Edexcel IAL Physics · Unit — Electricity
Resistance, Resistivity & Potential Dividers
Electrons moving through a material keep bumping into stuff — how much they bump into, and how the wire's shape and material affect that, determines resistance. Wire that up in a smart way (a potential divider) and you can split a voltage into exactly the piece you want, or build a sensor that reacts to light or heat.
Quick Summary
- Resistance is caused by free electrons colliding with fixed ions as they drift through a conductor, transferring kinetic energy as heat.
- Resistivity (ρ) is a property of the material itself (not its shape) — it tells you how strongly a material opposes current flow. R = ρL/A links resistance to length, area, and resistivity.
- Core Practical 7 finds resistivity by measuring resistance at different wire lengths, plotting R vs L, and using the gradient (= ρ/A).
- Drift velocity is the average speed of charge carriers, linked to current by I = nqvA.
- Potential difference is energy transferred (or work done) per unit charge, measured with a voltmeter connected in parallel.
- Potential dividers split a supply voltage between two series resistors in the ratio of their resistances: Vout = [R₂ / (R₁+R₂)] × Vin.
- Potentiometers, LDRs and thermistors turn potential dividers into variable-voltage or sensor circuits (light and temperature sensing).
1. Electrical Resistivity
Why does resistance exist at all?
Picture a wire as a crowded corridor full of people standing still (these are the fixed metal ions) while a stream of people (the free electrons) try to rush through to the other end. They can't move in a straight line — they keep bumping into the standing people, bouncing off in random directions, and losing a bit of energy (as heat) every time they collide. That's what's happening inside a metal wire carrying current.
Since current is just the flow of charge, anything that gets in the way of that flow — i.e. the ions resisting the electrons — is what we call resistance. The more collisions there are, the more resistance there is, and the more electrical energy gets converted into heat (this is why wires and resistors get warm).
➜ ELECTRONS ➜
− (•)(•)(•)(•)(•)(•) +
− (•)(•)(•)(•)(•)(•) + (•) = metal ion (fixed, in the way)
− (•)(•)(•)(•)(•)(•) + • = electron (small, darting between them)
Electrons zig-zag past ions, losing energy on every collision → this IS resistance.
What resistance actually depends on
Resistance isn't just about the material — it's also about the wire's shape. Three things matter:
- Length (L): a longer wire means more ions to crash into on the way through → more resistance.
- Cross-sectional area (A): a thicker wire gives electrons more "lanes" to travel down at once → less resistance.
- Resistivity (ρ): a property of the material itself, independent of its shape, that says how strongly it opposes current.
Think of it like a motorway
length
cross-sectional area
Resistivity of common materials (room temperature)
| Category | Material | Resistivity ρ / Ω m |
|---|
| Metals | Copper | 1.7 × 10⁻⁸ |
| Gold | 2.4 × 10⁻⁸ |
| Aluminium | 2.6 × 10⁻⁸ |
| Semiconductors |
| Silicon |