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 resistance of a wire vary with its area of cross section?

(e) Why are copper and aluium wires usually employed for electricity transmission?

(a) 

Electric lamps heat up with time as they are used. Since, tungsten has very high melting point, it does not melt even at very high temperatures. Hence, the lamp in not destroyed.

(b) 

Electrical heating devices are required to produce a lot of heat and maintain high temperatures. Since, heat generated is proportional to resistivity of device, materials of higher resistivities are preferred in the construction of heating devices. Alloys have more resistance than metals and hence are preferred for the same.

(c) 

When resistors are connected in series, potential difference across each resistor is smaller than the applied voltage. Hence, the power delivered to each device is reduced and hence they don't work at full efficiency.

(d)

$R=\rho l/A$

Hence, resistance is inversely proportional to the cross section of the resistance of the wire.

(e) 

Cables used for electrical transmission should use up as small power as possible so that most of the power is delivered to the electrical equipment. Power consumed in the wire is proportional to resistivity. Copper and aluium have very low resistivity and hence well suited for the purpose of being used as wires.