question_answer 18)

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

Answer:

(a) Tungsten has a high melting point (3380°C) and becomes incandescent (i.e., emits light at a high temperature) at       2400 K. (b) The resistivity of an alloy is generally higher than that of pure metals of which it is made of. (c) In series arrangement, if any one of the appliances fails or is switched off, all the other appliances stop working because the same current is passing through all the appliances. (d) The resistance of a wire (R) varies inversely as its cross-sectional area (A) as R  1/A. (e) Copper and Aluminium wires possess low resistivity and as such are generally used for electricity transmission.