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dogjones
30th December 2005, 07:19 AM
Could someone explain to me, in the most "reductionist" way possible, the difference between heat energy and light energy?

Ta!

Genesius
30th December 2005, 07:31 AM
Just different frequencies of the same energy. Heat is infra-red light - light with a wavelength longer than the red end of the visible spectrum.

Reductionist enough?

dogjones
30th December 2005, 07:34 AM
Really? Is that it? Infra-red = heat? But microwaves produce heat, do they not?

kmortis
30th December 2005, 07:39 AM
Really? Is that it? Infra-red = heat? But microwaves produce heat, do they not?
Nope. Well, kinda...microwaves vibrate dipole molecules (usually water) in the item being microwaved. When the water vibrates faster, it produces heat.

Microwaves are even higher frequency than infrared, but less than X-ray.

dogjones
30th December 2005, 07:39 AM
It is different, but everything that has heat emits infrared light.

I split this from "Help with ID discussion".

If the above is true and that heat & infrared are not equivalent but just "constantly conjoined" (apologies to Hume), then original question still stands please!

Genesius
30th December 2005, 07:42 AM
Nope. Well, kinda...microwaves vibrate dipole molecules (usually water) in the item being microwaved. When the water vibrates faster, it produces heat.

Microwaves are even higher frequency than infrared, but less than X-ray.

Nope - microwaves are the other end of the spectrum. From http://home.howstuffworks.com/microwave1.htm:

Microwaves are radio waves (http://home.howstuffworks.com/radio-spectrum.htm). In the case of microwave ovens, the commonly used radio wave frequency is roughly 2,500 megahertz (2.5 gigahertz).

Otherwise correct.

dogjones
30th December 2005, 07:45 AM
When the water vibrates faster, it produces heat.

And this heat is defined as...?


Microwaves are even higher frequency than infrared...

Begparding, but I don't think that's true... I'm sure they are lower frequency, between radio and infrared?

Genesius
30th December 2005, 07:47 AM
I split this from "Help with ID discussion".

If the above is true and that heat & infrared are not equivalent but just "constantly conjoined" (apologies to Hume), then original question still stands please!

Not sure what you mean by "constantly conjoined". Heat energy and infra-red energy are the same thing.

"Heat" is a measure of the motion of the molecules in a substance. The more the molecules move around, the more heat. The more heat, the more infra-red energy they give off.

Genesius
30th December 2005, 07:49 AM
Dogjones, for a some good info on the topic, have a look at http://encarta.msn.com/encyclopedia_761575286/Heat_(physics).html

kmortis
30th December 2005, 07:50 AM
And this heat is defined as...?



Begparding, but I don't think that's true... I'm sure they are lower frequency, between radio and infrared?

Nope - microwaves are the other end of the spectrum. From http://home.howstuffworks.com/microwave1.htm:



Otherwise correct.
BAH! You weren't supposed to notice that. THat'll teach me to be too lazy to turn around an look a the frelling freq chart I have hanging in my office. Gamn and blast.

Genesius
30th December 2005, 07:52 AM
BAH! You weren't supposed to notice that. THat'll teach me to be too lazy to turn around an look a the frelling freq chart I have hanging in my office. Gamn and blast.

Jeez, I'd think biomechanoids would have this kind of info preprogrammed. Return for software updating at once!

:D

kmortis
30th December 2005, 07:58 AM
Jeez, I'd think biomechanoids would have this kind of info preprogrammed. Return for software updating at once!

:D
Unfortunatly, my User hasn't seen fit to do so. Ratbastage.

For those in my same predicament (http://www.ntia.doc.gov/osmhome/allochrt.pdf)...

