Physics · Thermal Properties Of Matter · NEET
The two metals are firmly joined and heated to the same temperature. But each metal has a different coefficient of linear expansion (alpha). The metal with larger alpha grows longer than the metal with smaller alpha for the same temperature rise. Since they cannot slide apart, the longer metal is forced to take the outer curve and the shorter metal the inner curve, so the whole strip bends into an arc.
It bends toward the metal with the SMALLER alpha (the one that expands less). That metal ends up on the inside of the curve, and the metal with larger alpha ends up on the outside. For example, with brass (high alpha) and iron (low alpha), on heating the strip curves toward the iron side, so brass is on the convex outer face.
On cooling, the metal with larger alpha shrinks more than the other. So it becomes shorter and moves to the inside of the curve. The strip bends the opposite way compared to heating. This two-way bending (one way when hot, the other way when cold) is what makes it useful as an automatic on/off switch.
The bending depends on the difference in their expansion. If both metals had the same alpha, both would expand equally and the strip would stay straight, only getting longer. A large difference in alpha (like brass and invar) gives a large bend for a small temperature change, which makes the device more sensitive.
The strip is placed so its free end touches an electrical contact. When temperature rises past a set value, the strip bends and breaks (or makes) the contact, switching the circuit off (or on). When it cools, it bends back and restores the contact. This keeps devices like electric irons, ovens, refrigerators and fire alarms at a controlled temperature automatically.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Two different metal strips (commonly brass and iron, or brass and invar) of equal length riveted or welded firmly together along their length so they act as one strip.
Brass has a high coefficient of linear expansion while invar has a very low one. This large difference in alpha makes the strip bend a lot for a small temperature change, so the device is very sensitive.
Yes, the same principle applies, but the direction of bending reverses. On heating it curves toward the low-alpha metal; on cooling it curves toward the high-alpha metal, giving automatic two-way switching.
Thermostats in electric irons, ovens, geysers and refrigerators; fire alarms; automatic temperature-controlled switches; and older flashing-light circuits (like turn indicators).
It is an application of thermal (linear) expansion in the Thermal Properties of Matter chapter. NEET can ask conceptual questions about the direction of bending and why it happens, so understand the alpha difference clearly.
