If temperature is related to average kinetic energy or speed of the particles, why is fast flowing water not hot?


Is it because the velocity of one water molecule relative to the other water molecules is low? When I stick my hand into a cold stream, why do I feel that it’s cold even though the water is moving fast into my hand? Shouldn’t the water flowing fast mean more collisions with my hand (which I thought is how we sensed temperature)?

In: Physics

7 Answers

Anonymous 0 Comments

The part where you’re putting your hand in to the water is partly what makes it deceiving.

Your hand is warmer than the water (obviously). When you stick your hand into the water, you’re heating up more water (and water flows by your hand, your hand heats up that water, and that water flows by. Then you hand heats up more water.) So the flowing water will actually feel colder than still water. The same sort of thing happens with wind chill factors.


You’ve also inadvertently come across the idea of *special relativity*. It’s something that Einstein and other scientists have thought about.

Say that you’re on a riverbank, and you’re watching the water flow by. You have a thermometer on a stick, and you measure the temperature of the water.

At the same time, someone else is floating down the river on a boat. They are also measuring the temperature of the water with a thermometer on the side of the boat.

When the boat passes by your thermometer, which one is right?

To you, the boat thermometer is wrong because it’s moving, and has slightly more kinetic energy. But from the boat captain’s point of view, the boat and the water are going the same speed, and your thermometer is wrong.

But obviously, the water isn’t two different temperatures at once.

The short answer is that the difference between the two isn’t significant and don’t worry about it. But nifty and weird things happen when closer to the speed of light.

Anonymous 0 Comments

The macroscopic movement does not factor into the kinetic theory of temperature, it is the average kinetic energy from an independent frame of reference stationary to the object/liquid/gas you’re looking at. Otherwise temperature would be completely dependent on the frame of reference – any water stationary from your point of view is still moving at 30km/s from the sun’s point of view, should that mean the water is extremely hot?

Anonymous 0 Comments

Think of it like you’re on a packed train.
If you all stand still, you’ll all stay mostly cool even though the train is moving.
But if you all start jumping up and down, you’re all gonna start feeling pretty warm.

Anonymous 0 Comments

Get that water speed near the speed of light then tell me what temperature it is. Comparatively speaking, your hand and the water are moving at the same speed.

Anonymous 0 Comments

It does heat up, very slightly

Water in liquid form can hold 3-5 times its capacity, meaning the molecules are somewhat spaced out,

And all flowing in the same direction.
So while they produce kinetic energy, they don’t produce Heat Energy on their own.

Striking your hand does generate a minuscule amount of heat energy, but the over all temp of the water takes that heat near instantly and dissipates it

Anonymous 0 Comments

The fastest estimate of the speed of a river is 3.1 m/s.

One liter (=1 kilogram) of water with this speed has a kinetic energy of 4.805 joules, being that it has 1kg mass and the kinetic energy equation is 1/2 m v^(2).

This amount of kinetic energy were it converted purely to heat energy, as an airplane’s brakes do, could hope to improve the temperature of one kilogram (=1 liter) of water by 0.0011 degrees Celsius. For you see, it takes 1 calories (not Calories, but calories) to raise one gram of water by 1 degree Celsius, and it takes 1000 calories to raise 1000 grams of water by 1 degree Celsius. And 1000 calories is 4184 joules. Easy division.

This is not a lot at all, and though this kinetic energy does constantly convert into the water’s temperature energy, it does not do so fast enough to make it “hot” by a typical human measure.

Anonymous 0 Comments

What we consider “fast” flowing water is not actually “fast”. The molecules are moving around at the about the speed of sound, so for the water to “feel fast” it would have to be moving at around that speed. If you stick your hand in water moving at the speed of sound, you will have other problems though.