How does movement affect the transfer of temperature?


E.g. I fill a bowl with hot water and then place a ball in it, the ball will heat up. Does the rate at which the ball heats up change if that ball is spinning on the surface or moving around in the water, vs. sitting still?

In: 1

In practice, yes, because the ball is going to induce motions in the water. That prevents a boundary layer (“thin shell around a thing”) of cooler water from surrounding the ball, making the heat transfer from the bowl of water more efficient. But how much it does so depends on properties of the water (e.g. viscosity) and of the ball (how smooth is its surface?) – an idealized frictionless ball wouldn’t create any current and thus wouldn’t affect heat transfer.


Moving the ball will being more got water into contact with the ball’s surface more often and speed up heat transfer. This is why we stir things to equalize the temperature.

Without motion, the water touching the ball will quickly cool down and you need convection to drive the circulation of new hot water. Movement speed that up, like the fan in a convection oven.

Probably, but it may depend on what the ball is made of, since it might also affect how fast the heat transfers out of the ball into the air.

When something moves within a fluid, even just spinning in place, it moves the fluid around it. That results in convection, in this case, moving the water that has just dumped its heat into the ball away and replacing it with still-hot water. But the reverse would happen, too, with air. Remember, in physics, the term “fluid” refers to liquid *and* gas. So the air gets convected around, too, carrying heat away from the ball. Which one wins out, and how quickly, depends on the specific heat capacity of the ball, how deep it floats, and even which way it’s spinning (if it spins vertically, it would carry water up onto the air side as it turns, insulating it from the air a bit.