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Thermodynamics is the field of physics studying how thermal energy can be converted to or from other forms of energy, usually mechanical or electrical energy. The steam engine is an example of a thermodynamic system, where heat drives a shaft to pump water or power a train. The laws of thermodynamics describe the physics behind thermodynamic processes such as the steam engine. The zeroth law implies that no heat will flow between two objects at the same temperature, which means that the two are in thermal equilibrium with each other. The first law is a thermodynamic restatement of energy conservation, where the change in energy of a system equals the heat added to it and the work done on it. The second law states that entropy cannot decrease for any naturally occurring process. This law implies that, when heat is transferred in a thermodynamic process, the process is not reversible. Heat spontaneously flows from hotter objects to colder ones, but the reverse never occurs. The third law states that absolute zero is the minimum possible temperature and the temperature at which the entropy of a system also drops to its minimum. 
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thermodynamics, zeroth law of thermodynamics, first law of thermodynamics, entropy, second law of thermodynamics, reversibility, third law of thermodynamics
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Review problems and questions |
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- Jean wants to build a machine that has a wheel that will never stop turning. She claims that it will work because energy is conserved. Is Jean right? Use the first law of thermodynamics to explain.
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Jean is not right. Although energy is conserved, there is friction in every process. Eventually, her wheel will slow down because of the energy lost as the wheel experiences friction with the air and mount, causing heat to be exchanged. Thus some of the rotational energy is lost. 
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- When a container of gas expands, is positive work done on the gas or is the gas doing positive work on the surroundings?
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Work is done on the surroundings. When the gas expands, it pushes up on the cylinder, doing positive work on the surroundings and negative work on the gas. The gas therefore loses energy.
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- Kaleb fills a glass with half lemon-lime soda and half root beer. The two drinks mix together. Explain why Kaleb’s drinks will not naturally separate themselves from one another after being mixed.
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From the second law, entropy of a closed system will not decrease as time moves forward. The entropy of the two mixed drinks is much higher than that of the two drinks separated, so the system will not naturally move toward the separation of the two drinks. Statistically, the chance of the drink spontaneously “unmixing” is so low that it will never occur.
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- After learning Newton’s first law, Natasha figures that if she gets her car up to the speed she wants, it will continue moving at that speed until she applies the brakes. When she performs an experiment to test this, however, her car slows down and eventually stops without her pressing the brakes. When she gets out to examine her car, she notices that the tires are warm. What happened?
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The car’s kinetic energy was gradually converted into heat, and after all the kinetic energy was converted, the car came to a stop. This heat was transferred to the air and the road, although some of it is in the tires, as Natasha noticed.
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- Match each statement below to the corresponding law of thermodynamics.
- No engine can be 100% efficient.
- A system can only have zero entropy if it is cooled to absolute zero (0 K).
- Energy is conserved in thermodynamic systems.
- If A and B are at the same temperature, and B and C are at the same temperature, then A and C must also be at the same temperature.
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- second law of thermodynamics (the law of entropy)
- third law of thermodynamics
- first law of thermodynamics (conservation of energy)
- zeroth law of thermodynamics
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- Simon received several balloons filled with pure nitrogen gas for his birthday. He left them in his room for a couple days, and they all deflated. Will the nitrogen gas that escaped ever separate itself from the other gas in his room?
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No. The entropy of the mixed gas is much higher than that of the separated gas, so the gases will not naturally separate from one another. Since most of the Earth’s atmosphere is composed of nitrogen gas, the nitrogen released from his balloon is now indistinguishable from the rest of the gas in the room.
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