Explain how current varies with potential difference for the following…

The idea

This statement asks you to link the shape of a current–potential difference graph to resistance for three devices. For a fixed resistor at constant temperature, current is directly proportional to potential difference, so the graph is a straight line through the origin and resistance stays constant. For a filament lamp, increasing potential difference raises the current, which heats the filament; the resistance increases, so the graph curves and flattens at higher values. For a diode, current flows easily in one direction once a small threshold p.d. is exceeded, but almost no current flows in reverse, giving a very high resistance in the reverse direction. In each case resistance follows from R = V ÷ I, so a steeper graph means lower resistance. Multiple-choice questions test whether you can match a device to its graph and explain the resistance change.

What earns the marks

• Identifies that a fixed resistor gives a straight-line current–p.d. graph through the origin because resistance is constant.
• Explains that a filament lamp's graph curves because heating increases the filament's resistance as current rises.
• States that a diode conducts in one direction only, giving very high resistance in the reverse direction.
• Uses R = V ÷ I to relate the gradient of a current–p.d. graph to resistance, noting a steeper line means lower resistance.
• Distinguishes the three devices by matching each to the correct shape of current–potential difference graph.

Common mistakes and how to put them right

• Claims a filament lamp obeys Ohm's law at all currents.

  Put it right: Explain that its resistance changes as the filament heats.

• Treats a diode as conducting equally in both directions.

  Put it right: State that it conducts in one direction only.

• Reads a steeper current–p.d. line as higher resistance.

  Put it right: Apply R = V ÷ I so a steeper line means lower resistance.

Exam tips

• Read the graph axes carefully before choosing an option, since current–p.d. and p.d.–current graphs look different.
• Eliminate options that contradict the constant-resistance behaviour of a fixed resistor.
• Check whether the question asks about the shape of the graph or the change in resistance, as these are different points.