State the equation that defines resistivity in terms of resistance, cross-sectional area and length.

ρ = RA/L, where ρ is resistivity, R is resistance, A is cross-sectional area, and L is length.

In the resistivity equation ρ = RA/L, what does each symbol represent?

ρ is resistivity, R is resistance, A is cross-sectional area, and L is length.

When using ρ = RA/L to determine resistivity, which quantities must be measured or known?

Resistance (R), cross-sectional area (A), and length (L) of the conductor.

Calculate the resistivity when resistance = 19.5 Ω, cross-sectional area = 2.5 × 10⁻⁷ m², length = 0.8 m.

Resistivity = 19.5 × 2.5 × 10⁻⁷ ÷ 0.8 = 6.09 × 10⁻⁶ Ω m.

AQA A-Level Physics 7408

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Electricity

Resistivity

4 things to know
Papers 1 and 3

Overview

The resistivity equation, the effect of temperature on a metal, and superconductivity.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

How is resistivity defined, what do its symbols mean, and what is its unit?

Process

Why does the resistance of a metal rise with temperature?

Key term

What are superconductors used for, and does critical temperature vary between materials?

Key term

Which instrument is used to measure the diameter of a wire when determining its resistivity?

Questions from this subtopic

Each one with the model answer underneath.

1
State the equation that defines resistivity in terms of resistance, cross-sectional area and length.
Model answer
ρ = RA/L, where ρ is resistivity, R is resistance, A is cross-sectional area, and L is length.
2
In the resistivity equation ρ = RA/L, what does each symbol represent?
Model answer
ρ is resistivity, R is resistance, A is cross-sectional area, and L is length.
3
What is the SI unit of resistivity?
Model answer
Ohm metre (Ω m).
4
When using ρ = RA/L to determine resistivity, which quantities must be measured or known?
Model answer
Resistance (R), cross-sectional area (A), and length (L) of the conductor.
5
Calculate the resistivity when resistance = 19.5 Ω, cross-sectional area = 2.5 × 10⁻⁷ m², length = 0.8 m.
Model answer
Resistivity = 19.5 × 2.5 × 10⁻⁷ ÷ 0.8 = 6.09 × 10⁻⁶ Ω m.
6
How does the resistance of a metal conductor change when its temperature increases?
Model answer
The resistance increases.

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