(a) What is meant by saying that the potential difference two points is `1` volt ? Name a device that helps to measure the potential difference across a conductor.
(b) Why does the connecting cord of an electric heater not glow hot while the heating element does ?
( c) Electrical resistivities of some substances at `20^@ C` are given below :
Silver `1.60 xx 10^-8 Omega m`
Copper `1.62 xx 10^-8 Omega m`
Tungsten `5.20 xx 10^-8 Omega m`
Iron `10.0 xx 10^-8 Omega m`
Mercury `94.0 xx 10^-8 Omega m`
Nichrome `100 xx 10^-6 Omega m`.
Answer the following question in relation to them :
(i) Among silver and copper, which one is a better conductor ? Why ?
(ii) Which material would you advise to be used in electrical heating devices ? Why ?

Explanation

(a) One volt is the potential difference between two points in a current-carrying conductor when 1 Joule of work is done to move a charge of 1 coulomb from one point to the other m i.e.,1 volt `= (1 "Joule")/(1 "coulomb")`
The potential difference is measured by the device called voltmeter.
(b) The connecting cord of an electric heater is usually made of copper which has a low resistivity and does not produce much heat as compared to the heating element. The resistivity of the heating element, on the other hand, is high and it produces large amount of heat which is retained by the element which thus gets very hot and emits light.
( c) (i) Silver is a better conductor than copper. This is due to the reason that the resistivity of silver is less than that of copper.
(ii) Nichrome should be used for making electrical heating devices because (a) it has very high resistivity which produces high temperature when current flows through it and (b) it does not react with air when it is red hot `(800^@ C)`.