Explain the following.
a] Why is 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 aluminum wires usually employed for electricity transmission?

Explanation

a] Tungsten has very high melting point `[3380^(@)C]`. So, it can.t melt up to a high temperature. This is the reason why tungsten is used almost exclusively for filament of electric lamps.
b] Resistivity of an alloy is generally higher than that of its constituent metals.
c] Tn series arrangement, the potential drop is divided into different appliances. So, some appliance can not get sufficient potential drop for its proper working. This is possible only in parallel connection. Hence, series arrangement is not used for domestic circuits.
d] The resistance of a wire is inversely proportional to its area of cross-section,
`RpropI/A`
e] Copper and aluminum have low resistivity. Therefore, there will be less energy loss. Hence, they are usually employed for electricity transmission.