$(a)$ What is meant by saying that the potential difference between two points is $1$ volt?
$(b)$ Why does the connecting cord of an electrical heater not glow while the heating element does?
$(c)$ Electrical resistivities of some substances at $20^{\circ}C$ are given below:

Silver	$1.60 \times 10^{-8} \, \Omega m$
Copper	$1.62 \times 10^{-8} \, \Omega m$
Tungsten	$5.2 \times 10^{-8} \, \Omega m$
Iron	$10.0 \times 10^{-8} \, \Omega m$
Mercury	$94.0 \times 10^{-8} \, \Omega m$
Nichrome	$100 \times 10^{-6} \, \Omega m$

Answer the following questions in relation to them:
$(i)$ Among silver and copper, which one is a better conductor? Why?
$(ii)$ Which material would you advise to use in electrical heating devices? Why?

(N/A) The potential difference between two points is said to be $1$ volt if $1$ joule of work is done in moving a charge of $1$ coulomb from one point to another.
$(b)$ The heating element of an electrical heater is made of an alloy (like nichrome) which has high resistivity. Due to high resistivity, it produces a large amount of heat when current passes through it, causing it to glow. In contrast, the connecting cord is made of copper, which has very low resistivity, so it does not produce significant heat and does not glow.
$(c)$ $(i)$ Silver is a better conductor than copper because it has lower electrical resistivity ($1.60 \times 10^{-8} \, \Omega m$ for silver vs $1.62 \times 10^{-8} \, \Omega m$ for copper).
$(ii)$ Nichrome is advised for electrical heating devices because it has a very high resistivity compared to other materials listed, which allows it to generate more heat according to Joule's law of heating $(H = I^2Rt)$.