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Why is Total internal energy of a system or external energy only the sum of kinetic and potential ? Why not some some other quantity as well present.

Is it because there can be only scenarios present. Energy possessed by a body due to virtue of its motion and other at rest?

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  • $\begingroup$ i think that your prof might have used total mechanical energy insetad of total internal energy $\endgroup$
    – Anonymous
    Dec 4 '20 at 13:08
  • $\begingroup$ We were talking about heat transfer.So he used total internal energy actually. $\endgroup$
    – user281503
    Dec 4 '20 at 14:12
  • $\begingroup$ But my question was that is there a chance that we may miss some form of energy when calculating total energy of any system.@Anonymous $\endgroup$
    – user281503
    Dec 4 '20 at 14:12
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The total energy does indeed include all portions of energy, not just kinetic and potential but also thermal, chemical etc. and everything which is contained in the body.

But it is rarely all relevant. Typically you would only find the total energy in order to use the energy conservation law. And there, only an energy difference is actually important. So you can spare yourself from the tough internal energies often in your calculation and only include the energies that may be changing.

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  • $\begingroup$ What if there are some other energies which we don’t know yet ? $\endgroup$
    – user281503
    Dec 4 '20 at 11:21
  • $\begingroup$ How do we identify that you know , there are no other forms of energy which we can consider.Is there some device telling us that there are no other changes happened ? $\endgroup$
    – user281503
    Dec 4 '20 at 11:23
  • $\begingroup$ Or Is it like we think about all forms of energy which we think can be considered. $\endgroup$
    – user281503
    Dec 4 '20 at 11:24
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    $\begingroup$ @user279337 You include those that are relevant. And that is a choice from experience and expertise, yes. For instance in classical mechanical circumstances, it is well known which energies that will be involved. We would there typically ignore/neglect heat generation and thus not have to consider, say thermal energy changes. And with no material changes, we would typically ignore all internal changes as well and also magnetic influences, solar irradiation and such. We simply ignore those whose changes presumably are negligible - and that is an educated estimate/guess. ... $\endgroup$
    – Steeven
    Dec 4 '20 at 11:32
  • $\begingroup$ ... In some scenarios we can't ignore internal changes, such as during car crashes where energy is spent on deforming materials, melting/burning objects and ripping apart chemical structures. Or simply when friction is involved which generates heat. Then the energy conservation law is not useful since we then don't know the total energy, unless we have other means (special formulas or measurements) to know exactly how much energy that is being converted between those various forms. $\endgroup$
    – Steeven
    Dec 4 '20 at 11:32

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