---
title: "Resistivity and What Sets a Wire's Resistance"
description: "A wire's resistance grows with its length and falls with its cross-section, and dividing out both leaves a number belonging to the material alone. That number is why copper is used for cables and why "
canonical: https://lightmysky.com/learn/science/resistivity-and-what-sets-a-wires-resistance-mt_A4f8THP-xm
source: https://lightmysky.com/learn/science/resistivity-and-what-sets-a-wires-resistance-mt_A4f8THP-xm.md
retrieved: 2026-09-12
---

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# Resistivity and What Sets a Wire's Resistance

A wire's resistance grows with its length and falls with its cross-section, and dividing out both leaves a number belonging to the material alone. That number is why copper is used for cables and why a strain gauge works at all.

Subject: Science · Area: Energy · Ages 16 to 17
Page: https://lightmysky.com/learn/science/resistivity-and-what-sets-a-wires-resistance-mt_A4f8THP-xm

## Ready when they can

- Predicts how resistance changes when a wire is stretched, doubled in length or swapped for a thicker one
- Calculates a resistivity from measurements of resistance, length and diameter, with the diameter squared handled correctly
- Compares two materials by resistivity and says which suits a heating element and which a connecting lead

## Lesson: Why wires fight current

One wire owns its drag; its stuff owns a deeper mark. The tie is R as rho L by A. Twice the run means twice the drag; twice the cut means half.

**Example.** Stretch a wire to twice its run at same cut and drag doubles. Fold it back to half run with twin cut and drag drops to a fourth. Small shifts in width swing wide since cut grows with width squared.

Heat turns use high-mark blends such as nichrome, which turn flow into glow. Links use low-mark copper that wastes scant power. Big rho fits fires; small rho fits links.

**Tip.** To weigh stuff, log drag, run, and width, then work rho as R A by L. Gauge width with fine jaws and square with care. Long, thin, hot, or high-mark lines all lift drag.

**Recap.** Long, thin, hot, or high-mark lines all lift drag, and rho alone tells which stuff fits which post.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Resistance in Series and Parallel](https://lightmysky.com/learn/science/resistance-in-series-and-parallel-mt_B29m3h5B75)
- [Current-Voltage Graphs for Components](https://lightmysky.com/learn/science/current-voltage-graphs-for-components-mt_n_CiOa-05J)

## Opens up

- [Electromotive Force, Internal Resistance and the Potential Divider](https://lightmysky.com/learn/science/electromotive-force-internal-resistance-and-the-potential-divider-mt_g3zYJ9qC53)