Temperature Scales & Thermal Expansion

Three scales are in common use. Celsius puts water's freezing point at 0 and its boiling point at 100. Fahrenheit puts the same two points at 32 and 212, so a Fahrenheit degree is only 5/9 as big and the two scales cross at −40. Kelvin uses the same size of degree as Celsius but starts at absolute zero, −273.15 °C, the point at which molecular motion stops. Kelvin is the scale that must be used in gas-law calculations, because only it has a true zero.

Heat also makes materials expand. A bar of original length L grows by ΔL = αLΔT, where α is the coefficient of linear expansion for the material. The numbers look tiny — steel is about 12 millionths per degree — but multiply by a long member and a large temperature swing and the movement becomes very real. Aluminium expands roughly twice as much as steel for the same rise, and titanium rather less than either.

This is why structures need expansion gaps, why pipelines have loops, and why joining two dissimilar metals matters: a bimetallic strip bends precisely because one side grows faster than the other, and that is exactly how a simple thermostat and many fire-warning switches work. An airframe that leaves a hot ramp at +45 °C and cruises at −55 °C swings through a hundred degrees, and every clearance and fastener has to tolerate it.

• Try this: Set 100 °C. What is that in Fahrenheit and in Kelvin, and which two numbers differ by a constant?
• Try this: Set −40 °C. Why do the Celsius and Fahrenheit readings agree there?
• Try this: Compare steel and aluminium on a 10 m bar over the same temperature rise. Which needs the bigger gap, and by how much?