I'm studying black bodies for IB Physics and I'm having trouble understanding the full concept of black body radiation. I understand that a black body in thermal equilibrium with its surroundings will emit all radiation that is incident upon it. My question is, why must this radiation be emitted at all? Why can't the object continue to heat up from the absorbed radiation? If energy is transferred to the black body, why can't it's temperature increase rather than the energy being emitted? I'm barely starting to cover quantum mechanics, so my knowledge on the subject is bare which is probably the source of my confusion. Any help would be greatly appreciated. Thank you in advance.
[Physics] Why must black bodies in thermal equilibrium emit the amount of radiation absorbed
equilibriumthermal-radiationthermodynamics
Best Answer
The definition of thermal equilibrium is that at equilibrium there is no net transfer of heat, so it is this requirement that forces the blackbody to emit as much energy as it absorbs.
In general an object can heat up (or cool down) by absorbing (respectively emitting) radiation, but then the object is not in thermal equilibrium and will have a lower (respectively higher) temperature than its surroundings.