I–V Graphs

Ohm's Law states that the current through a conductor is directly proportional to the potential difference across it, provided physical conditions like temperature remain constant. This is expressed as $V = IR$.

Components that follow this law are called Ohmic conductors. Their I–V graphs are straight lines passing through the origin, meaning the resistance remains constant regardless of the voltage applied.

The gradient of an Ohmic I–V graph is equal to $\frac{1}{R}$. Therefore, a steeper line represents a lower resistance, while a shallower line represents a higher resistance.

A filament lamp is a non-Ohmic component because its resistance increases as the current increases. This happens because the flowing current heats the metal filament.

At higher temperatures, the metal ions in the filament's lattice vibrate more vigorously. these vibrations increase the frequency of collisions between the ions and the flowing electrons, which opposes the flow of charge.

Visually, the I–V graph for a filament lamp curves away from the current axis at high voltages, forming an 'S' shape. This indicates that as voltage increases, the current increases at a decreasing rate.

A diode is a semiconductor device that allows current to flow in only one direction. This behavior is highly non-linear and asymmetric.

In forward bias (the direction of the arrow in the circuit symbol), the diode has very low resistance once a specific threshold voltage is reached, resulting in a sharp increase in current.

In reverse bias, the diode has an extremely high resistance, effectively preventing any current from flowing. On an I–V graph, this is shown as a flat line along the x-axis in the negative voltage region.

Check the Axes: Always verify if the graph is $I$ vs $V$ or $V$ vs $I$. If $V$ is on the y-axis, the gradient represents the resistance ($R$); if $I$ is on the y-axis, the gradient is $\frac{1}{R}$.

Identify the Origin: A component only obeys Ohm's Law if the graph is a straight line and passes through $(0,0)$. If it doesn't pass through the origin, there may be a systematic error or the component is active.

Interpret Curvature: If an $I-V$ graph curves toward the voltage axis (becomes flatter), the resistance is increasing. If it curves toward the current axis (becomes steeper), the resistance is decreasing.

Sanity Check: For a filament lamp, remember that 'hotter means harder' for electrons to pass through, so the resistance must go up as the graph extends.