15.1 The First Law of Thermodynamics
A cylinder is divided in half by a movable disk in the middle. Each half is filled with an equal number of gas molecules, but one half is at a higher temperature than the other. Which choice best describes what happens next?
- The high temperature side expands, compressing the low temperature side.
- Heat moves from hot to cold, so the low temperature side will gradually increase in temperature and expand
- (b) happens quickly, but after that (c) happens more slowly.
Imagine a solid material at the molecular level as consisting of a bunch of billiard balls connected to each other by springs (this is actually a surprisingly useful approximation). If we have two blocks of the same material, but in one the billiard balls are shaking back and forth on their springs a great deal, and in the other they are barely moving, which block is at the higher temperature? Using what you know about conservation of momentum in collisions, describe which block will transfer energy to the other, and justify your answer.
A system has 300 J of work done on it, and has a heat transfer of -320 J. Compared to prior to these processes, the internal energy is:
- 20 J less
- 20 J more
- 620 J more
- 620 J less
Find a snack or drink item in the classroom, or at your next meal. Find the total Calories (kilocalories) in the item, and calculate how long it would take exercising at 150 W (moderately, climbing stairs) at 20% efficiency to burn off this energy.
A potato cannon has the fuel combusted, generating a lot of heat and pressure, which launch a potato. The combustion process _____ the internal energy, while launching the potato _____ the internal energy of the potato cannon.
- increases, increases
- increases, decreases
- decreases, increases
- decreases, decreases
Describe what happens to the system inside of a refrigerator or freezer in terms of heat transfer, work, and conservation of energy. Confine yourself to time periods in which the door is closed.
15.2 The First Law of Thermodynamics and Some Simple Processes
Consider process CD in Figure 15.43. Does this represent work done by or on the system, and how much?
A thermodynamic process begins at 1.2 × 106 N/m2 and 5 L. The state then changes to 1.2 × 106 N/m2 and 2 L. Next it becomes 2.2 × 106 N/m2 and 2 L. The next change is 2.2 × 106 N/m2 and 5 L. Finally, the system ends at 1.0 × 106 N/m2 and 5 L.
On Figure 15.42, this process is best described by
The first step of a thermodynamic cycle is an isobaric process with increasing volume. The second is an isochoric process, with decreasing pressure. The last step may be either an isothermal or adiabatic process, ending at the starting point of the isobaric process. Sketch a graph of these two possibilities, and comment on which will have greater net work per cycle.
Look at Figure 15.42, and assign values to the three pressures and two volumes given in the graph. Then calculate the net work for the cycle ABCFEDCFA using those values. How does this work compare to the heat output or input of the system? Which value(s) would you change to maximize the net work per cycle?
15.6 Entropy and the Second Law of Thermodynamics: Disorder and the Unavailability of Energy
Equal masses of steam (100 degrees C) and ice (0 degrees C) are placed in contact with each other in an otherwise insulated container. They both end up as liquid water at a common temperature. The steam ___ entropy and ___ order, while the ice ___ entropy and ___ order.
- gained, gained, lost, lost
- gained, lost, lost, gained
- lost, gained, gained, lost
- lost, lost, gained, gained
A high temperature reservoir losing heat and hence entropy is a reversible process. A low temperature reservoir gaining a certain amount of heat and hence entropy is a reversible process. But a high temperature reservoir losing heat to a low temperature reservoir is irreversible. Why?
15.7 Statistical Interpretation of Entropy and the Second Law of Thermodynamics: The Underlying Explanation
A piston is resting halfway into a cylinder containing gas in thermal equilibrium. The layer of molecules next to the closed end of the cylinder is suddenly flash-heated to a very high temperature. Which best describes what happens next?
- The high temperature molecules push out the piston until their energy is reduced enough that the system is in equilibrium.
- The molecules with the highest temperature bounce off their neighbors, losing energy to them, and so on until the system is at a new equilibrium with the piston moved out.
- The molecules with the highest temperature bounce off their neighbors, losing energy to them, and so on until the system is at a new equilibrium with the piston where it started.
- The high temperature molecules push out the piston until their energy is reduced enough that the system is in equilibrium, and then the piston gets sucked back in.
Design a macroscopic simulation using reasonably common materials to represent one very high energy particle gradually transferring energy to a bunch of lower energy particles, and determine if you end up with some sort of equilibrium.