A system consists of different forms of energy like thermal energy, mechanical energy, chemical energy, nuclear energy etc. If these energies are to be transferred to another system (call it system 2), it can either be done as heat or work (or mass but here I take system approach) and again at the other system (system 2) it will be held as (change the) one of the forms of energy (thermal, mechanical chemical etc).

So when the second law implies that heat cannot be completely converted to work is it actually implying that thermal energy of a system cannot be completely transferred as work to another system? or does it mean energy that is transferring as heat cannot be changed to transfer-of-energy-as-work mid transfer?

It cannot be about the quality of energy because heat and work are not energy they just imply transfer of energy.


2 Answers 2


The 2nd law implies that heat can't be completely converted to work using a cyclic process. Obviously, heat can be converted to work if the process does not have to be cyclic. An example is isothermal reversible expansion of an ideal gas.

  • $\begingroup$ Yes...but is it heat cannot be converted to work or thermal energy cannot be converted to work in cyclic process? $\endgroup$
    Nov 19, 2017 at 6:33
  • $\begingroup$ What is your definition of the term "thermal energy?" Is it internal energy U? $\endgroup$ Nov 19, 2017 at 12:34
  • $\begingroup$ I would say the mico kinetic energy content of the system(sensible and latent heat content of the system). $\endgroup$
    Nov 19, 2017 at 16:05
  • $\begingroup$ Thermal energy is a part of internal energy would be my understanding . $\endgroup$
    Nov 19, 2017 at 17:16
  • $\begingroup$ Thermal energy as you describe it is exactly internal energy. And, in a cyclic process, since internal energy is a function of state, the change in internal energy over each cycle is zero. So we are talking about heat and work in your statement of the 2nd law. The precise statement of the second law (Moran et al, Fundamentals of Engineering Thermodynamics) is as follows: It is impossible for any system to operate in a thermodynamic cycle and deliver a net amount of energy by work to its surroundings while receiving energy by heat transfer from a single thermal reservoir. $\endgroup$ Nov 19, 2017 at 17:34

@Chester Miller So if energy is transferred to a system(as heat or work) and there is no transfer of energy from this system, then the energy transferred to the system will be saved as one of the forms of energy of the system ie increase the energy of the system(which may be thermal energy, mechanical energy, chemical energy etc). If the energy transfer was as work then this can be completely be saved as either mechanical energy of the system or thermal energy of the system (we won't consider other energy of the system). But if the energy transfer was as heat then this can be saved completely as the thermal energy of the system but the energy transferred cannot be completely be saved as mechanical energy. Is this the case or can the complete energy transferred to the system as heat can also be saved as mechanical energy of the system?(as mechanical energy can be completely be changed to work easily)

  • $\begingroup$ Could not post this whole as a comment $\endgroup$
    Nov 20, 2017 at 4:29
  • $\begingroup$ It can also be completely saved in the system as mechanical energy(e.g., potential energy) if there is equipment included in the system to implement the conversion) $\endgroup$ Nov 21, 2017 at 17:04
  • $\begingroup$ @ChesterMiller Thank you for taking the time to answer. I got to understand something new. $\endgroup$
    Nov 23, 2017 at 5:07

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