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Heat loss and mass

  1. Feb 14, 2014 #1


    "Kinetic theory of matter: All matter is made up of atoms and molecules that are constantly moving.

    When heat is added to a substance, the molecules and atoms vibrate faster. As atoms vibrate faster, the space between atoms increases. The motion and spacing of the particles determines the state of matter of the substance. The end result of increased molecular motion is that the object expands and takes up more space.

    Mass of the object remains the same, however. Solids, liquids and gases all expand when heat is added. When heat leaves all substances, the molecules vibrate slower. The atoms can get closer which results in the matter contracting. Again, the mass is not changed."

    I am just wondering: If matter loses heat, it loses photons (because heat is made up of photons). But if it is loosing energy, how come it isnt losing mass? What happend to E=mc2, where energy is a form of mass?

    Electrons lose energy because of the resistence in different materials. This energy comes out as heat. Why would matter not lose mass when losing heat (energy)?

    PS. The name of the website is https://www.mansfieldct.org/Schools/MMS/staff/hand/atomsheat.htm
  2. jcsd
  3. Feb 14, 2014 #2


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    It does lose mass when it loses heat. If everything else is identical, a colder object has less mass than a hot one. The amount of mass lost or gained is so small that we typically ignore it though. That's probably why that site is saying the mass doesn't change.
  4. Feb 14, 2014 #3


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    You are right, an object loses mass according to ##E=mc^2## as it cools. This effect is so small that it is generally completely ignored, and that's what the source you're citing has done.

    (Although not directly relevant here, heat is not made up of photons and a cooling object does not necessarily give up its energy by emitting photons. That's one way it can happen, but not the only way)
  5. Feb 14, 2014 #4

    I beleive heat can come in a couple of forms. But when I say heat consists of photons I mean in the sense of thermal radiation.

    From Britannica:

    thermal radiation, process by which energy, in the form of electromagnetic radiation, is emitted by a heated surface in all directions and travels directly to its point of absorption at the speed of light; thermal radiation does not require an intervening medium to carry it.

    Thermal radiation ranges in wavelength from the longest infrared rays through the visible-light spectrum to the shortest ultraviolet rays.

    Here is the link: http://www.britannica.com/EBchecked/topic/591461/thermal-radiation

    Therefore heat is made up of photons, when it is a electromagnetic radiation (thermal radiation).
  6. Feb 14, 2014 #5
    Strange that they would teach a false statement to students. If it does lose mass, then it is false to say that it doesnt.

    Thank you, I was quite frustrated there for a while.
  7. Feb 14, 2014 #6

    Vanadium 50

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    No, heat is not made of photons. Just because a hot object emits photons doesn't mean it's made of photons. A skunk emits stink but a skunk is not made of stink.
  8. Feb 14, 2014 #7


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    Because it is impossible to teach kids everything simultaneously, it is necessary to include simplifications. Just be aware of that and don't let it frustrate you: They aren't trying to mess with you, they are just trying to control the torrent of information thrown at you at once.
  9. Feb 14, 2014 #8
    I just want to point out that it is possible for a substance to actually shrink instead of expand when heat is added. Water does that in the temperature range from 0°C to 4°C
  10. Feb 14, 2014 #9


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    Please note that this site was created by the teacher of an 8th grade general-science class in the USA. So it's intended for 14-year-olds, who have not even started high school yet; and it's written by someone who is not a professional textbook author and probably has a degree in chemistry or biology rather than in physics.

    I think it would be rather unusual to discuss relativity in a general-science class at this level in the USA. Maybe in some other countries...
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