Eli5: why is it so easy to make something hot, but it’s a lot harder to make something cold?


Like, I can just rub my hands together and make them hot, but I don’t know how to make stuff colder

In: Physics

Coolness is actually a lack of energy whereas heat is a concentration of energy. Entropy (the eventual breakdown of matter ie why icecream melts and coffee gets cold) to cool something requires that you displace the energy in something whereas heating something just requires adding to it. I suppose it’s easier to add energy than to displace it.
Hope this helps!

Under the right circumstances you _can_ make stuff colder just as easily. It comes down to _thermal conductivity_, and material-material thermal transfer. It comes down to what materials swap thermal energy most efficiently.

When you hold a flame or candle up to a metal thing, the heat byproduct of the chemical reaction going on in the candle is directly transferring to the metal from the excited molecules of the chemical reaction’s waste byproducts and ambient air heated by the reaction as well. Remove that heat and the metal can only transfer the thermal energy into the air – and it turns out air isn’t a very good thermal conductor. Case in point, sticking your hand in a hot oven takes a while to hurt your hand – air doesn’t transfer the heat to things like your hand very well. But metal to hand transfer is really efficient, so when you touch a baking sheet or the side of the oven you get burned (very quick thermal transfer).

So if you need to heat something very fast (but only up to 100^o C) you’ll do it faster if you boil some water and put the thing in the hot water – water transfers heat much better.

Likewise if you want to cool something quickly, yes, you can put it in your freezer, but unless its resting on something that is already cold, the heat in your thing will transfer slowly to the cold air of the fridge. To speed things up, wet a towel with water and wrap your thing in that, then make sure the towel makes contact with the sides of the freezer. The water will lose its thermal energy to the sides of the freezer very fast and in turn absorb the thermal energy of your thing a lot faster than the cold air will. So pro-tip, if your beer is warm, dunk it in a sink of ice water, or wrap it in a wet towel before you put it in the freezer.

We’re good at moving our bodies which generates friction which adds heat to a thing.

If you got really good at taking energy away from a thing (ex. blowing air to take heat away) you’d have some really *cool* skills.

When we deal with thermodynamics, one way to define temperature is average kinetic energy.

Imagine a box with a lot of little balls bouncing around in it like a game of brick breaker. Shaking the box around will most likely increase the speed of the balls on average. To slow them down you need to be much more careful and precise – you can’t just hit them randomly.

What’s an easy way to slow them down then? Put some slower balls in the box! Over time, the fast balls will give off energy to the slow balls until they even out somewhere.

This is the phenomenon you are asking about: fast = hot and slow = cold. Almost every interaction that isn’t fine tuned will add heat to the system and raise the temperature. Lowering it requires either a fine tuned process or using a colder material to reach equilibrium in a temperature mid point.

It’s natural for things to become the same temperature. If you put a cold thing in a hot environment it becomes hot. If you put a hot thing in a cold environment it becomes cold. In that sense they are equally easy.

But like you said, you can rub your hands and make them hot. That is because it’s possible to convert potential energy (chemical energy in your body from food, in this case) into heat. This conversion of useful energy into heat is natural. But it is not natural for heat to turn into useful energy. You won’t see decayed matter turn into food that you can eat, and you won’t see water and carbon dioxide turn into sugar. Those processes aren’t natural and need an input of useful energy (like sunshine giving energy to a plant to make sugar through a complex system) and generate heat in the process.

It’s easy to make things hot because energy naturally turns into heat. It’s hard to make things cold because any energy you use in the process generates heat and you have to be thoughtful about redirecting that heat away from the thing you want to make cold.

Or put another way, making a thing cold requires making something else hot. But you can make something hot simply on its own.