Does cold radiate outwards? What’s the process by which a hot object becomes cold?
In: Physics
Cold is the absence of heat. Heat always flows from a hot region to a cold region. That is an idea so basic that they had to go back and make it a law after they had already named the first law (energy can not be created or destroyed) so the law that heat flows from hot to cold is called the “zeroth law of thermodynamics”
So we know that a hot object will give its heat to all the cold things around it. There are three ways that heat can move. It can be conducted from one object to another just because the objects are touching. Also, heat will be conducted from the hot part of an object to the cooler part of the same object. To feel this in action, get a copper wire and heat one end. Copper is a great conductor of heat: you will feel the cool end warm up quite quickly.
The second way heat can move is by convection. That is when a fluid, like air or water, flows past the hot object and picks up heat as it goes by. A hot fluid can also transfer heat to a cooler object that it is flowing past. In fact a car radiator is a good example of convection: hot water flows through the tubes of the radiator and heats up the metal. Then cool air flows over the outside of the radiator and picks up the heat and carries it away. Convection inside and outside.
The third way to transfer heat is by radiation, and oddly, a radiator does not use this method. Radiation is the way heat from the sun gets to Earth even though the space between is a vacuum. The heat is carried by electromagnetic waves, including good old light, and waves not quite long enough for humans to see, called infrared waves. Light can travel through a vacuum, and it carries the heat with it. It is worth noting that to get any decent amount of heat by radiation, the hot object has to be really hot.
Heat radiates outward. Cold is a lack of energy in a certain environment. Heat will always go from it’s source into the environment, adding energy until all of it has been equalized. Think of it like this. If you sit near a fire, it warms you. But an air conditioner needs a fan to push the cooled air around. Or even if you sat around a big ice cube it wouldn’t cool you. It would instead pull in heat from the air around it and melt.
Heat is a type of energy (thermal energy). In this scenario you can think of it like water. It ‘flows’ from a hot thing to a cold thing in the same way that water flows downhill from a higher area to a lower one
But eventually, the water levels out. Same with thermal energy. So if there are two objects next to each other and one is 100 degrees and the other is 0 degrees, eventually they’ll both be 50 degrees.
Cold doesn’t radiate anywhere. Cold is just a lack of heat. In heat terms, you can think of cold as nothing – a vacuum. Heat always wants to move to where cold is.
A hot object becomes cold because its heat leaks into the cold. Everything around it technically becomes a little bit warmer, but that heat spreads out into the entire world, and it quickly becomes so diluted it makes no measurable difference for the most part. It’s a bit like if you took a cup of red paint and poured it into the ocean. At first you’d be able to see it, but soon it would get so mixed in with the billions and trillions of gallons of water in the ocean that you wouldn’t be able to see any red any more.
Cold isn’t really a thing that physically exists. An object is cold when it is a lower temperature (typically by a decent amount) than a reference point (often what’s comfortable for a person). When it’s in reference to humans, there’s the added caveat of it needing to be able to transfer heat relatively well (fun little experiment, put a piece of plastic in the freezer with a piece of metal and see which one feels colder after a few hours). Heat flows from areas of high concentration (hot things) to areas of low concentration (cold things).
The atoms and molecules in a substance are moving and have a certain amount of energy contained in them. On a macroscopic scale, this is thermal energy. So the molecules in a hot piece of metal are moving faster and contain more energy than the ones in a lower temperature piece of metal. This energy is lost through three methods.
Conduction: This is when a solid is in physical contact with another solid and the atoms transfer energy from one solid to another.
Convection: This is when heat is transferred by moving fluids at a macroscopic level. This can be a solid to a fluid (or vice versa) or fluid to fluid. At an atomic or molecular level this occurs by atoms and molecules bouncing off of each other.
Radiation: This is when individual atoms loose energy by emitting light (think of a hot piece of metal). At lower temperatures this isn’t much and is often way less than conduction and convection. At high temperatures, you get things like the sun. In space conduction and convection don’t really work, leaving radiation.
Cold is the absence from heat (energy). If something becomes cold it loses heat to something with even less heat.
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