question_answer 31)

                An electron and a proton are moving under the influence of mutual forces. In calculating die change in the kinetic energy of die system during motion, one ignores the magnetic force of one oil another. This is because,                 (a) the two magnetic forces are equal and opposite, so that produce no net effect.                 (b) the magnetic forces do no work oil each particle.                 (c) the magnetic forces do equal and opposite (but non-zero) work on each particle.                 (d) the magnetic forces are necessarily negligible.                

Answer:

                (b) Change in K.E. = Work done by a force. Work done by magnetic force is zero.