Difference between thermal radiation and radiation?

[vitality00]

thank you in advance, i understand that all particles have "thermal motion" i believe (something along those lines) and thus they are always emitting radiation (even neutrons? <--- yes that is a question so answer that too please if you can) as long as it is above absolute zero...... and thermal motion means thermal emission of radiation (thermal radiation)

 

 

[Strange]

Particles do not always emit radiation. Only the acceleration of charged particles causes radiation to be emitted.

[swansont]

A collection of particles will emit thermal radiation, with some approximation to a blackbody spectrum with the power being given by the Stefan-Boltzmann law. It's electromagnetic radiation, i.e. light, which can be in the visible region if the surface is hot enough (incandescence)

 

Radiation in general is any energetic particle emitted by something, so that is not only EM, but also electrons, neutrons, protons and alpha particles.

 

—

 

Strange's point is most easily seen by considering photons or neutrinos. They would not emit thermal radiation.

[Strange]

A collection of particles will emit thermal radiation, with some approximation to a blackbody spectrum with the power being given by the Stefan-Boltzmann law.

 

I have never thought much about the relationship between this and the behaviour of individual particles. I assume this thermal radiation arises because individual atoms (and the charged particles within them) are constantly being accelerated by their interactions. That is pretty cool.

[swansont]

 

I have never thought much about the relationship between this and the behaviour of individual particles. I assume this thermal radiation arises because individual atoms (and the charged particles within them) are constantly being accelerated by their interactions. That is pretty cool.

 

That's my understanding, too — while an atom is neutral, when it collides (or vibrates in a lattice) there's an induced dipole moment, and that's the source of the radiation. The accelerations span a continuous spectrum, so BB radiation does as well.

[Sensei]

I have never thought much about the relationship between this and the behaviour of individual particles. I assume this thermal radiation arises because individual atoms (and the charged particles within them) are constantly being accelerated by their interactions. That is pretty cool.

 

Month ago I was test driving IR camera, were this was visible on 120 frames-per-second recorded movie (or real-time).

Optris PI160 http://www.optris.com/thermal-imager-pi160 (3.3k euro with VAT)

Software coming with IR camera were using Stefan-Boltzmann law to calculate temperature.

I was picking up location using mouse on still frame, or animation, or in real-time (camera observing hot object, sending data through USB to computer), and it's showing what temperature has that spot of image.

It is also showing graph, one axis temperature, second quantity/percentage of image.

 

 

I will have camera probably in December. Now making list of experiments to perform.

 

Of course it's not exactly "constantly being accelerated", because temperature of hot object is going down (as long as it's not heated from external source of energy). The more photons it emitted the closer temperature will be to temperature of environment, ~20 C for room temperature.

Once average temperature of environment is reached, there is reached thermal equilibrium, object can't release more energy, because hotter air molecules will give it back (you can place 0 C water in front of camera and see how it's going up to room temperature, air molecules are accelerating water molecules, and cold water molecules are decelerating air molecules, so soon "blue" color on IR camera image is spreading around where was cold object, also on the ground touching it).

[swansont]

 

Of course it's not exactly "constantly being accelerated", because temperature of hot object is going down (as long as it's not heated from external source of energy).

 

How can the atoms not be constantly accelerated? The center of mass is stationary, but the atoms are in motion. How can you possibly achieve that condition without internal motion that includes acceleration of the atoms?

[Enthalpy]

The random motion of atoms, electrons and so on, lets them radiate even at room temperature, then in the far infrared around 10µm. The emitted power is drawn from the heat stored in the body; particles that emit photons in a shock rebound less strongly. In space in shadow, a body cools down through radiation.

 

In a lukewarm environment, the body that radiates due to its temperature also receives heat from the environment, in the form or thermal radiation. Because heat doesn't move spontaneously between bodies at the same temperature, this implies that a body at the same temperature as its environment receives as much thermal radiation as it emits.

 

A consequence is that the body's ability to emit thermal radiation (the emissivity) equals its ability to absorb it (absorptivity), for any given wavelength, polarization - for any attribute that permits to filter the radiation. Though, the emissivity and absorptivity do differ at varied wavelength, so copper that absorbs some visible light but emits little infrared gets hot at sunlight (especially without atmosphere) while glass that absorbs little sunlight but emits infrared well gets cold.

[Sensei]

 

How can the atoms not be constantly accelerated? The center of mass is stationary, but the atoms are in motion. How can you possibly achieve that condition without internal motion that includes acceleration of the atoms?

 

I think you should go back to original sentence, that I was commenting:

"I assume this thermal radiation arises because individual atoms are constantly being accelerated by their interactions."

 

"constant acceleration" reminds me saying "releasing energy forever". But there is finite amount of energy in hotter object.

 

IMHO thermal radiation arise because hotter object is appearing in colder environment.

Acceleration of one molecule, and deceleration of other molecule is just method of transfer energy.

Hotter object is sharing energy with colder environment, until both have equal temperature (in idealized closed system).

 

We can place hotter object in vacuum, so there will be no direct interaction between it and colder environment, and the only way hotter object won't go to infinite temperature/infinite energy (which is obviously not possible) is to emit photons. Relatively cold hot object (in vacuum) will emit photons in infra red spectrum, warmer hot object will emit visible photons.

 

Our normal environment also is emitting thermal radiation, but it's equal with all objects.

Edited October 29, 2014 by Sensei

[Strange]

"constant acceleration" reminds me saying "releasing energy forever". But there is finite amount of energy in hotter object.

 

I agree that "continuous acceleration" is not clear. However, the atoms will continue to experience random accelerations even when the body is at room temperature; it will still radiate energy at the same rate it absorbs it from the environment.

[Sensei]

 

I agree that "continuous acceleration" is not clear. However, the atoms will continue to experience random accelerations even when the body is at room temperature; it will still radiate energy at the same rate it absorbs it from the environment.

 

Agreement.

[swansont]

IMHO thermal radiation arise because hotter object is appearing in colder environment.

 

I think this has been adequately addressed by Strange (and you are in agreement with that), but it's a concept that others might need to get a better grasp of: a body in radiative thermal equilibrium emits as much radiation as it absorbs. There's a perhaps subtle but important distinction between not radiating any net energy and not radiating any photons.