# Question: A bimetallic strip is made of two ribbons of dissimilar metals

A bimetallic strip is made of two ribbons of dissimilar metals bonded together.
(a) First assume the strip is originally straight. As they are heated, the metal with the greater average coefficient of expansion expands more than the other, forcing the strip into an arc, with the outer radius having a greater circumference (Fig. P19.54a). Derive an expression for the angle of bending - as a function of the initial length of the strips, their average coefficients of linear expansion, the change in temperature, and the separation of the centers of the strips (Δr = r2 - r1).
(b) Show that the angle of bending decreases to zero when ΔT decreases to zero and also when the two average coefficients of expansion become equal.
(c) What If? What happens if the strip is cooled?
(d) Figure P19.54b shows a compact spiral bimetallic strip in a home thermostat. The equation from part
(a) Applies to it as well, if Ө is interpreted as the angle of additional bending caused by a change in temperature. The inner end of the spiral strip is fixed, and the outer end is free to move. Assume the metals are bronze and Invar, the thickness of the strip is 2Δr = 0.500 mm, and the overall length of the spiral strip is 20.0 cm. Find the angle through which the free end of the strip turns when the temperature changes by one Celsius degree. The free end of the strip supports a capsule partly filled with mercury, visible above the strip in Figure P19.54b. When the capsule tilts, the mercury shifts from one end to the other, to make or break an electrical contact switching the furnace on or off.

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