Q.Sketch and describe the general shape of the $V$-$I$ characteristic for

For an ohmic conductor (e.g. a metallic resistor at constant temperature), $V=IR$ with $R$ constant, so plotting $V$ against $I$ gives a straight line through the origin; the SLOPE of this line is exactly the resistance $R$, and the line is identical in both the first quadrant (forward current) and third quadrant (reverse current), since $R$ does not depend on the direction of current flow.

For a p-n junction diode, the graph is very different: in forward bias, almost no current flows until the applied voltage exceeds a small threshold, after which current rises sharply and non-linearly with voltage; in reverse bias, only a tiny, almost constant leakage current flows over a wide range of applied voltage. The graph is therefore neither straight nor symmetric about the origin, so a single constant resistance cannot describe …

Compare the defining feature of an ohmic device (straight line, constant slope) against the diode's …

• Assuming every electrical device obeys Ohm's law just because $V=IR$ can always be written down at one particular operating point. …