exam1soln - Physics 262/266 - exam#1 1. (30 pts) Answer the...

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Physics 262/266 - exam#1 3/12 1. (30 pts) Answer the following questions. (Use the space provided below and on the next page if needed.) a). (6 pts) An ideal gas is being taken from an initial state i to a final state f through three different thermodynamic processes: a, b, and c . Which process ( a, b, or c ) will result in i) the largest work done by the gas? ii) the largest heat absorbed by the gas? b). (4 pts) You have two ideal gases with the same number of moles in your laboratory. You know that one of them is monatomic and the other is diatomic but you don’t know which one is which. You are asked to distinguish them by heating the gases up and to observe the temperature changes. For the same amount of heat absorbed by the two gases, you observed that gas #1 results in a higher temperature rise than gas #2. Which one is the diatomic ideal gas (gas #1 or gas #2)? c). (6 pts) Four different Carnot heat engines operate between the following ranges of temperatures: i) [100K,300K], ii) [200K,400K], iii) [50K,150K], and iv) [50K, 200K]. If an equal amount of heat Q H is being absorbed by the four heat engines from the high temperature reservoir, which heat engine will produce the largest net amount of work done in one cycle, W net ? d). (8 pts) In an experiment, an idea gas at temperature T i is being put through the following different expansion processes, what will its final temperature T f be in relation to its initial temperature T i (larger, smaller, or unchanged) for each case: i) Reversible adiabatic expansion ii) Reversible isothermal compression iii) Adiabatic free expansion iv) Reversible isobaric compression e). (6 pts) In a thermodynamic experiment, a system is interacting with its environment through an unknown process. A student measured the entropy change S sys for the system to be negative. i) Is this physically possible (yes or no)? ii) What can the student conclude on the corresponding entropy change for the environment S env (circle one)? ,,, 0 env sys env sys env sys env SS o rS   
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Physics 262/266 - exam#1 4/12 SOLUTION: a) i. Work done by the gas is given by the area under the curve. Among the three different processes, process a will result in the largest amount of work done by the gas. ii. From the 1 st Law of Thermodynamics, we have Q = U + W. Since all three processes move from an isotherm at a lower temperature to one at a higher temperature, U > 0. We also know that W > 0 for all three expansions. Lastly, since all three processes begin with the same initial state and end at the same final state. Their change in internal energy U are the same. Therefore, the process with the largest heat absorbed Q will be the one with the largest W and process a will also result in the largest amount of heat absorbed. b) According to Kinetic Theory of Gases, a diatomic ideal gas will have a larger heat capacity so that for the same amount of heat absorbed, it will result in a lower temperature raise (dQ=nCdT). Thus, gas #2 is the diatomic gas.
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This note was uploaded on 05/10/2010 for the course PHYSICS 262 taught by Professor Paulso during the Spring '10 term at George Mason.

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exam1soln - Physics 262/266 - exam#1 1. (30 pts) Answer the...

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