# Why does heat flow from hot to cold?

• PGaccount
In summary: The second law of thermodynamics is about the tendency of entropy (a measure of disorder) to increase in a system. The more collisions that happen, the more entropy will be created. This is why we see heat flow from hot to cold - because it's easier for entropy to increase in the hot than the cold.
PGaccount
What is the origin of the fact that heat flows from hot to cold? Does it have something to do with fluctuations?

This is part of the second law of thermodynamics. Entropy wants to reach equilibrium, to make the contents of the system a constant temperature.

Not sure what you mean by origin.

PrashantGokaraju said:
What is the origin of the fact that heat flows from hot to cold?

Isn't that just a definition?

weirdoguy
I'm not the person to explain the whole theory. You can ask Google or wait for another post here. But...

The answer here is essentially the same as this experiment: Take a box and carefully place many red and blue balls into the box so that all of the red balls are on one side and all of the blue balls are on the other side. Then shake the box. Observe the location of the different colors. If you imagine the red ones as hot molecules and the blue ones as cold molecules, this is a pretty good analogy.

Notice that the red didn't "move to the blue" and vice-versa, they just averaged out, moving at random to a new part of the box. It's not that the blue balls attract the red ones. It's just the nature of the world that when things are mixed up they tend towards uniform distributions.

Edit: Plus, it's the nature of the world that things do tend to get mixed up over time.

Consider a container with a removable wall in the middle. On the right are many gas molecules moving relatively slowly (cold). On the left are many gas molecules moving relatively quickly (hot). If we remove the wall the hot and cold molecules will begin to collide with each other. The hot molecules, upon collision with their cold counterparts, will transfer some of their speed and momentum and thus cool down. This means, of course, that the cold molecules heat up.

In order for heat to flow from the cold to the hot these collisions would require that the slow molecules move even slower after a collision and the fast molecules move even faster. This is an impossibility in isolated collisions, and a rarity in multi-body collisions.

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russ_watters, sponteous, Vanadium 50 and 2 others

## 1. Why does heat flow from hot to cold?

Heat flows from hot to cold because of the second law of thermodynamics, which states that heat will naturally flow from a higher temperature to a lower temperature. This is due to the fact that molecules in a hotter object have more energy and are moving at a faster rate, leading to collisions with molecules in a cooler object and transferring heat energy.

## 2. Is there any exception to heat flowing from hot to cold?

There are some exceptions to heat flowing from hot to cold, such as in the case of a refrigerator or air conditioner, where heat is actually being removed from a cold space and transferred to a warmer space. However, this is achieved through the use of external energy, such as electricity.

## 3. How does heat transfer occur?

Heat transfer can occur through three different mechanisms: conduction, convection, and radiation. Conduction is the transfer of heat through direct contact between molecules, convection is the transfer of heat through the movement of fluids, and radiation is the transfer of heat through electromagnetic waves.

## 4. Can heat flow in a vacuum?

No, heat cannot flow in a vacuum because it requires the presence of molecules to transfer energy. In a vacuum, there are no molecules present to transfer heat energy, so heat cannot flow.

## 5. What is the relationship between temperature and heat flow?

The greater the temperature difference between two objects, the greater the rate of heat flow between them. This means that the larger the temperature gradient, the faster heat will flow from the hotter object to the cooler object.

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