Heat radiation...

[Externet]

How does heat radiate ? What is that emission of heat ?

Is it some sort of rays, electromagnetic waves, particles, molecular vibration ?

 

A body placed between a radiated heat source and its target can block or absorb it. What is actually traveling trough the air from the hot body to the being heated one ?

 

Miguel

[swansont]

Electromagnetic radiation. All materials have some amount of free charge that radiate when they undergo acceleration, which happens due to collisions or vibrations. If the material is hotter than the surroundings, it will radiate more energy than it absorbs; this varies with the difference of the fourth power of the temperature. In equilibrium, it absorbs the same amount as it radiates.

[insane_alien]

heat can propagate in 3 ways, convection, conduction and radiation.

in convection hot gas/liquid/plasma moves up and cold gas/liquid/plasma moves down. there are usually atleast 2 conduction/radiation steps for this to keeps cycling

 

in conduction the heat is transmitted by molecular vibrations. if there is no matter however the heat transfers by the emmission of infrared or in extreme cases like stars UV/vis and some Xrays as well

[5614]

Mind if I expand?

 

Conduction: When something is hot the molecules are vibrating. If these vibrating molecules touch something else they will transfer some energy to it or make those molecules vibrate. Remember that vibrate = heat, so now the heat has been transferred. If you touch a hot rod and your hand gets hot it is because the molecules of the rod were vibrating and then when you touched it made the molecules in your hand vibrate.

 

Convection: This only happens in a fluid (liquid, gas, plasma etc.) not a solid though. Basically when molecules are hot in a fluid they move around quickly. The hotter they are the quicker they move. So if you have a load of fluid then the hotter parts will be moving quicker, and so the hotter molecules will be more spaced out, and so the hot areas will be less dense and so they "float" and rise to the top. Or if you want to think of it more simply then you can imagine the slow moving molecules which are all close to each other and the fast moving molecules all a long way apart, the cold, slow moving ones would then naturally sink to the bottom.

 

Radiation: When anything is hot it will release electromagnetic waves. This is often infra-red.

 

Conduction only happens when you touch something - you could argue that you can "touch" things through the air, so the rod makes the air molecules vibrate which then make your molecules vibrate.

 

Convection only happens when the molecules can move, ie. not in a solid.

 

Radiation is like giving off light. It can go through a vacuum and requires no physical contact. It is how we get heat from the sun.

[Externet]

Thanks all for the explanations.

I was interested in the radiation form of heat transmission only, it is difficult to understand how simple friction of two electrically inert bodies can radiate electromagnetic waves (heat).

[5614]

It's because if an atom has a lot of energy then it's electrons are in a higher energy level - you can think of it as the electrons have jumped up to a higher shell. However electrons would prefer to be as low as they can possible be, so they jump back down, in doing so they release energy, in the form of a photon.

[swansont]

It's because if an atom has a lot of energy then it's electrons are in a higher energy level - you can think of it as the electrons have jumped up to a higher shell. However electrons would prefer to be as low as they can possible be, so they jump back down, in doing so they release energy, in the form of a photon.

 

That's not what's going on, though. A blackbody spectrum is continuous and typically at much lower energy than the first excitation of most atoms or molecules.

 

Atoms and molecules are (usually) neutral by themselves, but during a collision, they will form dipoles, and it's during this interaction (i.e. the acceleration) that they radiate (and note that molecules can have a permanent dipole moment). So there's a charge distribution present, and AFAIK this is a source of radiation for materials without free charges. In metals, which are a lot closer to ideal blackbodies, you do have free charges that undergo collisions, even though the material itself is neutral.