Explain the following:
(a) Why is the tungsten used almost exclusively for filament of electric lamps?
(b) Why are the conductors of electric heating devices such as bread toaster and electric irons, made of an alloy rather than a pure metal?
(c) Why is the series arrangement not used for domestic circuits?
(d) How does the resistance of a wire vary with its area of cross-section?
(e) Why are copper and aluminium wires usually employed for electricity transmission?
(a) Melting point of tungsten is very high. So it does not oxidise (burn) at high temperatures. Secondly, the resistivity of tungsten is high. So thin wire of tungsten which has high resistance which produces more heat enough to emit light.
(b) This is because of the high resistivity and high melting point of the alloy. Also, they do not oxidise when red hot.
(c) Domestic circuits are not connected in series because they do not work at the same voltage and also if there is a short- circuiting in one distribution circuit, its fuse will not blow and other circuits will also be affected.
(d) Larger the area of cross-section means a large number of free electrons can move in the conductor, thereby decreasing the resistance.
(e) Copper and aluminium wires are good conductors of electricity. So they have low resistivity. They offer low opposition to the flow of current from one point to the other, hence employed for electricity.