DrMatt
30th December 2005, 08:01 AM
Light is an electro-magnetic wave, specifically in a particular frequency range that triggers our eyes. Heat is the kinetic energy of moving molecules. Light moves at the speed of light. Unless heat is converted into another form of energy along the way, the fastest that heat can travel is the speed of sound.

hammegk
30th December 2005, 08:06 AM
Leave it to someone to furnish heat, not light, with convection. ;)

dogjones
30th December 2005, 08:10 AM
Dogjones, for a some good info on the topic, have a look at http://encarta.msn.com/encyclopedia_761575286/Heat_(physics).html

Marvellous - thanks.

dogjones
30th December 2005, 08:12 AM
Leave it to someone to furnish heat, not light, with convection. ;)

There are no convections stronger than religious ones - Gloria in Excelsius Deo.

hammegk
30th December 2005, 08:24 AM
You've never met an atheist.

DrMatt
30th December 2005, 08:25 AM
Carpus deum! Goldfish of the gods.

Xeriar
30th December 2005, 08:29 AM
Could someone explain to me, in the most "reductionist" way possible, the difference between heat energy and light energy?

Ta!


Heat energy is the internal vibration of molecules within a substance. It gives off blackbody radiation - normally infrared light - and that is how we 'feel heat'.

Though normally associated with infrared light, it doesn't end there. Stars are blackbodies. As the molecules vibrate faster, they give off higher and higher frequency light - red hot, white hot, etc. Eventually, at around 5000 degrees fahrenheit, molecules are no longer possible - they simply break down.

DrMatt
30th December 2005, 08:29 AM
You've never met an atheist.

The ones I have met have been warm. But so far I've never met a person who radiated visible light. Since recombinant DNA may have made it possible to splice firefly genes into the genes of other species, I don't rule out the possibility that some day there will be glow-in-the-dark people.

BillHoyt
30th December 2005, 08:38 AM
dogjones,

DrMatt's post (today, 11:01am) is the answer to which I'd alluded on the other thread.

dogjones
30th December 2005, 08:57 AM
The ones I have met have been warm. But so far I've never met a person who radiated visible light. Since recombinant DNA may have made it possible to splice firefly genes into the genes of other species, I don't rule out the possibility that some day there will be glow-in-the-dark people.

Doubtless a glow-in-the-dark person would be aurafully smug - he probably wouldn't even say "halo" to someone like me.

Ziggurat
30th December 2005, 08:58 AM
Heat energy is the internal vibration of molecules within a substance. It gives off blackbody radiation - normally infrared light - and that is how we 'feel heat'.

Not exactly. Heat is highly random energy. It comes in different forms: the vibrations of atoms in a solid, the kinetic energy of gas molecules, AND electromagnetic waves. Blackbody radiation isn't simply about energy being radiated away: the derivation of blackbody spectra comes from treating the vacuum ITSELF as having a temperature (and even a heat capacity per unit volume) based on the energy of photons zipping around in it. Blackbody radiation can be treated as heat flow from a hot object to a colder surrounding vacuum (even if it's got something transparent like air in it too), just as heat gets transmitted from a hot object to a cold one when placed in physical contact.

In other words, blackbody radiation is just another form of heat.

phildonnia
30th December 2005, 09:20 AM
Infrared light is not heat, although that myth is probably still being taught even today. This probably got started because radiant heat felt from warm objects is primarily from infrared radiation.

Heat is molecular motion. Light is an electromagnetic disturbance.

dogjones
30th December 2005, 09:31 AM
Infrared light is not heat, although that myth is probably still being taught even today. This probably got started because radiant heat felt from warm objects is primarily from infrared radiation.

Heat is molecular motion. Light is an electromagnetic disturbance.

Is it fair to say that in sufficient quantities, any frequency EMR can create heat?

TonyL
30th December 2005, 09:41 AM
Could someone explain to me, in the most "reductionist" way possible, the difference between heat energy and light energy?

Ta!

Well, the problem is that what we call "heat" is not necessarily due to one form of energy, or may not be synonomous with what we classically measure when we take a temperature.

