I'm a high school student, I just read something about black body. So I wanna know if I understand it correctly that black body is an ideal perfect absorber and emitter in sense that
a normal object, which is not under a thermal equilibrium condition, absorbs energy carried by EM radiation and emits them again. The object can't absorb all the energy from EM and neither emit all the energy. However, a black body can absorb and emit energy with highest efficiency.

If what I understand is correct, when an object is not under thermal equilibrium condition, it will not emit all the energy it absorbs, so does it just store the energy? In what form? Kinetic energy of an electron?


1 Answer 1


Matter in all its forms has a temperature, which can be measured in various scales. This temperature expressed in degrees Kelvin is directly related to the average kinetic energy of the atoms and molecules and lattices that compose matter.

average kE

. $E$ is the mean kinetic energy in joules (J)

. $k_B = 1.3806504(24)×10−23 J/K$ is the Boltzmann constant

. $T_k$ is the kinetic temperature in kelvins (K)

This kinetic energy is in the form of vibrations and rotations in the solid or liquid.

At the given temperature, any body of matter will emit radiation. As charged atoms and molecules vibrate, they interact with the spillover fields of each other and emit electromagnetic radiation, i.e., photons, characteristic of what is called the black body spectrum.

The Stefan-Boltzmann law tells us how much energy is radiated away :


where $j*$ is the total power radiated per unit area, $T$ is the absolute temperature and $\sigma = 5.67×10^{−8} W m^{−2} K^{−4}$ is the Stefan–Boltzmann constant

In space without any incoming radiation, a body will finally cool to absolute zero.

If incoming radiation falls on the body it will absorb it and distribute it as kinetic energy in the various degrees of freedom of the atoms, molecules and lattice, raising its temperature.

A body in a heat bath will be in equilibrium, radiating and absorbing the same amount of energy and keeping a constant temperature.

Note: gases behave with a different energy loss functional form than the SB above, because the degrees of freedom for vibrations and rotations are less in gas and it is only the translational degrees and the soft collisions that are responsible for the radiation of photons and the diminution of kinetic energy and subsequent cooling.


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