Electricity: Question 2

Worked solution

Step 1: Read the resistance behaviour from the shape of the graph

At any point on an I–V graph, the resistance is: $R = \frac{V}{I}$

The description states that for equal increases in $V$, the increases in $I$ get smaller and smaller. This means the ratio $V/I$ is increasing as $V$ (and $I$) increase, so the component’s resistance is increasing, not constant. A graph with constant, increasing resistance cannot be a straight line through the origin, so the component is not behaving like an ohmic conductor.

Step 2: Why option A fits

In a filament lamp, the current $I$ dissipates power $P = I^2R$ as heat in the filament, so the filament’s temperature rises as $I$ increases. For a metal, resistivity $\rho$ (and hence resistance $R = \rho L/A$, with $L$ and $A$ fixed) increases with temperature, because the increased thermal vibration of the metal ions scatters the free electrons more often. This growing resistance is exactly why, at higher $V$, each additional volt produces a smaller additional current, the curve bends over towards the $V$-axis, matching the description given.

Step 3: Why the other options are wrong

• B (reverse-biased diode): a reverse-biased diode carries a current that is negligibly small (essentially zero) at almost all voltages, but the description here shows a current that keeps growing substantially, just more slowly, which does not match “almost no current flows”.
• C (thermistor at constant temperature): if the temperature (and hence resistance) really were constant, $R = V/I$ would be the same at every point, giving a straight line through the origin, but the question explicitly describes a changing gradient, so the resistance is not constant here.
• D (ohmic conductor): an ohmic conductor’s defining feature is a straight-line I–V graph (constant resistance). The curve described bends, so it cannot be this component.

Final answer

• The component is a filament lamp, option A.