B.5.4—Ohmic behaviour

Ohm’s law

At constant temperature, an ohmic conductor has $V\propto I$ , so $R=V/I$ is constant. Its I–V graph is a straight line through the origin when plotted with V and I consistently.

Non-ohmic behaviour

A non-ohmic component has a changing resistance, so current is not directly proportional to potential difference. Filament lamps, diodes and thermistors can be non-ohmic.

Why temperature matters

Heating can change a conductor’s resistance. Apply Ohm’s law only under the stated constant-temperature condition; otherwise the slope or ratio changes as the component operates.

Common trap

A curved I–V graph is not automatically wrong. It is evidence that the component is non-ohmic under those operating conditions.

How it is assessed

The evidence asks learners to explain why a component is non-ohmic, so the answer must connect the graph or data to non-constant resistance or failure of direct proportionality.

Command terms

Outline / Explain

What earns marks

For an ohmic device at constant temperature, state that V is directly proportional to I and resistance is constant. For a non-ohmic graph, point to the changing gradient or changing V/I ratio rather than merely saying the graph is curved.

Watch for

Calling a component non-ohmic only because its graph is curved, without explaining that V/I or resistance changes.

Representative question

[Maximum number: 1]

Outline why component X is considered non-ohmic.

Source and transfer

Cells provide emf $arepsilon$ , the energy transferred per unit charge by the source. Electrical energy transferred in a circuit is $E=VIt$ , and power is $P=VI=I^2R=V^2/R$ . Keep emf, terminal potential difference, energy and power distinct.

Current and circuit laws

Conventional current is the direction positive charge would move, with $I=\Delta q/\Delta t$ . In DC, the direction is constant; in AC, it reverses periodically. Apply Kirchhoff’s junction rule to charge conservation and the loop rule to energy conservation.

Resistance model

Use $R=V/I$ for a component, $R= ho L/A$ for a uniform conductor, and the correct series or parallel combination rule. Ohmic behaviour means constant resistance at constant physical conditions; non-ohmic behaviour requires reading the gradient or ratio from the graph at the stated point.

Real and variable components

For a real cell, $arepsilon=I(R+r)$ and $V= arepsilon-Ir$ . A variable resistor changes circuit resistance; LDRs and thermistors use a stimulus-dependent resistance. Before calculating, draw or inspect the circuit, identify the fixed quantity, and state the relevant assumption.

B.5.4—Ohmic behaviour

Distinguish Ohmic and Non-Ohmic Behaviour

B.5.4 Exam Analysis

Assessment in practice

Retrieve the B.5 Current and Circuits Model

Question 1