# Joule's experiment with a nonviscous fluid

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?

• Very much related because these answers explain viscosity on a molecular level. physics.stackexchange.com/questions/129676/… Commented Feb 12, 2018 at 11:45
• It cannot be just $(1)$ because you have collisions with the back of the paddles where there is a decrease in momentum. Commented Feb 12, 2018 at 11:46