Resistors: Students should be able to use graphs to explore whether circuit elements are linear or non-linear and relate the curves produced to their function and properties.

The idea

Plot current on the y-axis against potential difference on the x-axis for each component. A straight line through the origin shows a linear element obeying V = IR with constant resistance; its gradient equals 1/R. A fixed resistor at constant temperature gives this straight line. A filament lamp curves because increasing current heats the filament, raising resistance, so the gradient falls. A diode conducts only above a forward threshold, giving near-zero current in reverse and a steep rise forwards, so it is non-linear. Compare gradients and shapes to link each curve to the element's function and properties, such as constant resistance, heating, or one-way conduction.

What earns the marks

• Plot I–V data with current on the y-axis and potential difference on the x-axis, then judge linearity by whether the points form a straight line through the origin.
• For a linear element, state that resistance is constant and calculate it from the gradient using R = 1 ÷ gradient, or from V ÷ I at a point.
• For a filament lamp, explain that increasing current transfers more energy, heating the filament, so resistance increases and the curve flattens.
• For a diode, describe the forward threshold above which current rises steeply and the very small reverse current below it, showing one-way conduction.
• Relate each curve shape to the component's function: fixed resistor for steady resistance, lamp for lighting where heating changes resistance, diode for rectification or one-way flow.

Common mistakes and how to put them right

• Plotting potential difference on the y-axis instead of current.

  Put it right: Keep current on the y-axis so the gradient gives 1/R and the curve shapes match the specification.

• Describing a filament lamp as ohmic because it is a resistor.

  Put it right: Its resistance changes with temperature, so it is non-linear.

• Treating a diode as a normal resistor in reverse.

  Put it right: In reverse bias the current is extremely small until breakdown, so the diode blocks current.

Exam tips

• Always label axes with quantity and unit, and draw a smooth curve or best-fit line rather than joining points dot-to-dot.
• When asked to compare, quote the gradient or a resistance value for each element and state whether it stays constant.
• Use the phrase 'resistance increases as temperature increases' for the filament lamp, and 'threshold potential difference' for the diode.