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The One and Only Quantum of Heat

Discussion in 'General Electronics' started by Anomaly Magnetism, Oct 9, 2003.

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  1. I would be a bit more careful. Empty space, (or the thin plasma that passes
    for empty space here abouts), doesn't have a nice polite temperature, but
    heat sure as heck leaves through it at night, and arrives through it in the
    daytime. I would say heat travels through space well enough. It just has to
    move to radiation to do it. I have yet to figure out just how one would
    define a single temperature for a vacuum in any consistent manner, but the
    physics is straight enough.
     
  2. daestrom

    daestrom Guest


    Well, I would say 'energy' travels through empty space. The fact that such
    energy *often* is converted to 'heat' (molecular kinetic energy) when it
    interacts with matter is true. But *not* always. Some e-m radiant energy
    can be converted to chemical energy directly. (photosynthesis comes to
    mind). Or in a PV cell to another form of electrical energy.

    The fact that e-m radiation *often* is converted to heat, doesn't mean they
    are the same thing, any more than the kinetic energy of a car (often
    converted to heat through brakes and turbulance) is heat.

    daestrom
     
  3. ....
    The question you have to ask is whether there is a sensible "temperature".
    Sitting in the hot sun, it would be nice to have a temperature to attribute
    to the discomfort, eh? As I said, the physics is easy, but finding a
    definition of the sensibility of "hot" may be a bit more complex. Perhaps it
    just doesn't work. If you refuse to include radiant energy as heat, then
    most of the heat transfer phenomena that require radiation to be included
    are not longer simply classified as heat transfer, in spite of the
    sensibility that hot moved from here to there. I think the question is not
    the physics, which doesn't care whether you call radiant energy heat or not,
    but the human propensity to simplify the problem and model it in terms of
    common experience. It would be desirable if heat transfers could be
    associated with a form of heat, just to make it clear and match our senses.
    Not necessary, but nice.
     
  4. daestrom

    daestrom Guest

    There can be no 'temperature' unless you have matter. Then you have e-m
    radiation interacting with matter.

    One of the falacies of the lay person is in describing how 'cold' space is.
    Except for the very sparce particles, there isn't any matter to 'have' a
    temperature. But put any *thing* in space and its temperature will be
    driven towards the point where radiant input matches radiant output. So in
    the shadow of the earth, energy input is small and the *thing* loses energy
    until its radiant output matches the small radiant input. This doesn't
    happen until its temperature is low. And conversely, when exposed to full
    sunlight, its temperature rises until it radiates the same amount of energy
    out as it absorbs.
    People talk of converting potential energy to kinetic and back again by
    working in a gravitational field all the time. It isn't that much to talk
    about converting molecular kinetic energy ('heat') into e-m energy and back
    again.

    It is true that thermal radiation is discussed with conduction and
    convection in many texts. It is, after all, a fundemental way that objects
    gain/lose heat. Just as 'friction' is a fundemental way objects lose
    kinetic energy. You don't see people trying to call the energy transfered
    from a rotating shaft to a stationary bearing, 'kinetic energy'. We say the
    kinetic energy is transformed into another form (heat).

    The fact that every piece of matter (above absolute zero) radiates e-m
    radiation doesn't make e-m radiation 'heat'. Not anymore than the fact that
    anything suspended above the floor means potential energy is kinetic because
    it can easily be converted (let the object fall).

    Infra-red 'radiant heaters' simply radiate a lot of energy towards whatever
    it is aimed at. If the object is highly reflective to the IR, it doesn't
    really get hot. A perfect example is the reflector mounted on the back of
    many such heaters. Only when the radiated e-m is absorbed by an object is
    it converted to heat.

    Ah, well, I suppose to most it's just symantics. It is a subtlty that can
    be 'glossed over' most of the time with little loss in accuracy.

    daestrom
     
  5. You do not need material particles. For example, a perfectly conducting (I
    know that is an approximation) cavity can contain a gas of photons with a
    temperature.

    Bill
     
  6. AES/newspost

    AES/newspost Guest

    Radiation -- specifically blackbody radiation -- can absolutely have a
    totally meaningful and physically significant temperature.

