(a) State the laws of electromagnetic induction
(b) Explain how one of the laws illustrates the principle of conservation of energy
(c)(i) Draw a labelled diagram of a simple d.c. electric motor and explain how it works.
(ii) State two reasons why the efficiency of an electric motor is less than 100%.
Explanation
(a) Whenever there is a change in the magnetic flux linked with a circuit an electromotive force is induced, the strength of which is proportional to the rate of change of the flux linked with the circuit. Lenz’s law states that the direction of the induced current is always such as to oppose the charge producing it.
(b) Lenz’s law illustrates the law of conservation of energy. In this case mechanical energy i.e kinetic energy, due to the motion of the bar magnet is changed to electrical energy in the form of induced current.
(c)
A simple d.c. electric motor can supply direct current (d.c.) by using a split – ring commutator.The brush (-) touches one half of the split ring X while the other brush (+) touches the other half Y. As the coil rotates it cuts through the magnetic flux linking the magnets and current is induced which flows through the coil as shown in the diagram. As the coil rotates past the vertical Y moves round to touch (-) while X moves round to touch (+). This action reverses the connections to the external resistance connected to (-) and (+). The reverse happens when the current is just zero (coil vertical) so instead of the current reversing in the external resistance, it continues to flow in the same direction.
c(ii) (1) Part of the current is wasted in form of heat due to the resistance of the coil and this reduces efficiency;
(2) Friction between the moveable parts of the motor makes the efficiency to reduce.