Explain the following :
(a) Why is tungsten used almost axclusively for filament of incandescent lamp ?
(b) Why are the conductors of electric heating devices, such as 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 cross-sectional area ?
(e ) Why are copper and aluminimum wires usually employed for electricity transmission.

Explanation

(a) Tungsten is used almost exclusively for making the filaments of electric lamps (or electric bulbs) because it has very high melting point (of `3380^(@)C)` due to which the tungsten filament can be kept white-hot without melting away. Moreover, tungsten has high flexibility and low rate of evaporation at high temperature.
(c) The series arrangement is not used for domestic circuits because of the following disadvantages:
(i) In series arrangement, if one electrical appliance stops working due to some defect, then all other appliances also stop working (because the whole circuit is broken).
(ii) In series arrangement, all the electrical appliances can have only one switch due to which they cannot be turned 'on' or 'off' independently.
(iii) In series arrangement, all the appliances do not get the same voltage (220V) as that of the power supply line (because the line voltage is shared by the appliances). Due to this, the appliances do not work properly.
(iv) In series arrangement of electrical appliances, the overall resistance of the circuit increases too much due to which the current from power supply is low. Because of this, all the appliances of different power ratings cannot draw sufficient current for their proper workings.
(d) The resistance of a wire is ineversely proportional to its area of cross-section. That is: `R prop (1)/(A)`. So, when the area of cross-section of wire increases (or thickness of wire increases), then its resistance decreases. And when the area of cross-section of wire decreases (or thickness of wire decreases), then its resistance increases.
(e) Copper and aluminium wires are usually employed for transmission of electricity because copper and aluminium have low electrical resistivity (due to which they are very good conductors of electricity).