My professor while teaching Mechanical energy said all the mechanical works are performed at at a constant temperature and said this is the main difference between the thermodynamic work and mechanical work that thermodynamic work typically involves work and heat transfer. Taking an example of motor, so far I know, we say it cannot perform at 100% efficiency some work is converted to heat. How can we justify the statement?
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$\begingroup$ I think your professor is trying to explain the difference between mechanical energy, which is kinetic and potential energy of an object as a whole, and internal energy, which is kinetic and potential energy of the molecules of an object. A temperature increase when performing mechanical work would mean some mechanical energy has been lost to internal energy. $\endgroup$– Bob DCommented Apr 18, 2022 at 15:14
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2 Answers
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The statement: the work done by the net force on an object is equal to the change of the kinetic energy ($E_k$) is always valid.
When the force is conservative (as gravity for example), there is a well defined conserved quantity $E = E_p + E_k$, where $E_p$ is called potential energy. And there is no temperature change involved.
But in the more general case, (as an object with an initial velocity that stops after some time due to friction with the ground), there is not that conserved quantity. It is necessary to deal with the increase of temperature of object and ground due to friction, and also the heat lost to the surroundings.
answered Apr 18, 2022 at 21:58
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Mechanics typically deals with point particles or rigid bodies, for which the internal energy is constant, and defines work as force acting through a distance causing a change in kinetic energy. (The change in potential energy is just a simpler way to evaluate the negative of the work done by a conservative force.)
Thermodynamics considers the change in the internal energy of a system and has a broader definition of work. See my answer to Where is (mechanical) energy conserved?.
answered Apr 19, 2022 at 13:19