EASA/GCAA Part-66 · Module 2 — Physics · Thermodynamics

Thermal Expansion — the Bimetallic Strip

Objective: materials expand when heated by ΔL = α·L₀·ΔT. Bond two metals with different expansion coefficients and the strip curls when heated — the basis of the thermostat and thermal circuit breaker. Heat it up and watch it trip.

Bimetallic strip & thermostat contact

Temperature 20 °C

Thermostat trip point 90 °C

Temperature

20°C

Tip deflection

0mm

Δα (per K)

0×10⁻⁶

Circuit

Closed

ΔL = α·L₀·ΔT · the strip curls toward the metal with the smaller α

Heat makes the atoms in a solid vibrate more and sit slightly further apart, so it expands — by an amount ΔL = α·L₀·ΔT, where α is the coefficient of linear expansion. Bond two metals with different α and heat them: the higher-α metal grows more, so the strip must curl, bending toward the lower-α side. That controllable, repeatable movement is used everywhere — a thermostat opens a heater contact when the room is warm enough, and a thermal circuit breaker trips when overcurrent heats the strip. Cool it and it straightens back.

Try this: Heat the strip and watch it curl and lift off the contact — the circuit opens and the lamp goes out.
Try this: Compare metal pairs — a bigger difference in α (Δα) curls the strip more for the same temperature.
Try this: Lower the trip point and the thermostat opens at a cooler temperature.
Try this: Cool it back down and the strip straightens and the contact re-closes.