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For each of the situations described below, the object considered is undergoing some changes. Among the possible changes you should consider are: (Q) The object is absorbing or giving off heat. (T) The object's temperature is changing. (U) The object's internal energy is changing. (W) The object is doing mechanical work or having work done on it. For each of the situations described below, identify which of the four changes are taking place and write as many of the letters Q T U W (or none) as are appropriate.

1. A cylinder with a piston on top contains a compressed gas and is sitting on a thermal reservoir (a large iron block). After everything has come to thermal equilibrium, the piston is moved upward somewhat (very slowly). The object to be considered is the gas in the cylinder.
2. Consider the same cylinder as in part a, but it is wrapped in styrofoam, a very good thermal insulator instead of sitting on a heat reservoir. The piston is pressed downward (again, very slowly), compressing the gas. The object to be considered is the gas in the cylinder.
3. An ice cube sitting in the open air is melting.

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Page last modified October 30, 2002: H&T03