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Is this an energy conversion error?

  1. Aug 26, 2011 #1
    Lets say we had a twister, below it there's a lit candle, after a while the blades of the twister move. I know its because the air around the flame is heated and so the air rises. But can we say heat energy converts to kinetic energy in this situation. Becuause its not becuz the air got heated so it moved it is because the air there got lighter(less dense) so the air above it moved down and it moved up. I mean lets say we had a little particle in a vaccum its not go gonna move up just because its heated right?
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  3. Aug 26, 2011 #2


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    We have to be careful here as macroscopic quantities, such as pressure and density don't really work when we have a small number of particles - we need many particles to work with these quantities.

    What we can say is this: The air in the immediate vicinity of the candle is heated and gains kinetic energy. Therefore, the air moves faster and the same number of air particles occupy a greater volume. Thus, the density of the air decreases. The now less-dense air surrounding the candle rises through the surrounding more-dense air. This "block" of rising air collides with the blades of the twister and causes the twister to, well, twist.

    Another explanation, without requiring density is that when the air is heated it gains more kinetic energy and can then (via brownian motion) move to a higher gravitational potential, i.e. move upwards.
    Last edited: Aug 26, 2011
  4. Aug 26, 2011 #3
    I am not sure we can say that when such air is heated up it gains kinetic energy, not in the context that you are trying to address it...after all, you could also heat up gas inside a balloon tied down to a chair...is the balloon going anywhere? is the air going anywhere? did it gain kinetic energy?

    I think that you are forgetting that the work require to move the heated air is being done by the weight of the surrounding (denser) air and gravity...nothing to do with the candle itself...you could let helium (without being heated up) out of a can and it will also move up without having to "gain kinetic energy" or being "pushed" by a candle.

    So, you are "correct" to think that if your heated air was in a vacuum, it wouldn't move up since it wouldn't be surrounded by denser air to push it up.
  5. Aug 26, 2011 #4


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    Whilst the balloon doesn't gain kinetic energy, the air inside the balloon certainly does. After all, we know that the average kinetic energy of an ideal gas is proportional to the temperature. Therefore, if you increase the temperature of a volume of gas, the gas molecules will, on average, have more kinetic energy.
    The point here it that the helium filled balloon started less dense. However, for the air to become less dense, it had to gain kinetic energy. How else to you propose that the density of heated air decreases?
    Heat air in a vacuum?! Are you serious? A vacuum, by definition, will be [classically] empty and have no air in it.
  6. Aug 26, 2011 #5
    But this is internal kinetic energy, not kinetic energy in the translation sense that would produce motion! It is not like the candle is actually "pushing" the air, like I said before...it is just heating it up and raising its temperature...the candle is NOT causing the air to move! I think this is the context of kinetic energy the OP was concerned about.

    Yes, I am "serious"...just trying to help the OP visualize the problem that if you take the surrounding air, then the heated air does not move....let the OP be the judge of such visualization.
  7. Aug 26, 2011 #6


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    Just to clarify, I never said that the candle "pushes the air upwards".

    There is no such quantity known as "internal kinetic energy". There is a quantity known as internal energy, which is composed of kinetic energy and potential energy. The kinetic energy accounts of the translational, rotational and vibrational motion of the molecules. Kinetic energy is always associated with motion, so I don't understand what you are referring to.

    Answer me this: When the candle increases the temperature of the air, does the kinetic energy of the air increase?

    Second question: If the candle isn't causing the air to move, would it still move upwards if we removed the candle?

    You cannot have air inside a vacuum, nor can you discuss the notion of macroscopic temperature inside a vacuum, certainly not the temperature of a gas.
  8. Aug 26, 2011 #7
    Without getting into a semantics war, I am going to reply one more time to you Hootenanny and that's it; after all, I think by now, the OP most probably got an answer to his/her question.

    The main thing that I was trying to indicate (and separate) is that when the OP asked if the heat from the candle turned into kinetic energy, which then caused the "bubble" of hot air to move up...clearly, he/she had in mind that kind of kinetic energy that is typically related to translation motion of an entire body (e.g., a ball thrown, a bullet shot, etc...think kinetic energy in ridig bodies) and NOT the kinetic energy of the individual particles of the gas...in other words, kinetic energy as defined http://en.wikipedia.org/wiki/Kinetic_energy" [Broken].

    No, the kinetic energy included into the internal energy actually excludes the kinetic energy due to motion of the entire system. Please refer to http://en.wikipedia.org/wiki/Internal_energy" [Broken] definition and read carefully.

    Yes, the kinetic energy that is part of the internal energy, also known as THERMOdynamic energy, increases....but this has nothing to do with the kinetic energy that entire "bubble" of hot air could acquire as it moves up due to external forces (surrounding denser air pushing it up).

    The candle is not the SOLE reason why the "bubble" of hot air moves up...the candle makes the "bubble" of hot air lower its density, but it is the fact that such "bubble" of hot air is surrounding by denser air under gravity that actually pushes the "bubble" of hot air up....this is nothing but Archimedes principle all over again.

    And that's when I brought up the "bubble" of helium example...if you release a balloon of helium, you don't need a candle since helium already has lower density than air and so, it gets pushed up.

    And that's when I brought up the example of a "bubble" (don't be a purist and just bare with me) of air surrounded by nothing (in vacuum?)...if you heat it up, it is not going anywhere...

    That's all.


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  9. Aug 26, 2011 #8


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    Just to clarify: it was you that began this "semantics war" by saying that I was incorrect in asserting that the kinetic energy of a gas increases as its temperature increases.
    I'm sorry, but this is where you're flat out wrong. There are no "types" of kinetic energy - kinetic energy is kinetic energy. It doesn't matter if its an oil tanker, a plant, a fly, a car, a molecule or a proton. If it moves, it has kinetic energy. Obviously, the interpretation of an objects motion and hence kinetic energy, depends on the context. However, I reiterate: There are no "types" of kinetic energy.
    I never said it didn't. Temperature, is defined in the rest frame of the system under consideration. This is well known. You'll find no contradictory comment in my above posts. I'll say again, there are no "types" of kinetic energy.
    I never said it did. See my above comments. At some point you have to refer to the kinetic energy of the air molecules, how else are you to explain how the air becomes less dense?
    I'm not being a purist, this just doesn't make any sense whatsoever and is a very poor analogy to what is or isn't happening.
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