The figure below shows a basic kind of motor where the chemical energy of the ba

Question

The figure below shows a basic kind of motor where the chemical energy of the battery is converted to the mechanical movement of a rod through the magnetic field. The conducting rod is free to slide on horizontal conducting rails which are inside a uniform magnetic field (B= 0.5T) directed into the page as shown. A 6.0V battery connected across the ends of the rails drives a current around the loop made by the rails and the rod. Assume that the resistance of the rails and rod are zero. A resistor of R=1ohm is in series with the battery. The distance between the rails is L=25cm. (a) What direction does the rod travel in (left or right) ? (b) The magnitude of the magnetic force required to push the rod at a constant speed, v, is F= 0.25N. How big is the current in the loop made by the rails and rod ? (c) What is the voltage drop across the resistor, R ? (d) Therefore, what is the back e.m.f. generated by the moving rod ? (e) What is the speed, v, at which the rod is travelling ?

Explanation / Answer

a) right (according to right hand rule)

b) use, F = B*I*L

==> I = F/(B*L)

= 0.25/(0.5*0.25)

= 2 A

c) V = I*R

= 2*1

= 2 volts

d) 2 volts

e) induced emf = B*v*L

v = inuced emf/(B*L)

= 2/(0.5*0.25)

= 16 m/s

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