    The temperature of the radiation in a region of "empty" space will come
    to thermal equilibrium -- or at least, move toward thermal equilibrium
    --with the temperature(s) of the matter that surrounds the region, or
    with which the region interactions.
     
  7. It is important to understand that heat and temperature and two
    *totally* different things.
     
  8. Temperature doesn't. Heat does.
     
  9. In an old classic text by Kennard and Rictmeyer, a gas of photons is used as
    the working fluid in an engine. The cylinder is made of perfectly reflecting
    material. It is shown how by doppler shifting off of the moving piston, the
    black body character of the photons is maintained. The temperature, however,
    changes according to the work injected or extracted by the piston.

    Bill
     
  10. Folrget the heat! it is energy that transfers through empty space. In the
    enc, except for understanding the process, the detail is unimportant. Energy
    is conserved. Please do not throw relativity at me to claim otherwise.

    Bill
     
  11. C-O-R-R-E-C-T-I-O-N:

    It is important to understand that heat and temperature ARE two
    totally different things.


    I wrote in flaw:
     
  12. Leo Guinto

    Leo Guinto Guest

    Hi, I am a student at Beliingham Technical College in Bellingham, WA.
    I'm currently doing some research on latest news information on the
    development of Quantum Computing. If possible, if you can provide me
    with any brief information regarding my subject and any Internet
    resources site links that you may know of. Any information you provide
    is greatly appreciated! Thank You!
     
  13. daestrom

    daestrom Guest

    I'm sorry, at what point did I confuse the two? Of course they are
    different things. We were discussing how accurate it is to consider e-m
    radiation (a form of energy) as 'heat' (a different form of energy).

    The energy profile of the e-m radiation given off from an object is a
    function of its temperature. And a temperature change is *one* of the
    parameters in measuring the change of heat energy in an object.

    daestrom
     
  14. daestrom

    daestrom Guest

    Here I disagree. It *is* true that a broad spectrum of photons are said to
    have a 'temperature', but that isn't quite right. What is meant by this is
    that the energy distribution of the e-m radiation '...is similar to that
    radiated by an ideal blackbody of a particular temperature.'

    Since 'temperature' is an affect of molecular kinetic energy, and photons
    have zero rest mass, they cannot have 'kinetic energy' in the traditional
    sense (1/2 mv^2).

    Even deep space has low-level e-m radiation from the Big Bang. These
    microwaves are said to be at 'temperature of approximately 3 degrees
    Kelvin.' But what that means is the temperature profile of this radiation
    is similar to that of a 3 degreeK blackbody. Any body, placed in such a
    field, regardless of emissivity will tend to reach that temperature over
    time (assuming its own radiation is not reflected back towards itself
    perfectly).

    Like I said, it is mostly semantics. But e-m radiation has a frequency
    profile, or specific-energy photons, not a temperature. It only is useful
    to talk of 'temperature' if the energy profile is similar to that of a
    blackbody radiator of that temperature.

    Take for example, a radiation field of one specific energy (perhaps a radar
    antenna or microwave). Would you say it has a 'temperature' that
    corresponds to the energy level of these photons? If you put a body in this
    field, it will warm until its emissions equaled the total energy absorbed
    (assuming for discussion there is no conduction or convection cooling the
    object). But since the object radiates over a broad spectrum, and the
    incoming energy is just a narrow frequency/wavelength, it's equilibrium
    temperature is *not* the 'temperature' of the radiation. Not very useful to
    say the radiation temperature is 'X', but any object in equilibrium with
    that radiation source is 'Y'. Nor to say that sometimes you can
    characterize radiation by giving a temperature reading, and other times you
    cannot.

