Explain the following :
(a) Why is tungsten used almost exclusively 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 aluminium wires usually employed for electricity transmission.

Explanation

(a) The melting point and of Tungsten is an alloy which has very high melting point and very high resistivity so does not burn easily at a high temperature.
(b) The conductors of electric heating divices such as bread toasters and electric irons are mode of alloy because resistivity of an alloy is more than that the metals which produces large amount of heat.
( c) In series circuits voltage is divided. Each component of a series receives a small voltage so the amount of currnet decreses and the device becomes hot and does not work properly. Hence, series arrangement is not used in domestic circuits.
(d) Resistance (R) of a wire is inversely proportional to its area of cross-section (A), i.e. when area of cross section increases the resistance decreases or vice versa.
(e) Copper and aluminium are good conductors of electricity also they have low resistivity. So they are usually used for electricity transmission.