Correct Response: D. A bimetallic-strip thermostat contains a flat strip of metal formed in the shape of a coil. As the ambient temperature changes, the coil winds tighter or looser according to the direction of the temperature change. The metal strip consists of two different metals bonded together: one metal is on the inside of the bend of the strip, and the other is on the outside. Since the two metals have different coefficients of thermal expansion, they expand and contract at different rates as the temperature changes. This causes the strip to bend one way or the other, actuating heating and/or cooling switches. Answer choice A might make sense for very fast and extreme changes in temperature since the metal with the greater specific heat would react more slowly to the temperature change. However, even under those unlikely circumstances, the coil would eventually return to its original position as temperature equilibrium is reached. This return would make the thermostat ineffective. Answer choice B deals with the friction of two objects sliding across each other's surfaces. This phenomenon is not involved in the operation of a bimetallic-strip thermostat. Answer choice C deals with the density of materials. This characteristic is not pertinent to the operation of a bimetallic- strip thermostat.
