Why do the two metals in a bimetallic strip bend when temperature changes?

A bimetallic strip bends when the temperature changes because it is composed of two different metals joined together, each possessing a unique thermal expansion coefficient.1

Because these two materials expand or contract at different rates in response to temperature fluctuations, the strip is forced to flex or curve to accommodate the unequal changes in length. This mechanical movement is proportional to the temperature change, which allows the strip to function as a sensor in feedback control systems, such as those found in a thermostat. In such applications, the bending action can physically trigger a switch to regulate heating or cooling processes.12