How does it happen that i just put a bottle of water in the fridge and when i take it out it’s cold? How can we just “charge“ stuff with coldness? Warmth is energy but what is it when something is cold? Energy aswell?
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Your fridge is basically a machine that pumps heat out of its insides. The things in your fridge bleed their heat to the air in the fridge, the fridge cools the air in itself by transferring its heat to the outside.
It does this by basically taking advantage of the evaporation and condensation of a fluid. If you want to learn more about that, [how stuff works](https://home.howstuffworks.com/refrigerator.htm) describes it better than I would.
Cold is just a lack energy.
When something is hot that means it has a lot of energy.
When something is cold that means it has less energy.
So the way we make something cold is by sapping away all the warmth from an object. That’s why you can feel hot air blowing out of your fridge, that’s all the heat that has been sapped away from the inside of the fridge.
Technically, we don’t make things cold; we remove the heat from them. Of course it’s the same thing in practical terms, but yeah. The refrigerant in the coils absorbs the heat and then takes the heat outside the refrigerator.
Let’s talk about warmth. It’s really “heat” in physics. You’re right that it is energy. Where does that energy come from? Well, everything is made of particles like atoms and molecules. They move around. If they move around faster and more chaotically, that’s more heat energy. A quirk of Physics is things don’t “get cold”, they “lose heat”. But if I say “gets cold” you know what I mean.
If you put a hot glass of water in a cool room, those fast, chaotic particles bump into the air around them. The air near the glass will feel a little warmer. But there is a LOT of air, and its particles are moving more slowly, so each moment that passes more collisions happen and the water loses a little bit of its heat energy into the air because when the water particles “push” on the air particles they slow down a tiny bit. Given a long enough time, the water and air will be the same temperature and stop “pushing” on each other.
That’s what happens in a fridge, but how did the air inside get cold? Most use the same basic principle as air conditioners. The loud part of a fridge is a system that uses a motor to pump a special fluid under pressure through a series of pipes. At one part of the pipe, the fluid is forced to fill a bigger space, which causes a big pressure drop. Since this makes the particles farther apart, and that means less collisions, this makes the fluid a lot colder. This cold fluid is pumped through some coils and fridge air is blown over it. If the fridge air is warmer than the fluid, its particles collide with the colder coil and the fridge air loses a little bit of heat. Then the fluid gets pumped back outside the fridge and repressurized. Cramming it into a smaller space makes the particles bump together more and it gets a lot warmer. Then it’s pumped through some coils outside of the fridge, where the heat moves from the fluid to the air inside your home. (Yep, a fridge heats your home! That’s why leaving the door open is bad, and why air conditioning units have a part that is outside so the heat can go there.)
Or, put a shorter way: hot things in a fridge lose their heat to the colder air. When the air gets warm, the fridge starts running the fluid system. That system takes heat out of the fridge and moves it outside of the fridge.
“Temperature” is a measure of how much thermal energy something has. “Cold” is not a thing, it’s the absence of heat aka the state of having low thermal energy.
When you put a bottle of water into the fridge, it loses thermal energy into the cold surroundings and therefore has less energy remaining itself.
When you take out the cold bottle (remember, cold just means having less heat), the room loses heat into the bottle, so the bottle gradually gains heat and warms up.
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