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I am having trouble understanding why a division is made between heat and work.

The units for energy is $\text{kg} \, \text{m}^2 \, \text{s}^{-2}$. The only way to obtain this unit is if you exert a force over a distance - that is, if you do work. In other words, the only way to change the energy of something is if you do work on it. However, in thermodynamics, we are taught that there are two ways of changing the energy of a system: (1) heat transfer and (2) work. This got me thinking about how something other than work can change the energy of a system. But, given that heat transfer is just a transfer of energy through collisions at the microscopic scale, can we go as far as to say that heat is just work done at a microscopic level?

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You are correct: heat transfer can be thought of as work done at a microscopic level. Consider the simplest example of two molecules colliding. There will be some electrostatic repulsive force for the very short duration of time when they are colliding, and this force will act over a very small distance. As a result, one molecule will gain energy and the other will lose energy. Averaged out over many molecules in a gas, this microscopic phenomenon could give rise to heat transfer between two parts of a gas.

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    $\begingroup$ Correct, and the distinction is made to account for energy transfer associated with temperature difference. $\endgroup$
    – garyp
    Jun 7, 2021 at 1:38
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    $\begingroup$ "at a microscopic level" must be changed to "at a single-particle level" for this answer to make sense. Once more than one pair of particles is considered, the energy exchange clearly does not occur in concert, which is the defining nature of work. Put another way, you need to define a threshold where you stop calling it "work" and start calling it "heat." This threshold is not the microscale–macroscale threshold. $\endgroup$ Jun 7, 2021 at 2:38
  • $\begingroup$ That is a very useful clarification @Chemomechanics. $\endgroup$ Jun 7, 2021 at 15:15

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