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-toasters 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 resistances of a wire vary with its area of cross sections?
(e ) Why are copper and aluminium wires usually employed for electricity transmission?

Explanation

(a) Tungsten us used for filament of electric lamps because its melting point is extremely high (`3380^@C)`
(b) Condutors of electric heating devices are made of an alloy rather than a pure metal due to the following reasons:
(i) Resistivity of an alloy is generally higher that of pure metals hence for a given resistance we need a conductor of less length.
(ii) At high temperature an alloy does not oxidise (burn) readily.Hence, heating element prepared from an alloy has longer life.
(c ) Series arrangement is not used for domestic circuits due to the following reasons:
(i) In series arrangement same current will flow through all the appliances, which is not required.
(ii) Total resistance of domestic circuit will be sum of the resistances of all appliances and hence current drawn by the circuit will be less.
(iii) We cannot use independent on/off switches with individual appliances.
(iv) All appliances are to be used simultaneously even if we do not need them.
(d) Resistance (R) of a wire is inversely to its cross section (A).
(e ) Copper and aluminium wires are usually employed for electricity transmission because these are extremely good conductors having a low value of resistivity. More over, these are ductile and can be drawn in the form of fines wires.