A student carries out an experiment and plots the V-I graph of three samples of nichrome wire with resistances $R1, R2, R3$ respectively. Which of the following is true?

 

$I-V$ characteristic of a copper wire of length $L$ and area of cross-section $A$ is shown in figure. The slope of the curve becomes

The following graph shows the $I\mathit{-}V$ characteristics of

Is Ohm’s law universally applicable for all conducting elements? If not, give examples of elements which do not obey Ohm’s law.