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What is resistance?

Resistance measures how strongly a component opposes the flow of electric current, and is defined as potential difference divided by current.

Resistance explained

Resistance depends on material, length and cross-sectional area: longer wires resist more, thicker wires resist less. Resistivity captures the material property itself.
Resistors in series add directly; in parallel the reciprocals add, so the total is always smaller than the smallest branch.

Key formula

R=VI,R=ρLAR = \dfrac{V}{I}, \qquad R = \dfrac{\rho L}{A}

Worked example

Two 6 Ω6\,\Omega resistors in parallel give 3 Ω3\,\Omega; in series they give 12 Ω12\,\Omega.

Examiner tip

For parallel combinations, remember to invert at the end. Stopping at 1/R is the most common single-mark loss in circuit questions.

Resistance: common questions

Why does resistance increase with temperature in metals?
Hotter ions vibrate with larger amplitude, so electrons collide with them more often and lose energy more readily.
What is a thermistor?
A component whose resistance falls sharply as temperature rises, widely used in temperature-sensing circuits.

Related terms

Ohm's law

Ohm's law states that the current through a metallic conductor is directly proportional to the potential difference across it, provided temperature stays constant.

Electric current

Electric current is the rate of flow of electric charge, measured in amperes — one ampere is one coulomb per second.

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