Why does a bimetal strip bend as the temperature changes?

Why does a bimetal strip bend as the temperature changes?

• A. Bimetal is composed of two different metals bonded together; copper or brass expands faster than Invar, causing the shape change.
• B. Because two metals are bonded and heat has no effect.
• C. Because copper expands slower than Invar.
• D. Because magnets cause bending during heating.

Answer: (A)

A bimetal strip bends because two bonded metals have different rates of expansion with temperature. When heat is added, the metal with the larger coefficient of thermal expansion lengthens more than the other, and since they’re fixed together, the strip must curve to accommodate the mismatch. Invar barely expands with temperature, while copper and brass expand more. So, as the strip heats, the copper/brass side grows longer than the Invar side, causing the strip to bend toward the Invar side. This is exactly how thermostats and other temperature-sensing controls work. Magnets don’t influence this bending, and heat clearly does affect the metals, so those statements don’t fit.

A bimetal strip bends because two bonded metals have different rates of expansion with temperature. When heat is added, the metal with the larger coefficient of thermal expansion lengthens more than the other, and since they’re fixed together, the strip must curve to accommodate the mismatch. Invar barely expands with temperature, while copper and brass expand more. So, as the strip heats, the copper/brass side grows longer than the Invar side, causing the strip to bend toward the Invar side. This is exactly how thermostats and other temperature-sensing controls work. Magnets don’t influence this bending, and heat clearly does affect the metals, so those statements don’t fit.