Let's say I supply heat Q to a system. If it's internal energy increases by U and the work done on it is W then by the first law of thermodynamics, Q = U + W. Now, let's say that due to the thermal expansion, the gravitational potential energy of the system also increases by P. Would it be right to say Q = U + W + P? What if the kinetic energy also increases(by K), is it right to say Q = U + W + P + K

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Let&#039;s say I supply heat Q to a system. If it&#039;s internal energy increases by U and the work done on it is W then by the first law of thermodynamics, Q = U + W. Now, let&#039;s say that due to the thermal expansion, the gravitational potential energy of the system also increases by P. Would it be right to say Q = U + W + P? What if the kinetic energy also increases(by K), is it right to say Q = U + W + P + K

zastenjkcy · Accepted Answer

Yes.The 1st law of thermodynamics comes from basic energy conservation covering all involved energies:      E    1    +  W  +  Q  =      E    2      or rewritten:    Δ  E  =  W  +  Qwhere E represents all present energy (the sum of kinetic, potential, chemical, thermal...) and W and Q represent all energy added (work or heat). We could, if we wanted to, expand the formula into its parts:  Δ  K  +  Δ      U    gravity    +                              Δ                      U            elastic                    +          Δ                      U            chemical                    +          Δ                      E            thermal                          ⏟                    internal energies       Δ              U        i              +  ⋯  =  W  +  QSome of these may be considered internal, such as thermal and chemical energies, and possibly elastic energy, and more and they are often grouped together and symbolised       U    i   or similar.In typical thermodynamic applications (when dealing with refrigeration system, pumps, heating systems etc.), gravitational potential energies, kinetic energy and other macro-scale energies are not relevant or negligible. Therefore, typically, only internal energies are left. And that&#039;s why you most often see the 1st law of thermodynamics written like this:  Δ      U    i    =  W  +  Q

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