If you want the most reductionist description possible, let's consider an ideal gas. A bunch of particles flying around and bouncing off each other. For this system, the "heat" is related to the average kinetic energy of the particles. Now, put this gas inside a container made of a crystal material. The gas particles bounce off the material and deliver a little of their energy to the material. This causes the component particles of the crystal to move. Because these particles are bound together, they vibrate and the vibration spreads to the other particles. For the container material, the "heat" is due to the average vibrations of all the particles. (which is again, a kinetic energy.) (liquid works along a similar line.) Now when we measure temperature, we generally measure the average energy of particles in a gas, liquid, or solid and call that heat. However, anything that can increase the kinetic energy of those particles will heat it up. So, if you shine light on the container of an ideal gass, some of the light will bounce off or be absorbed by the particles, increasing it's heat. Likewise, Since most of those particles have various charge distributions, when they vibrate or accelerate in any fashion (ie. change direction) they will radiate electromagnetic energy (light). The distribution of light will look something like the distribution of energy in the system (this isn't exactly true, but I'm staying overly simplified to convey the general idea.). For most objects at the temperatures we commonly encounter, most of the light will be in the infrared band. Hence, when an object radiates infrared radiation it cools down a little and a nearby object that absorbs the radiation heats up a little. We generally call this radiation "heat", because it's the stuff that an object gives off when it cools and the stuff that warms up a nearby object receiving it.

So basically, what whe call "heat" could be kinetic energy of molecules, or it could be energy from electromagnetic waves. It's really just any form of energy that produces the physical sensation on our sking that we call "hot".

well, that wasn't as clear as I'd wanted it to be, but hopefully it helped.

BillHoyt
30th December 2005, 09:45 AM
Is it fair to say that in sufficient quantities, any frequency EMR can create heat?
Actually, with the right frequencies, controlled just so, EMR can cool. Sorry to make life complicated, but this one goes back to the first experiments that cooled a single atom to absolute (or near-absolute) zero.

I'll go away now.

Xeriar
30th December 2005, 09:45 AM
Is it fair to say that in sufficient quantities, any frequency EMR can create heat?

As far as I know there is no substance that is perfectly transparent or reflective across any given frequency, so eventually, enough photons passing across a molecule, one will excite it.

It's a bit dodgy to make the comparison, though eventually it can be kind of moot. A red CO2 laser firing at an iron block in a dark room is essentially just creating heat, even if the iron is reflecting most of the light to be absorbed by other parts of the room.

dogjones
30th December 2005, 10:01 AM
Actually, with the right frequencies, controlled just so, EMR can cool. Sorry to make life complicated, but this one goes back to the first experiments that cooled a single atom to absolute (or near-absolute) zero.

I'll go away now.

Any idea where can I read about that?

BillHoyt
30th December 2005, 10:08 AM
Any idea where can I read about that?
Delighted to oblige your curiosity (http://prola.aps.org/abstract/PRL/v62/i4/p403_1)
The men who first achieved it (http://physics.nist.gov/News/Nobel/OtherSites/phyback97.html) (and whose (Phillips') article I first read in 1987.)

Ziggurat
30th December 2005, 10:24 AM
Actually, with the right frequencies, controlled just so, EMR can cool. Sorry to make life complicated, but this one goes back to the first experiments that cooled a single atom to absolute (or near-absolute) zero.

Which fits with my definition of heat as highly-random energy. That includes things like random molecular vibrations as well as the random spectra of blackbody radiation, but does not include laser cooling, where the very well-tuned frequency of the laser light makes its energy very non-random. Such non-random energy CAN produce heat (for example, absorb it with a hunk of black material and it will turn into random vibrations), but as in the laser cooling case, it does not need to.

dogjones
30th December 2005, 10:24 AM
Delighted to oblige your curiosity (http://prola.aps.org/abstract/PRL/v62/i4/p403_1)
The men who first achieved it (http://physics.nist.gov/News/Nobel/OtherSites/phyback97.html) (and whose (Phillips') article I first read in 1987.)

Fascinating, thanks. I go now to try and ram it into my woolly brain.

dogjones
30th December 2005, 10:27 AM
Thanks all, this has been most illuminating!