    In fact, if the field strength is high enough, you could heat the object
    above the 'temperature' of the incident radiation. Simple proof is to heat
    a cup of coffee in a microwave. The 'temperature' of microwaves is far
    below freezing, yet there is your nice hot cup of coffee.

    daestrom
     
  15. daestrom

    daestrom Guest

    No, heat does not travel through space. It doesn't 'travel' except by
    phonons (not photons) between molecules of matter or by moving the matter
    itself. What travels through 'empty space' is e-m radiation. And it is
    only converted to heat if it is absorbed by matter.

    daestrom
     
  16. AES/newspost

    AES/newspost Guest

    True -- at least, it's only rigorously valid to talk of temp in this
    case -- but various possible other responses to this:

    1) Rigorously interpreted, temperature only applies -- to anything, of
    any sort -- in equilibrium. Things tend to move toward equilibrium, but
    never get there. The universe is not in equilibrium, nor are any
    subcomponents of it (since nothing is really *absolutely* isolated from
    everything else. Temperature is, therefore, a meaningless concept, not
    just for radiation, but for anything?

    2) The "heat" in a hot solid (or gas) equally implies a frequency
    profile, or "specific-energy phoNons" in your wording. This profile, in
    a gas or solid sample, is usually close to the expected blackbody or
    Maxwell or Boltzmann or whatever distribution corresponding to a given
    temperature, but can also be slightly different from it.

    3) Useful calculations can be, regularly are, done treating e-m
    radiation as a kind of substance: blackbody radiation in an enclosure
    has a certain specific heat, entropy, all the other thermodynamic
    quantities, including temperature. You can take two radiation baths at
    different temperatures; use the hotter one to pump an idealized maser or
    laser transition; use the other one to cool the "idler" transition of
    the masing or laser system; and the conversion of heat to energy will
    obey What's-his-name's principle of conversion of heat to work.

    In short, thermodynamic concepts, including temperature, apply with
    equal rigor (and equal usefulness) to radiation or matter
     
  17. E-M radiation at infrared frequencies?

    This is confusing. So many science books say that infrared radiation
    is heat radiation. Yet, medical journals dealing with radiation injury
    state that exposure to higher frequencies results in further
    concentration of generated heat in the surface of the skin, while
    lower-frequencies allow generated heat to dissapate faster.

    Higher-frequencies have a poorer Dissapation-to-Generation Ratio (DGR)
    than lower-frequencies.

    "DGR" is the amount of the heat dissapated versus the amount of heat
    generated.

    Given a constant amplitude (wattage), the overall amount of heat
    produced is the same. However, due to the poor DGR of short-wave
    radiation, burns associated with higher-frequencies often show further
    thermal protein denaturation than burns associated lower-frequencies.
    Then I take it there is no way of measuring heat without the presence
    of matter.
     
  18. daestrom

    daestrom Guest

    I'm not sure you're understanding my use of the word 'phonon'. It has
    nothing to do with e-m radiation. It is a generally accepted term for the
    'waves' of kinetic energy transfer that occur *inside* crystalline
    substances. The transfer of kinetic energy from one molecule to the next
    (heat conduction) can be treated as a transfer of 'phonon's.

    The e-m radiation from an object, as you say is distributed in a shape close
    to that of the 'black-body' radiation predicted by Maxwell-Boltzmann. The
    slight differences from the 'ideal' are what make spectroscopy possible
    since the differences are unique for each chemical/elemental compound.

    But don't you still find it 'curious' that we can use microwaves, which many
    say are at a 'temperature' of only a few degrees K, to heat things to
    several hundred K?? So the 'temperature' aspect of e-m radiation is only of
    limited usefulness. Even in your maser/laser setup, the two baths must be
    of the same spectral 'shape' for your temperature calculations to work out.
    One cannot be a narrow spectrum created perhaps by a magnetron or x-ray and
    the other by thermal radiation. If they are different, then the
    'temperature' of each is not representative of their total energy
    influx/outflux.

    The underlying *assumption* when assigning a 'temperature' to e-m radiation
    is that it is broad-spectrum profile close to the Maxwell-Boltzmann
    distribution. Forget that assumption, and one can easily come up with
    garbage for a calculation.

    daestrom
     
  19. daestrom

    daestrom Guest

    E-m radiation from a thermal source is broad-band and contains a whole range
    of frequencies. The 'peak' of the distribution is a function of the body's
    temperature and is often in the infra-red but not always. Take for example
    the Sun. It radiates a wide range, the 'peak' distribution is in the
    'visible' light range of frequencies (evolutionary biologists might argue
    that the 'visible range' is only called that because we evolved eyes to use
    the most prevalent frequencies available from the Sun, but that's another
    story ;-)

    That is a simplification. Infra-red is e-m radiation, like any other except
    it is considered to be of a particular range of frequencies. In early
    works, each 'band' of e-m radiation was given a name by the folks that
    worked with it the most. 'Radio' waves, 'infra-red', 'visible', 'cosmic'
    are all names for e-m radiation of a general frequency range. Certain
    things about all of them are the same, other aspects change as the frequency
    of the e-m changes. All travel through vacumn, all are composed of
    alternating electric and magnetic fields at right angles. All show
    properties similar to 'waves' and 'particles' (called photons with a 't').

