Resistance and resistivity
Components have resistance, materials have resistivity
Resistance is a property of a particular component, like this resistor or this bulb. The symbol for resistance is R and it's measured in ohms (Ω).
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Resistivity is a property of a material, like copper or plastic. The symbol for resistivity is ρ (the Greek small letter rho - r for rho, r for resistivity). It's measured in 'ohms metre'. You can see why below.
When you talk about resistivity it doesn't matter how big or what shape the sample is. In this sense it's a bit like density because you can compare lead with wood without having to say which bit of lead or wood you're talking about.
When you say copper is a better conductor than plastic you're really saying that lead has a lower resistivity than plastic. In other words you're making a general statement about lead and plastic, rather than particular components.
Resistance, resistivity and size
The resistance of piece of wire is big if it's long and thin. In other words the resistance of a particular piece of wire depends on
- what metal it's made from (its resistivity, ρ)
- how long it is (its length, l)
- its cross-sectional area (its area, A, which decreases resistance the bigger it is)
We can write R = ρl/A. If we rearrange this we get ρ = RA/l. This has units of Ωm2/m, which is just Ωm (said 'ohms metre').
Changes of resistance and resistivity
Resistivity is normally taught in more advanced courses, which is why we tend to talk only about resistance. This becomes slightly clumsy because when you want to say 'the resistivity of a metal increases with temperature', you have to be more specific and say 'the resistance of a piece of metal increases with temperature'.