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Do all waves cause heat?

  1. Dec 27, 2011 #1
    Do waves (all types of waves) cause heat (however minimal) when intercepted?
     
  2. jcsd
  3. Dec 27, 2011 #2

    Simon Bridge

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    No.

    Think: what is heat?
     
  4. Dec 27, 2011 #3
    Waves transmit energy. So when this energy is intercepted by a barrier (variable depending on the type of wave), the energy is transmitted to the molecules of the barrier which as a result move faster. Hence since their average Kinetic energy increases, shouldn't the Temperature of the barrier increase too?

    Please correct me if I'm wrong.
     
  5. Dec 27, 2011 #4

    Simon Bridge

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    If a photon is scattered by a free electron, does the electron heat up?
     
  6. Dec 27, 2011 #5
    I think so, like phonon waves.
     
  7. Dec 27, 2011 #6

    Simon Bridge

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    @wdliwei: do you really think that a phonon (a collective vibrational mode) in a condensed-matter lattice exchanges energy like a photon in a vacuum? Don't you think the presence of all those other millions of particles may make a difference? Anyway - how can a single particle have heat - doesn't it just have kinetic energy? Isn't heat the randomized kinetic energy of many particles?

    Can you provide an example of a photon scattering from a free electron which shows energy loss due to heat?

    There are other kinds of waves too - like the probability waves of quantum wave-mechanics.
     
  8. Dec 27, 2011 #7
    maybe only phonon waves can cause the heat.
     
  9. Dec 27, 2011 #8

    Simon Bridge

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    "maybe" ?? - can't you reason it out?

    If you clap your hands together - you set up waves in your skin, muscle and bone - do you excite phonons? Do you get heat? You can set up waves in a gas - do gasses have phonons? Do microwave ovens heat water by exciting phonon states?
     
  10. Dec 27, 2011 #9

    russ_watters

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    It depends on what is meant by "intercepted". Since all waves carry energy, they all have an energy conversion when ABSORBED. For many types of waves, that means heat or other mechanical energy is dissipated. For some types of EM waves, it is converted to chemical energy.
     
  11. Dec 27, 2011 #10
    Let's not get tripped up on semantics here. All waves carry energy, and therefore have the potential to loose their energy to heat. Whether an individual wave in a certain case actually generates heat depends on the lossiness of the material that the wave is striking or going through. To a good approximation, many wave-material interactions are lossless (light through pure glass, sound through air, etc.) but in reality there is always going to be a little waste heat. Simon's extreme example of a single photon scattering off of a single free electron is lossless in theory, but such an example is unrealistic. The electron would have to be the only thing in the universe to be totally free and totally lossless. In reality, free electrons that are scattering photons bump into other free electrons as they oscillate and transfer their energy to heat in the process. Lay on the beach on sunny day and you will see that light can do a pretty good job of generating heat.
     
  12. Dec 27, 2011 #11
    Thank you Chris!
     
  13. Dec 27, 2011 #12

    Simon Bridge

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    Eventually. Some electrons can travel a long way before their energy can be dissipated ... maybe from one end of the lab to the other, maybe across stars. There are scales where "heat" is not a part of a useful model for what happens.

    What about probability waves?

    In macroscopic systems - every interaction creates some heat, waves being no exception. Similarly, it is always possible to simplify the physical situation so it is fine-grained enough that the concept of "heat" does not apply.

    So the simple answer to the question as posed is "no". With a "but" because waves are used to describe all kinds of things and the useful answer depends on the context of the question.
     
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