Constant and changing resistance

Resistance tells you how strongly a component opposes current. It is measured in ohms, Ω. From earlier electricity work, a larger resistance gives a smaller current for the same potential difference across the component.

In this spec point, the important question is not just "What is the resistance?" but "Does the resistance stay the same as the current changes?"

For some resistors, the value of resistance remains constant as current changes. For other components, the resistance changes when the current changes. That can happen because the component warms up, or because the component behaves differently in opposite directions.

The word component matters. A fixed resistor, a filament lamp and a diode can all be connected in circuits, but they do not respond to changing potential difference in the same way.

Ohmic conductors

An ohmic conductor is the simplest behaviour in this lesson. If the temperature is kept constant, the current through an ohmic conductor is directly proportional to the potential difference across it.

Directly proportional means that the ratio between potential difference and current stays the same. If the potential difference doubles, the current doubles. If the potential difference is halved, the current is halved.

That is why the resistance remains constant as the current changes. The condition "at constant temperature" is part of the idea. If a conductor heats up enough, its resistance may no longer stay constant.

For example, suppose a resistor at constant temperature has a current of 0.20 A when the potential difference is 3.0 V. If the potential difference is increased to 6.0 V, the current becomes 0.40 A because the potential difference has doubled.

Filament lamps

A filament lamp does not keep a constant resistance. The filament is a thin metal wire that becomes hot when current flows through it. As the current increases, the filament temperature increases.

In a metal, electrons move through a lattice of positive ions. When the filament gets hotter, the ions vibrate more. Moving electrons then collide with the vibrating ions more often, so the electrons lose energy more often and the opposition to current increases.

That is why the resistance of a filament lamp increases as the temperature of the filament increases.

Do not explain this by saying only "current increases, so resistance increases." The GCSE physics link is the temperature change in the filament. More current makes the filament hotter; the hotter filament has a higher resistance.

Diodes

A diode is different because direction matters. Current through a diode flows in one direction only. In the reverse direction, the diode has a very high resistance, so only a tiny current flows, or the current is treated as zero in ordinary GCSE descriptions.

The direction that allows current is called the forward direction. When a diode is connected the other way around, it is in the reverse direction. Reversing a diode is therefore not like reversing a fixed resistor; a fixed resistor behaves the same in either direction, but a diode does not.

This makes a diode useful when a circuit needs current to pass one way but be blocked the other way. For this lesson, keep the explanation qualitative: one direction has low enough resistance for current to flow, and the reverse direction has very high resistance.

Comparing the components

These three components are often confused because all can be placed in a circuit and all can have a resistance value at a particular moment. The difference is whether that value stays constant, changes with temperature, or depends on direction.

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The common exam trap is treating every component as if resistance is fixed. That is only safe for an ohmic conductor at constant temperature. For a lamp, mention the temperature of the filament. For a diode, mention the direction.