    But different frequencies carry different amounts of energy for each
    'particle'. The way each frequency interacts with matter is somewhat
    different. Some frequencies pass through most matter with attenuation
    determined by the density of the matter (x-rays). Other very narrow
    frequencies are reflected from objects to your eye giving different objects
    a different color. ('reflection' is even a simplification, but lets not get
    *too* complicated here)
    Higher frequency e-m radiation carries more energy per photon. In the range
    of infra-red, this energy simply raises the temperature of the molecule it
    interacts with.

    Now, if you have two sources radiating e-m in a broad spectrum as thermal
    emitters, and one is much hotter than the other, the hotter one will be
    radiating more higher-frequency photons.

    I don't know your medical text on DGR, but it seems how far the radiation
    travels *into* the tissue before interacting would be an important factor.
    If all the radiation interacts within the first few millimeters, than all
    the energy is deposited there. If the radiation doesn't interact as easily
    with the compounds of skin (water, amino-acids, whatever), then the
    radiation travels deeper and the energy is deposited in a larger volume of
    tissue. The energy is then able to be distributed from that larger volume
    more effectively before the temperature rises to the point of damage.
    Given the same wattage, 'short-wave radiation' would contain more higher
    energy photons. If the tissue is more 'opaque' to these higher energy
    photons, they will deposit their energy in the top-most layers of tissue.
    With the same energy being deposited in a smaller volume, its temperature
    will rise higher, no doubt causing more injury.

    But note that its the relative opacity and how much of the radiation that
    interacts with the tissue that is important. X-rays are much higher energy
    still than your 'short waves'. Yet much of the body is relatively
    transparent to them and most pass through with little interaction. Not that
    you should stand in front of a 100 Watt x-ray machine for as long as you
    would in front of an 'infrared heater' ;-)

    The other important point of e-m radiation and the human body is the exact
    nature of the interaction. Infrared will raise the kinetic energy of
    molecules (i.e. warm them). Radio and microwaves can excite water molecules
    to a high degree (such as in microwave ovens). X-ray, gamma and cosmic
    radiation can break the bonds holding molecules together causing
    free-radicals and a *potential* for gene failure. So the same amount of e-m
    energy aimed at the human body can have different effects depending on how
    that energy interacts with the matter making up the human body. And if the
    energy is deposited slowly over time, the body can repair the damage more
    readily (a low chronic dose of gamma rays over years is less harmful than
    one short massive dose).
    Actually, the point of the other discussion in this thread is whether you
    can measure 'temperature' without matter. Measuring 'heat' is saying,
    'measure the kinetic energy of molecules'. That is hard to do unless you
    *have* molecules ;-) But it is also hard to measure the energy of e-m
    radiation unless you cause it to interact with some matter and observe the
    effects ;-) Even your own eyes wouldn't work unless the e-m radiation
    (visible light) interacts with the structures in the retina.

    daestrom
     

  20. etc + much unrelated (to acoustics) discussion=======

    Dear other (than alt.sci.physics.acoustics) newsgroups Folks:

    Please *refrain* from coss-posting into acoustics unless it is
    directly on (acoustic) topic. As you all well know, such
    cossposting can diverge into tremendous useless traffic.

    Thanks, and have a good life!

    Angelo Campanella
    --
    --------- www.CampanellaAcoustics.com ---------

    "I have simply studied carefully whatever I've undertaken, and
    tried to hold a reserve that would carry me through." - Charles
    A. Lindbergh.

    "America is an experiment to be conducted by every generation."
    Washington, Madison and Lincoln.
     
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