So here’s a theory based on nothing scientific.

Water molecules are still attracted to each other. Albeit not as strongly as if It were a solid instead of a liquid. That’s why it doesn’t spread out into individual particles when you spill it on the floor. So to some extent, I bet the reason water doesn’t get hot when you stir it is the same as the reason my arm doesn’t get hotter when I wave it around in the air.

But they ARE moving relaitve to each other at least a little bit. Cause stuff mixes together when you stir.

So my point? I bet it actually WOULD get hot if you stirred real hard. Don’t you think? Just like my arm gets hot when I rub my hand back and forth over it for a long time.

It will heat up for sure, but the specific heat of water is pretty large so you have to put in a lot of energy (stir really hard) in order to be able to measure a temperature difference.

I suspect that it would take a special kind of stirring. If I just stir in a circle, surely that wouldn’t do anything. And even if I stir randomly, changing direction and whatnot, I could just be moving large chunks of water around, and not getting the right kind of energy into the water. I cup of water on a plane has a lot of energy, but not necessarily much temperature.

The cup of water will most definitely heat up. Any kind of stirring you like. Any energy input will raise the temperature of the water.

Why don’t you notice, like mentioned above its because the specific heat capacity of water is so high. Higher than any other common substance.

So what is specific heat capacity? Well is the amount of energy it takes to raise the temperature of 1 kg of a substance by one degree Celsius. For water this is around 4.2 KJ/kg°C.

O.k well what does this number mean in terms of stirring?

Lets take an example.

We have a cup of water (250ml = 0.25 kg)
A spoon

To raise the temperature of the water by one degree, we would need to put it 1.05 KJ (4.2*.25=1.05) of energy in by stirring. How much stirring is that?

The energy supplied (Work done) by the spoon is the Force applied * Distance its applied for.

O.k so here comes a little guess work, how much force would the average person apply in stirring, I’m guessing, not much, say about the amount of force to hold up an apple against gravity. Which is around 1 Newton of force.

So if we wanted to raise the temperature by _only _1 degree, we would need to stir for 1.05 kilometres! Which is a lot of stirring! :)

How you stir shouldn’t matter too much.Circular stirring should do the trick if you do it for long enough. Maybe with the random way, you might be applying more of force , than snooth stirring, and that would help.

I’d say if you put water in a blender, you should see a temperature rise of a few degrees after a while. If you knew the wattage (and how efficient it is in turning electrical energy to mechanical) of the blender you could work out how long.

David