Resistors: Students should be able to explain that, for some resistors, the value of R remains constant but that in others it can change as the current changes.

The idea

Resistance is defined by R = V ÷ I. For a fixed ohmic resistor at constant temperature, R stays constant as current changes. For others, like a filament lamp or diode, R changes as current changes because physical conditions alter. In a filament lamp, increasing current raises the filament temperature. This increases lattice ion vibration, making it harder for electrons to flow, so resistance increases. In a diode, resistance is very high in reverse bias and low in forward bias above the threshold voltage. To explain these effects, describe the component, state whether R is constant or changing, and link the change to a physical cause like temperature or current direction.

What earns the marks

• Resistance is calculated using R = V ÷ I, so a constant R means the ratio V ÷ I does not change as current changes.
• An ohmic conductor at constant temperature has constant resistance; its current–potential difference graph is a straight line through the origin.
• A filament lamp is non-ohmic because increasing current increases the filament temperature, which increases resistance.
• A diode has a very high resistance in reverse bias and a much lower resistance in forward bias once the threshold potential difference is exceeded.
• Explanations must link the observed change in resistance to a physical mechanism, such as increased lattice ion vibration at higher temperature.
• A component's resistance must be found from V ÷ I at a chosen point on its characteristic graph, not from the gradient.

Common mistakes and how to put them right

• Saying that all resistors have constant resistance.

  Put it right: Only ohmic resistors at constant temperature have constant resistance; filament lamps and diodes do not.

• Stating that resistance increases because current increases, without giving a physical reason.

  Put it right: Link the change to increased temperature in a filament lamp or to the direction of current in a diode.

• Calculating resistance from the gradient of an I–V graph.

  Put it right: Resistance is always V ÷ I at a specific point; the gradient is only equal to 1/R for ohmic conductors, not for non-ohmic components.

• Thinking that a diode has low resistance in both directions.

  Put it right: A diode has very high resistance in reverse bias and low resistance in forward bias above the threshold.

Exam tips

• When asked to explain, name the component first, then state whether R is constant or changing, then give the physical reason.
• Use the phrase 'at constant temperature' when describing an ohmic conductor, because temperature affects resistance.
• For calculations, show the equation R = V ÷ I, substitute values with units, and give the unit of resistance as ohms (Ω).