Do we feel heat only in the form of infrared waves?

[LogicalAcid]

Or can we feel heat in the form of visible light, not just infrared waves? Because I know heat is really kinetic energy (temperature), but why is it that we can't feel heat in the form of Radio waves or Micro waves, but then again Microwaves are used to heat food how? Main question hear is why can we interact with certain wavelengths of Radiation and not others?

 

[willem2]

We can feel heat in the form of visible light. In fact a large part of the rays of the sun are visible light, and that part will heat you up as well.
It's just that our eyes are very sensitive to visible light, so we might see a candle from 1 km away, even tough we can only feel the heat from 30 cm

 

[LogicalAcid]

We can feel heat in the form of visible light. In fact a large part of the rays of the sun are visible light, and that part will heat you up as well.
It's just that our eyes are very sensitive to visible light, so we might see a candle from 1 km away, even tough we can only feel the heat from 30 cm

Can you explain the physics of this? I feel I understand the basics.

 

[Phyisab****]

To try to reconstruct the absorption spectra of human skin from quantum mechanics is in practice nearly impossible. The reason heat is commonly associated with infrared radiation is because that is the em range in which most thermal radiation is emitted at roughly room temperature. All em radiation carries energy though, and it is reasonable to assume that many different wavelengths can be absorbed and felt by your skin, not just infrared. For an extremely rough idea of this, http://en.wikipedia.org/wiki/File:Water_absorption_spectrum.png . Obviously this does not correspond particularly well to your skin, given that skin is opaque and water is transparent. This can be seen in the low absorption intensities in the visual range.

 

[LogicalAcid]

To try to reconstruct the absorption spectra of human skin from quantum mechanics is in practice nearly impossible. The reason heat is commonly associated with infrared radiation is because that is the em range in which most thermal radiation is emitted at roughly room temperature. All em radiation carries energy though, and it is reasonable to assume that many different wavelengths can be absorbed and felt by your skin, not just infrared. For an extremely rough idea of this, http://en.wikipedia.org/wiki/File:Water_absorption_spectrum.png . Obviously this does not correspond particularly well to your skin, given that skin is opaque and water is transparent. This can be seen in the low absorption intensities in the visual range.

"The reason heat is commonly associated with infrared radiation is because that is em range in which most thermal radiation is emitted at roughly room temperature"

So at room temperature, there is more infrared radiation contacting me than visible light? Why Infrared radiation?

 

[cdotter]

Or can we feel heat in the form of visible light, not just infrared waves? Because I know heat is really kinetic energy (temperature), but why is it that we can't feel heat in the form of Radio waves or Micro waves, but then again Microwaves are used to heat food how? Main question hear is why can we interact with certain wavelengths of Radiation and not others?

Microwaves work by creating a changing electromagnetic field. Water is a polar molecule with one side more negatively or positively charged than the other. The water molecule continually rotates to be parallel with the changing field. This movement = heat.

 

[LogicalAcid]

Microwaves work by creating a changing electromagnetic field. Water is a polar molecule with one side more negatively or positively charged than the other. The water molecule continually rotates to be parallel with the field. This movement = heat.

Thats why they work with foods with H, or basically all?

 

[cdotter]

Thats why they work with foods with H, or basically all?

It doesn't work as well with less-polar molecules like fats.

 

[LogicalAcid]

It doesn't work as well with less-polar molecules like fats.

Due to...?

And why do microwaves change an electro magnetic field? Doesnt all rad do that?

 

[cdotter]

Due to...?

They aren't as polar. This rotation is more pronounced when the charges are larger, meaning the molecule is more polar.

 

[cdotter]

Due to...?

And why do microwaves change an electro magnetic field? Doesnt all rad do that?

You might be right. I'm just parroting what my physics professor told me.

From my understanding the field has to be a changing field. If it wasn't changing then the molecules would simply align, albeit with some oscillations about the equilibrium. A changing field ensures that the molecules are continuously in motion which in turn causes the temperature to continually rise (to a point.)

 

[LogicalAcid]

You might be right. I'm just parroting what my physics professor told me.

From my understanding the field has to be a changing field. If it wasn't changing then the molecules would simply align, albeit with some oscillations about the equilibrium. A changing field ensures that the molecules are continuously in motion which in turn causes the temperature to continually rise (to a point.)

Changing field? doesn't that mean the charge has to be able to move?

 

[cdotter]

Changing field? doesn't that mean the charge has to be able to move?

Maybe this picture will better explain it. A dipole can be thought of as two charges, one less negative or positive than the other, separated by a small distance.

[PLAIN]http://physicslearning.colorado.edu/PiraHome/ResourceCD/ResourceImages/PhysicsDrawings/Dipole_E_Field.gif

A torque from the force causes the dipole to rotate. If the electric field (or electromagnetic field) doesn't change orientations, the dipole will simply align to be parallel with it. A changing field means that the dipole will always try to rotate to be parallel with it, which means that the temperature will continue to rise (to a point.)

 

[DaleSwanson]

Have you read the wiki article on black body radiation?
http://en.wikipedia.org/wiki/Black_body"

Note that all objects with any heat energy emit electromagnetic radiation. The type and amount they emit depends on their temperature. As temperature increases, two things happen. First the total amount of radiation released increases. Second the peak of the curve of the type of radiation shifts towards the higher frequency, higher energy radiations.

So, low temperature objects emit primarily radio waves, warm them up and the peak becomes infrared, then visible light, then UV, X-ray, and finally gamma. The reason why we don't feel heat from radio waves is that the total energy is so low that we don't notice it (but it is still there). Similar to how you will not notice any heat from a single small LED light even if it is emitting the same frequency light as the Sun which you can easily notice.

 

[Lsos]

Not sure if that's the intent of the question, but I don't think you "feel" infrared radiation. You feel an a temperature on your skin. It just so happens that radiation is one way that can increase that temperature.

 

[thopsy]

Here's something quick and dirty, and also very useful. It will give you the wavelength corresponding to the maximum blackbody spectral emissive power for a given temperature, i.e. which portion of the EM spectrum emits the most energy at a given temperature. It's called Wien's displacements law, and is a basic result of the Planck distribution:

lambda*T = 2898 um-K

Where lambda is wavelength, T is temperature, um is micrometers, K is Kelvin.

So if you have a blackbody at T = 5780 K (the sun), the emissive power peaks at 0.5 um = 500 nm.

As a side-note, this is very close to the maximum sensitivity of the human eye. Is it a coincidence? Of course not, it is a result of biological evolution!

 

[Sabato]

Can you explain the physics of this? I feel I understand the basics.

All electromagnetic radiation is denominated in values of energy and according to the individual properties of matter, some frequencies of radiation get absorbed more or less than others. Some of the absorbed energy is used to do "work"(maybe cause a molecule to move a certain way, or to excite electrons). Not all the absorbed energy is used and a "certain" amount is converted to heat. Heat is just a method for transferring energy. Radio-waves, microwaves, infrared, optical waves, x-rays- all can, to a more or less extent (again, depending on the properties of the matter absorbing them) be transformed into heat. Infrared is not heat: it like the other radiant forms of energy can get transformed to heat. As far as a fire-place is concerned, you do not feel the infrared radiation from it. The temperature of it is radiating infrared radiation, and optical light as well. The heat of the fire constitutes a concentration of energy that is great than that of the room; the heat conducts to the surface area of the air it is in direct contact with; then this air begins to move with this heat, distributing it as it moves. This is transfer of heat energy by convection. Hope this helps.