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I learned that when energy is transfered it either produces work or it becomes thermal energy (heat).

Work implies a force that acts on an object producing changes in its position.

I'm learning these concepts from Khan Academy, and in this article they say:

A hot cup of coffee has a measurable amount of thermal energy which it acquired via work done by a microwave oven, which in turn took electrical energy from the electrical grid.

What work did the microwave oven if all the energy transferred became heat?

What is heat actually? What is so special about it?

Does heat imply movement of atomic particles and hence the microwave oven sucessfully did work on them?

Thanks for your patience.

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  • $\begingroup$ Seems like there're a lot of unrelated questions in your question, e.g. "what is heat actually?" is completely unrelated to "does work always imply that a force that acts on an object produces a change in its position?". $\endgroup$
    – Allure
    Commented Sep 11, 2022 at 10:52

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Work implies a force that acts on an object producing changes in its position.

Not necessarily. It is possible to do work on an object and only change its internal energy, not its position. Heating coffee in a microwave is one example; heating a wire by passing a current through it is another; compressing a spring is a third example.

Heat is simply the internal energy that a macroscopic object possesses by virtue of the microscopic motions of its constituent atoms or molecules.

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  • $\begingroup$ But change in movement is present in the definition of work. Does the microwave oven change the position of the particles of that object, changing its internal energy? $\endgroup$
    – Aleix
    Commented Sep 11, 2022 at 11:06
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    $\begingroup$ @Aleix The microwave oven increases the kinetic and vibrational energy of the molecules in the coffee, so in a sense it changes their microscopic motions, but that is not a useful way to think about what is happening to the coffee at a macroscopic level i.e. it is getting hotter. The microwave oven does not (usually) make the coffee jump out of the mug. $\endgroup$
    – gandalf61
    Commented Sep 11, 2022 at 11:58
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  1. Work in thermodynamics is usually associated with motion of the boundaries of the system. The standard example is a gas in a cylinder fitted with a piston. To move the piston to either compress or expand the gas involves work. But there are more ways to transfer work to a thermodynamic system and one example is #4 below.

  2. In the microwave example we have a series of energy conversions. Electrical work, by which we refer to moving electrons, is converted into MW radiation and then it is absorbed by the coffee as internal energy. In general, electrical work can be transformed into various energy forms, heat (via resistance), motion (via a motor), radiation, etc.

  3. Heat is energy that is transferred between systems at different temperatures. The molecules of a hot body move faster than those of a colder body. When the two bodies come into thermal contact, collisions transfer energy in such a way that the molecules of the hotter body slow down by passing some of their energy to the colder body, whose molecules now move a bit faster.

  4. Does heat imply movement of atomic particles and hence the microwave oven successfully did work on them? Yes, in the same sense that rubbing our hands does work on our palms that is manifested as higher temperature.

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