# Week 22 (Jan. 22 - 26)

Reading: Heat engines, Entropy, and the second law (Chap. 22)

Key Topics:
the second law of thermodynamics, reversible and irreversible processes, engine efficiency, absolute temperature scale, entropy, disorder,
Lab:

Homework Problems: due Tuesday, Jan 30 at noon.
1. Heat transport through fat: Suppose that heat from the inside of the human body must travel through a layer of fat 30 mm thick before reaching the skin. Suppose, further, that there are 1.7 square meters of skin on the surface of a person. On a 34 deg. C day, how much heat is conducted from the inside of the human body to the skin in one hour? You may assume that the thermal conductivity of fat is 0.20 watts per meter-degree C.
2. Two light bulbs are identical except that one has a filament that operates at 2500 K, while the other operates at 2200 K. Find the ratio of the power radiated by the hotter one to the power radiated by the cooler one.
3. Find the change in entropy that results when a 2.3 kg block of ice melts slowly (reversibly) at 273 K (0 deg. C).
4. Entropy Challenge problem: A 1 gram block of copper initially at 100 deg. C is placed gently into an insulated cup containing 10 grams of water initially at 20 deg. C. When thermal equilibrium is finally reached, (i) what is the temperature of the water and the block? (ii) what is the change in entropy of the water? of the block? (iii) Did the total energy of the system (block + water) increase, decrease or remain the same? (iii) Did the total entropy of the system increase, decrease, or remain the same?

General College Physics