Is it more difficult to cool than heat?

Question

While it seems there is a lot of ways to generate heat, using mechanical, chemical or electrical engineering solutions, it seems that generating cold is more difficult. Most cooling devices that exist generate heat in the process of cooling.

This is a layman’s observation from day to day experience. And based on this observation, it appears as if there are less ways to generate cold than heat.

Is this observation and hypothesis correct or not? And if so why? Could you please point to some basic literature that would help me learn on the subject if possible?

Answer 1

Your observation is correct, though, as you might expect, it's usually phrased in more precise terms. The reason has to do with the Second Law of Thermodynamics. A good article on the subject is here (https://www.livescience.com/50941-second-law-thermodynamics.html), and I'll present an argument in my own words below.

The Second Law of Thermodynamics concerns the behavior of a quantity called entropy. Entropy essentially keeps track of the number of possible states a system can be in. You will often hear this phrased as "the amount of disorder in a system;" this definition works as long as you don't consider anything particularly crazy, but "amount of disorder" is not a quantitatively measurable thing, so it's not a particularly good formal definition. That said, either of those definitions should work to explain your observations, so feel free to use whichever one is intuitive for now.

With a little bit of applied statistics, it turns out that, in general, cold objects tend to be more ordered (i.e. they have less possible states they can be in). Hot objects, on the other hand, tend to be more disordered; since their molecules are moving faster, there are more states that the system has access to. If you assume that, on the microscopic level, every state of the system's molecules is basically equally probable (which is a good assumption for the vast majority of real systems), then you would probably conclude that eventually, a system, when left on its own, should be far more likely to end up in a configuration that covers a lot of possible states (which means it's in a configuration with higher entropy). Congratulations, you just derived the Second Law of Thermodynamics! Formally, it states that the entropy of a closed system does not decrease with time. Colloquially, it means that a system that is initially ordered (with, for example, separation between hot and cold) will tend toward disorder if left alone (where, for example, everything has warmed to about the same temperature). Creating a cold region is equivalent to creating a region of higher order, or lower entropy; such a process is statistically unlikely, so it's hard to do.

But there's another caveat we have to worry about. I was careful to state that the entropy of a closed system does not decrease with time; but it turns out that in order for entropy to stay exactly the same with time, you have to act on your system infinitely quickly, which is obviously impractical! So, for any real process taking a finite amount of time, the entropy of a closed system will increase. What does this mean, practically? It means that anything you do to try and, for example, create a cold spot, will inevitably have to waste some energy increasing the entropy of its environment. This wasted energy is precisely the heat that you observe.

Answer 2

Good question! One way to think of it though is that cold is the absence of heat. A body will come into thermal equilibrium with its surroundings. So at normal temperatures, a hot body will cool off if it's in a cooler environment. This is fine for hot objects and room temperature environments. In that case, it is much easier to heat sense object cools on its own. However, sometimes you want to cool objects to many degrees below room temperature. Maybe even 270 degrees below zero! Here you must cool the object itself or make an environment that is that cold. Hence in cases like this, cooling an object requires a lot of effort and energy.

Answer 3

Perhaps in a simpler way...

1. The second law of thermodynamics basically states that entropy tends to grow.
2. Entropy is a complex term to understand because it imply changes, but a good notion can be grasped thinking of it as "energy dispersal". Therefore, the second law states that "energy tends to disperse".
3. Hot has more energy than cold (yes, in adequate conditions). So, if you put two bodies in contact, heat will flow from the hot body to the cold one and not inversely. Why? Because the hot body has more energy than the cold one, and "energy tends to disperse".
4. If you try to cool a body, you are basically trying to extract its energy. This can be done only if you concentrate its energy on a second body. But that's against the law! (well, the 2nd of thermodynamics). That is what cooling things is difficult.
5. The only way to concentrate dispersed energy is by using more energy. You can profit from the fact that gases expanding absorb heat (due to volume increase), or in other words, expanding gases take energy as heat. So, put your body and an expanding gas in contact. Voilà!... Seems you didn't used more energy. But wait... How did you got pressurized gas? There it is. You invested energy in putting it under pressure.

This means that if you leave the door of your refrigerator opened in order to cool your home, you will end up with a hotter place. Concentrating energy requires more energy. That's difficult for us. And that's why you can't cool a body down to 0 Kelvin using this method (or others): any expanding gas will always have some energy.

Answer 4

From the Newton law of cooling the rate of heat observe by the body dQ is directly proportion to the difference in the surrounding temperature (Q1 - Q2) where Q1 is the temperature of the surrounding and Q2 is the temperature of body so the cold body have rate of observation high and it temperature increases rapidly and so we can not keep the cool object for long time but for heating process we have different source that helps to easily generate heat....

Answer 5

Heating increases disorder and cooling decreases it. The latter will always be more difficult purely due to statistics.