Nichrome is used to make the element of an electric heater. Why?

What is meant by resistance of a conductor? Name and define its $SI$ unit. List the factors on which the resistance of a conductor depends. How is the resistance of a wire affected if
$(i)$ its length is doubled,
$(ii)$ its radius is doubled?

The $I-V$ graphs of two resistors,and their series combination,are shown below. Which one of these graphs represents the series combination of the other two? Give reasons for your answer.

Draw a schematic diagram of a circuit consisting of a cell of $1.5 \, V$, a $10 \, \Omega$ resistor, a $15 \, \Omega$ resistor, and a plug key, all connected in series.

$(a)$ Resistivity of iron is $10 \times 10^{-8} \; \Omega m$ and of mercury is $94 \times 10^{-8} \; \Omega m$. Which among iron and mercury is a better conductor? Give reason.
$(b)$ Resistivity of a material $A$ is in the range of $10^{10}-10^{14} \; \Omega m$ while that of $B$ is in the range of $10^{-6}-10^{-8} \; \Omega m$. Which of the two will behave as an insulator and why?

Three bulbs of $40\, W$,$60\, W$,and $100\, W$ are connected in series across the mains. Across which bulb is the potential difference highest? Across which is it lowest?