A metal disk of radius 5.0 cm is mounted on a frictionless axle. Current can flo

Question

A metal disk of radius 5.0 cm is mounted on a frictionless axle. Current can flow through the axle out along the disk, to a sliding contact on the rim of the disk. A forte at thegone iGetween the station ary electrical c onta ct and the rotating the surrent i5 3.0 A, the disk rotätes with constant angular velocity. What's the frictiona 825 N This problem focuses on two concepts. One is the Faraday Law, the other is the conservation law of angular momentum. When the disk rotates at a uniform angular velocity, the net torque should be zero

Explanation / Answer

magnetic moment u = I*A

torque due to magnetic field T1 = u*B*sintheta

theta = 90

T1 = I*A*B

torque due to frictional force T2 = -f*R

net torque = T1 + T2

but in constant angular motion net torque = 0

T1 + T2 = 0

I*A*B - f*R = 0

I*pi*R2*B - f*R = 0

f = I*pi*R*B

f = 3*pi*0.05*1.75

frictional force f = 0.825 N <<<--------ANSWER

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