# heat as a low quality energy

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?

• What do you mean by "low quality energy"? – ACuriousMind Apr 28 '16 at 14:25
• "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. – lucas Apr 28 '16 at 14:50
• 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." – ergon Apr 28 '16 at 15:07
• Note the distinction between 'net heat' and 'heat offered to the system'. – dmckee Apr 28 '16 at 15:17
• 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). – Emilio Pisanty Apr 29 '16 at 11:59

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).