Potential difference, current and resistance

What you will learn

V = IR; ohmic conditions, resistance calculations and graph interpretation.

Specification mapping: C2.1.2

Prerequisites: C1.4

Physical principles

V = IR relates the potential difference across a component to its current and resistance at a specified operating point. Rearrange to R = V/I or I = V/R. Ohm’s law is the additional statement that current is proportional to voltage when physical conditions such as temperature are constant for an ohmic conductor.

For a fixed resistor at constant temperature, an I–V graph is a straight line through the origin. Its gradient is I/V = 1/R. On a V-against-I graph the gradient is R. Axis choice reverses the interpretation. For a nonlinear component, V/I gives its static resistance at the chosen point; a local tangent slope describes a different incremental relationship.

Convert kΩ to Ω and mA to A before substituting. An increase in resistance reduces current if voltage is fixed; increasing voltage raises current if resistance is fixed. Do not assume both stay fixed while discussing a change. Heating at high current can alter resistance and invalidate a nominal constant-resistance model.

Scientific diagram and interpretation

Equations and relationships

V = IR

Worked examples and applications

Common misconceptions

Ohm’s law is not the claim that every component has constant resistance at every temperature.

Examination technique

Name the graph axes before using its slope to find resistance.

For an explanation, connect the physical cause to its effect. For a calculation, state the model, rearrange before substitution, retain intermediate digits, and finish with units. For a data question, refer to the actual readings and distinguish a trend from proof of a cause.

Check your understanding

Five questions. Choose an option, then Check Answer. Read the feedback and use Try Again to reset. Use Tab between controls, arrow keys within radio groups, and Enter or Space on buttons. These local practice answers are not saved.