This means that the resistance remains constant as the current changes.

The idea

Direct proportionality has a consequence worth stating separately: if current rises in exact step with potential difference, then V ÷ I is the same at every point, so the resistance stays constant as the current changes. Check it with numbers from a straight line through the origin. At 2 V the current might be 0.4 A, giving 5 Ω; at 6 V it is 1.2 A, giving 5 Ω again. That is what makes a fixed resistor useful, since it behaves predictably whatever the circuit does to it. The reverse test is just as useful: work out V ÷ I at two points on an unfamiliar graph, and if the answers differ the component is not ohmic and something, usually temperature, is changing.

What earns the marks

• Calculating resistance as V ÷ I at one point on the graph.
• A second calculation at a different point on the same line.
• Comparing the two values and stating that the resistance is constant.
• Linking constant resistance to a straight line through the origin.

Common mistakes and how to put them right

• Assuming a constant resistance without checking a second point on the graph.

• Using the gradient directly as the resistance when current is plotted on the vertical axis.

• Calculating V ÷ I using values read from two different graph lines.

• Rounding readings so heavily that two genuinely equal resistances appear different.

Exam tips

• Take readings from points where the line crosses grid intersections, so the values are easy to read accurately.
• Show both calculations in full; the comparison mark depends on seeing two values.
• Remember that with current on the vertical axis the gradient is 1 ÷ R, not R.