Quick questions on Ohm's law explained: O-Level Electronics Basic Circuit Concepts

Ohm's law states that, for a metallic conductor at constant temperature, the current through it is directly proportional to the potential difference across it. Written as an equation:

Resistance is defined as the ratio of the potential difference across a component to the current through it, $R = V/I$. For an ohmic component this ratio is constant, so doubling the voltage doubles the current and the resistance stays the same. The unit is the ohm ($\Omega$): one ohm is one volt per ampere.

An ohmic component obeys Ohm's law: its $V$-$I$ graph is a straight line through the origin, and its gradient ($\Delta I / \Delta V$) is constant. A fixed resistor or a length of metal wire at constant temperature behaves this way. The constant resistance is the reason fixed resistors are so useful for setting currents and voltages.

A non-ohmic component does not give a straight-line $V$-$I$ graph. Two common examples in the syllabus:

On a $V$-$I$ graph with voltage on the horizontal axis, the resistance at any point is voltage divided by current, which is $1$ divided by the gradient. A steep line means a small resistance; a shallow line means a large resistance. A straight line through the origin is the signature of an ohmic component.