(a) An electric motor runs on a d.c. source of emf e and internal resistance r. show that the power output of the source is maximum when the current drawns by the motor is `epsilon//2r`.
(b) Show that power output of electric motor is maximum when the back emf is one-half the source emf provided the resistance of the windings of the motor is negligible.
(c) Compare and contrast carefully the situation in this excercise with that in Example 43 above.