(a) Define: (i) reactance; (ii) impedance in an a.c. The diagram here illustrates an a.c. generator. When the coil is rotated, an e.m.f is induced in the coil.
(i) Explain why an e.m.f. is induced. (ii) State the purpose of the slip-rings. (iii) Name and state the law used to determine the direction of the induced current. (iv) State two ways to increase the induced e.m.f.
(c) A lamp is rated 12 V. 6 W. Calculate the amount of energy transformed by the lamp in 5 minutes.
Explanation
(a)(i) Reactance is the opposition to the flow of a.c offered by a capacitor or an inductor or both. (ii) Impedance is the overall opposition of a mixed circuit containing a resistor, an inductor and/or a capacitor.
(b) (i) Emf is induced because the coil rotates at a steady speed about a fixed axis in the magnetic field of the elctromagnet, it cuts through the magnetic lines of force. (ii) The direction of current is reversed in each half of the rotation of the coil. (iii) The law used to determine the direction of the induced current is Lenz's law. It states that the direction of the induced current is such as to oppose the change producing it. (iv) The induced e.m.f can be increased by any two of the following: increased speed of rotation, efficient core design, increased number of turns in the coil and increased area of coil.
