Chapter 25 review

    Conceptual questions
      Section 25.1
      1. What happens to the internal energy of a system if it does work on something else?

      2. Easy The hot coffee in a mug cools from 80°C to room temperature over the course of 15 minutes. An equal amount of hot coffee, also at 80°C, in an insulated travel mug cools to room temperature over the course of 4 hours. In which mug did the coffee experience a greater change in entropy?

      Section 25.2
      1. Describe what an adiabatic process is for a gas.

      2. Describe what an isothermal process is for a gas.

      3. What is the difference between a heat engine and a heat pump? Give one example of each.

      4. Imagine that the gas within a piston expands, pushing against an outside pressure. Which of the following statements are correct? (There may be more than one right answer.)
        1. The gas within the piston does positive work on its surroundings.
        2. The gas within the piston does negative work on its surroundings.
        3. The surroundings do positive work on the gas within the piston.
        4. The surroundings do negative work on the gas within the piston.


      5. Easy Which statement is consistent with the following equation for a Carnot cycle? efficiency=( 1− T 1 T 2 ) × 100%
        1. The efficiency of a Carnot engine is equal to the ratio of the two temperatures it is operating between.
        2. No Carnot engine with T1 above absolute zero can be 100% efficient.
        3. When the upper and lower temperatures of a Carnot engine are very close to each other, then it will have nearly 100% efficiency.
        4. By inverting the operating temperatures, a Carnot engine can operate at >100% efficiency.


      6. Easy Mirette put together a business model for a company that would build ideal Carnot engines that convert 100% of the input heat into energy. Would it be a good idea to invest your life savings in her company?

        Quantitative problems
          Section 25.1
          1. The gas within a piston expands from an initial volume of 25 mL to a larger volume of 125 mL, against a constant pressure of 100,000 Pa (approximate atmospheric pressure at sea level). How much work does the gas in the piston perform?

          2. A bearded dragon lizard basks in the desert Sun during the day and reaches a body temperature of 40°C. At night the air temperature quickly becomes 15°C. During the course of the night the lizard loses 100 J of heat while his body temperature equilibrates to the nighttime air temperature. What is his change in entropy?

          3. Easy A hot rock is plucked from a fire pit and dropped into a bucket of cold water. The rock has a temperature of 80°C, and it transfers an initial 500 J of thermal energy to the water. Assume that the temperatures of the rock and water remain unchanged during this first instant.
            1. Does the entropy of the rock increase or decrease? By how much?
            2. Does the entropy of the water increase or decrease? By how much?
            3. Does the total entropy of the rock+water system increase or decrease? By how much?

          Section 25.2
          1. Easy Suppose that an internal combustion engine operates between an internal temperature of 2,000 K and an exhaust temperature of 1,000 K. What is the engine’s maximum theoretical efficiency expressed as a percentage?

            2. Two <i>PV</i> diagrams depicting a thermal cycle of two pistons operating at different average pressures
          2. Medium These two PV diagrams each depict one thermal cycle of a piston containing an ideal gas. The pistons are physically identical, but Piston I operates at higher pressure, on average, than Piston II. The lower-left corner of the coordinate grid represents the origin (zero pressure and volume).
            1. Which piston does more (positive) work during the expansion phase (I, II, or a tie)?
            2. Which piston does more (negative) work during the contraction phase (I, II, or a tie)?
            3. Which piston does more (positive) work during one complete cycle (I, II, or a tie)?

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