What does it mean by average kinetic energy of molecules as temperature?

Question

We say kinetic of molecules as a function of temperature from the equipartition theorem. This means that more is the velocity or mass of the body , more the is the temperature.

My Q is why does increased motion or mass of molecules contribute to heat , increased temperature ?

From this. Is it also possible to say that when velocity of molecules = 0. Then , temperature is at its coldest measure ? Since even at -20K or so. There must be some speed of the molecules where I can say that they are just slow.

I kind of also feel fire and ice also confuse with how they can be compared here ?

To conclude. All these few Q are a subtopic of my major Q i.e why does increased motion or mass of molecules contribute to heat , increased temperature.

Answer 1

IMHO you are still confusing heat with temperature (despite the answer to this question or this ).

My Q is why does increased motion or mass of molecules contribute to heat , increased temperature.

I will try to answer in a way that address that fundamental difference.

Consider 1 kg of water in a well insulated vessel which is travelling with 10[m/s]. However the temperature of the water is:

• 200 Kelvin: So water is in solid (ice) state
• 300 Kelvin (approx 27 $^oC$): so its mostly in liquid state
• 400 Kelvin (approx 127 $^oC$): so its mostly in gaseous state.

The speed of the vessel does not have an immediate effect on the temperature.

It does however contribute to the overall energy of the system (this is to reply to your question) (NOTE: please edit your punctuation in the question).

Is it also possible to say that when velocity of molecules = 0. Then , temperature is at its coldest measure?

Regarding the part of your question:

Since even at -20K or so. There must be some speed of the molecules where I can say that they are just slow.

There can be no negative temperatures. Even, if you could reach absolute Zero, then the energy of the system would never be entirely zero due to quantum effects ( I've had a theoretical Laser physicist try to explain it to me, but my brain shut down after hearing a few times the word quantum).

---

Update for comment

If I put ice on my skin. Is it that the molecules of ice are trying to slow down the velocity of the molecules of my skin.

When you put ice on your skin, the molecules of water don't have much internal energy, so they are barely moving in the lattice. When you put them in contact with your skin (which has a lot of water molecules at higher temperature), what happens is that they exchange heat .

I am not an expect on the exact mechanism that heat is exchanged, but I tend to think about it the classical sense of two balls of water molecules hitting each other. The result is that energy is transferred in the impact and some energy is imparted (statistically from the fast moving molecule to the slow).

Because of this exchange, the hotter becomes cooler, and the cold becomes warmer (until eventually the exchange statistically is the same).

---

UPDATE 2: Fan example

Another example , AC or fan. Do they try to reduce the velocity of molecules of your body.

The reason you get cooling with the fan against your skin, is that the fan pushes to your skin air which is slightly cooler than your temperature.

The reason why the exchange is more severe, is that the more cooler molecules will hit your body and exchange heat in a unit of time (see forced convection.

The fan by itself will actually increase the temperature of the air and the room.