why is it easier to heat things up, than it is to cool them down?
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In a Physics sense it’s not, though if you look at gadgets meant to do the two things applying the Physics can be more complicated.
“Hot” and “cold” to Physics are just different amounts of energy. “Hot” things have more energy. “Cold” things have less. Let’s not worry about what kind of energy. Let’s just call it “heat energy” for simplicity. One thing we can exploit is something called “thermodynamics” explains the energy likes to balance. In ELI5 terms that means “hot things like to push their energy into cold things until they have the same amount of energy.” So to Physics, making something “colder” just involves finding a way to make it give heat energy to something else and making it hotter involves finding a way to make something else give it heat energy.
To make something hot, you have to add more heat energy to it. That’s pretty easy with electricity. If you force electrons through a material that has high resistance, the electrons will have to “push” harder and will lose some of the electrical energy. The energy isn’t really “lost”. It becomes heat energy stored in the material. We can do a lot of other things to create heat, like starting a fire or creating friction. Even our bodies generate a lot of heat. So “heat it up” devices usually seem pretty simple because all they have to do is convert some kind of energy into heat, which is usually pretty easy.
To make something cold, you have to remove heat energy from it. If we, say, wrap a hot thing in tubing and pump cold water through the tubing, some of the heat energy will move from the hot thing to the cold water. Then the cold water is pumped away so more cold water is on the hot thing and more heat moves out of it. As long as the water is colder than the thing, it will be “cooled” by the water.
But then we have a lot of hot water we have to deal with. If we pump that hot water back through the system, it’s not as cold so it may not be able to take as much heat energy on the next pass. So for this system to keep going, we have to figure out a way to get rid of heat in the water.
One way to do that would be to pump the hot water through some narrow tubes. The tubes suck in some of the heat, then the air around the tubes sucks heat out of them. That leaves colder water behind. Devices like this are usually called “radiators” and you’ll find them in cars and a lot of other devices meant to cool things.
Refrigerators and air conditioners use basically that idea, but instead of water they use a liquid that’s MUCH better at taking in and giving up heat called a “refrigerant”. One part of the system is the “evaporator”. It makes the refrigerant at lower pressure, which in Physics always makes things colder. Then it pumps that low-pressure cold refrigerant through coils that air is blowing over. The air is warm, and its heat gets sucked into the refrigerant through the coils. Now you have cold air. The warm refrigerant is pumped to a part of the system called “a condenser”. This squeezes it to higher pressure, which also makes it warmer. Then it runs that warm, high-pressure refrigerant through coils that air is blowing over. That air is cooler than the refrigerant so heat moves through the coils into the air and you’re left with cooler, high-pressure refrigerant. That gets pumped back to the evaporator and the cycle repeats.
So one end of a fridge/air conditioner makes cold air and the other end makes hot air. It’s making the cold air by moving heat from that air into the other air, and to do that it has to spend electricity to pump the refrigerant around and change its pressure. Modern heat pumps can reverse the direction so that it’s stealing heat from outside and pumping the heat inside.
It’s a complicated device to *build* compared to a plain old heater, but it works on very simple principles.
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