PHYSICS · Multi-part · CBSE · Physics · Magnetic Effects of Electric Current · Magnetic Effects of Electric Current
Problem
In a simple d.c. motor, the rectangular armature coil has its plane initially perpendicular to a uniform magnetic field. The current in the two long sides of the coil at this instant is from left to right on the front side and from right to left on the back side.
(i) Using a labelled sketch, indicate the directions of the magnetic forces acting on each of the two long sides.
(ii) Explain why these two forces do not produce any net translational motion of the coil but instead constitute a couple.
(iii) Describe the sequence of events that ensures continuous unidirectional rotation of the coil as it passes through the vertical position, making explicit reference to the role of the split-ring commutator.
(iv) Discuss two design factors (apart from increasing the current) that can be altered to obtain a larger turning moment on the coil and justify your answer on the basis of the force mechanism involved.