(a) List the factors on which the resistance of a conductor in the shape of the wire depend.
(b) Why are metals good conductors of electricity whereas glass is a bad conductor of electricity ? Give reason .
(c ) Why are alloys commonly used in eletrical heating devices ? Give reason.

Explanation

(a) Factors on which the resistance of a conductor in the shape of a wire depends.
(i) The resistance of a straight conductor is directly proportional to its length.
`R prop l`
(ii) The resistance of a straight conductor is inversely proportional to its area of cross-section .
`R prop l/A`
From (i) and (ii)
`R prop l/A`
`:. R = rho'l/A`, ...[`rho` = constant called resistivity]
(iii) the resistance of a conductor also depends upon the temperature of the conductor and nature of material used.
(b) Metals have free electrons which helps them in the conduction of electricity whereas glass has no free electrons therefore it is an insulator.
The resistivity of glass is very high `(=10^(10 ) "to" 10^(14) Omega m)` therefore it is a poor conductor of electricity wheares the resistivity of metals is lower, therefore metals are food conductors of electricity.
(c ) Alloys are commonly used in elctrical heating devices because -
the resistivities of alloys are much higher than those of the pure metals (from which they are made),
an alloy does not undergo oxidation (or burn) easily even at high temperature . Thus heating elements of electrical heating appliances such as electric iron, toaster, etc. are made of an alloy rather than a pure metal .