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I realised that I have a fundamental confusion in understanding Joule's experiment. When we say the paddles' motion causes a rise in temperature of the fluid, what do we mean-

(1) the paddles' motion imparts a greater momentum to the fluid molecules, which increases their energy and this reflects as temperature rise, or

(2) the molecules closest to the paddle heat up more than those farther away, due to friction, and this creates a temperature difference between the layers of fluid, transferring heat and thus the temperature increases

Now, which of these is the correct? I think the answer to this lies in doing the same experiment with a non viscous fluid. Can someone please explain what happens in this case? Or are the possible explanations I gave for heating both wrong?

valerio
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GRrocks
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1 Answers1

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Both of your mechanisms are correct. There is no such thing as an inviscid fluid; even ideal gases are viscous. So, even the smallest amount of viscous behavior is sufficient to convert coordinated kinetic energy (non-random) to random kinetic energy.

The rate of deformation of the fluid (which gives rise to viscous heat generation) is largest near the paddles and, to a lesser extent at the walls of the tank. So this is the region where, during the mixing, the temperature rise is greatest. But, the heat generated in these regions is transferred to other regions of the container by convection and conduction. Also, the fluid in these regions is replaced by fresh fluid as a result of the circulation.

Chet Miller
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