PHYS 102 Exams FALL 2014 Exam 3 (A) November 2014 The next two questions pertain

Question

PHYS 102 Exams FALL 2014 Exam 3 (A) November 2014 The next two questions pertain to the situation described below. A light bulb is attached to a frictionless, conducting track as shown in the figure. The tracks run through an area containing a magnetic field, Bex 4 T, pointing into the page. The tracks are L 0.45 m apart. The lightbulb produces 60 W when attached to a 115 Vpower source. A conducting bar is attached to the track. You push the bar with constant velocity v to the right as shown. 1) With what speed must the bar travel for the light bulb to dissipate 60 Wof power? a. v 13 m/s b. v 64 m/s c. v 200 m/s 2) Once the bar is pushed outside of the magnetic field area, the light bulb will continue to produce light. a. True b. False

Explanation / Answer

(1) The expression for the induced emf of a bar travelling in a magnetic field of strength B is given by -

E = B*L*v

where, B = strength of the magnetic field = 4 T

L = length of the conductor = 0.45 m

v = speed of the conductor

E = 115 V (given)

put these values -

115 = 4 * 0.45 * v

=> v = 115 / (4 * 0.45) = 63.9 = 64 m/s

Hence option (b) is the correct answer.

(2) (b) False, is the correct answer.

Explanation -

Once, the bar is outside the magnetic field then we have -

B = 0

So, value of induced emf = 0

So, the bulb will not glow.

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