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In a cyclic process, there is no change in internal energy. So the work done by the system must equal to the heat offered to the system. So if all heat is converted to work, how can heat by a low quality energy?

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    $\begingroup$ What do you mean by "low quality energy"? $\endgroup$ – ACuriousMind Apr 28 '16 at 14:25
  • $\begingroup$ "the work done by the system must equal to the heat offered to the system" and "all heat is converted to work" aren't true because of second law of thermodynamics. $\endgroup$ – lucas Apr 28 '16 at 14:50
  • $\begingroup$ Low quality energy is the one that cannot be converted fully into mechanical energy. @lucas: tell that to these people: web.mit.edu/16.unified/www/FALL/thermodynamics/notes/… "For a cyclic process heat and work transfers are numerically equal." $\endgroup$ – ergon Apr 28 '16 at 15:07
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    $\begingroup$ Note the distinction between 'net heat' and 'heat offered to the system'. $\endgroup$ – dmckee Apr 28 '16 at 15:17
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    $\begingroup$ Regarding your link, it's important to note that it does not contradict lucas's statement. For a process that's cyclic on the system, the net work performed by the system is equal to the net heat taken in by the system, but that is different from the total heat offered to the system from the hot reservoir (and which differs from the net heat taken in by the heat dumped into the cold reservoir). $\endgroup$ – Emilio Pisanty Apr 29 '16 at 11:59
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Energy stored as heat, by itself, is neither low- nor high-quality. What matters is the temperature at which the heat is stored, and the relationship of that temperature compared to the heat sink that will absorb the excess energy in the process.

To be more specific, say you have a heat sink at $T_S=20°\:\mathrm C$, such as the atmosphere for a car engine. Then the interesting comparison is between (say) $1\:\mathrm J$ of energy stored at $100°\:\mathrm C$ (such as a mass $m_{100}$ of water just below boiling point) and the same $1\:\mathrm J$ of energy stored in a bigger mass $m_{30}$ of water at a lower temperature of $30°\:\mathrm C$: although both samples have the same amount of energy, the one with a bigger temperature difference to the heat sink can operate a heat engine more efficiently and therefore can be used to perform more work (as opposed to simply handing most of its energy directly to the heat sink).

This is the reason heat is sometimes described as 'low-quality', when it is stored at low temperature (such as the heat created by friction between a car's wheels and the road) and therefore cannot be used to produce much useful work with the heat sinks we have available. Other sources of heat (like the exploding gas inside a car piston, or the glowing fuel rods in a nuclear reactor, and so on) are what you'd call 'high-quality energy' in that setting.

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  • $\begingroup$ since we can create almost absolute zero conditions, why don't we use that to take fully advantage of heat? $\endgroup$ – ergon Apr 28 '16 at 15:13
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    $\begingroup$ Because it requires work to cool to those conditions. I would recommend a good, long session with an introductory thermodynamics textbook. $\endgroup$ – Emilio Pisanty Apr 28 '16 at 15:19
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    $\begingroup$ One of the many variation of the 2nd Law says that a refrigerator can't be perfectly efficient any more than a heat engine can. It costs you energy to make a colder reservoir than you have naturally available, making the combination of refrigerator+heat engine less efficient than just building a heat engine to take advantage of the available reservoir. $\endgroup$ – dmckee Apr 28 '16 at 15:19
  • $\begingroup$ Emilio Pisanty I am really fed up with your basic GCSE-level approaches to most of my questions. My questions are more advanced that you tend to consider given your answers. Even if it takes work to create absolute zero, let's say X gigajoules. You will still be able to harvest the energy of the whole earth!!! $\endgroup$ – ergon Apr 29 '16 at 11:37
  • $\begingroup$ @ergon To be honest I do not recall having answered previous answers of yours (nor do I doubt having done so if you say I did, meaning that each answer has been independent of the others). If the answers seem pitched at a low level to you, then take that as constructive criticism that the question text as posed is also pitched at a low level; if you have a more sophisticated question then make sure it shines through in your text. As it is, on the text there is precious little evidence that you understand the material even at introductory level, I'm afraid. $\endgroup$ – Emilio Pisanty Apr 29 '16 at 11:54

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