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Why warm air raises?

  1. Jan 1, 2007 #1
    The intro physics book claims warm air raises. Why? Is it because we usually make our observations on ground level and warm air will go anywhere and there is more chance of it going up than down as there is more room at the top (from our point of view) .So we observe it going downwards.

    Or is it because warm air is able to go against gravity more than cooler air. It doesn't all go up but it can go up so even though its motion is random, it has the ability to go up relative to cool air. We observe an overall shift upwards of those warm air so we say warm air raises.

    The book then suggests that the fact that warm air rises is an explanation for why the freezing compartment of the fridge is at the top. I don't understand why this is the case. How are they related?
     
    Last edited: Jan 1, 2007
  2. jcsd
  3. Jan 1, 2007 #2
    Why is the freezing unit usually the top compartment?

    Many fridges have the freezing unit on top and the normal cooling compartment down the bottom. Why?

    The answers at the back of the book suggested it was because warm air rises. What has this got to do with this question?
     
  4. Jan 1, 2007 #3

    Gokul43201

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    Warm air "goes up" because the cooler air above it "sinks" into it, displacing it upwards. This happens for the same reason that a stone will sink in water, displacing the water upwards, or that a piece of wood held under water will float upwards when released - buoyancy.
     
  5. Jan 1, 2007 #4
    That is interesting. What is the 'fundalmental' physics behind it? Does Archemides principle work with gases as well (the answer is probably 'why not?')? It seems completely different my first explanation. Could my second explanation relate to what you say?
     
    Last edited: Jan 1, 2007
  6. Jan 1, 2007 #5

    Hootenanny

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  7. Jan 1, 2007 #6
    Yes, Archimedes's Principle work with gases as well. The fundamental physical property behind it is density. As hot air contain atoms and molecules with higher kinetic energy than cold air. As a result it has a lower density, and the cold air goes downwards.
     
  8. Jan 1, 2007 #7
    Under a constant pressure according to the gas law pV=nRT if you raise temperature T you raise volume V, i.e. a parcel of gas will expand as it gets hotter. When it expands its density decreases and it rises due to buoyancy as has already been said.

    Also the particle motion might appear random but you do not need to consider the motion of individual particles, at the scales discussed here physicists consider a parcel.
     
  9. Jan 1, 2007 #8

    russ_watters

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    Remember, when you heat a volume of gas, it expands, becoming less dense. So warm air is lighter than cold air.
     
  10. Jan 2, 2007 #9
    So gravity is the main reason behind this which basically is archemides principle applied to gases?

    What about this question "Why is the freezing unit usually the top compartment?" in my second post?
     
  11. Jan 2, 2007 #10
    simply put: the cooling compartment must be at the top - when slightly warmer air in the fridge rises (from foods or open door etc), it will come into contact with the cooling fins at the top. Here it will loose energy, become more dense (for a given volume) and consequently sink in the warmer air around it. Thos process ensures a convection current and is able to maintain an even temp (ish) throughout the fridge compartment. If the cooling compartment was not at the top, the air would literally consist of trapped warmer air in the top half.
     
  12. Jan 2, 2007 #11

    DaveC426913

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    Note that "warm air rises" is technically not true, and thus a bit misleading. Warm air falls under gravity like anything else. If you opened a can of warm air on the Moon, it would (on average, and eventually) fall.

    This is obvious, yes, but now put it back in the context of an atmosphere. The warm air still wants to fall, it just can't. The correct way to view the phenomenon is that cold air sinks more than warm air does.
     
  13. Jan 2, 2007 #12
    Heat can be transferred by conduction through the layers connecting the two compartments? Note: I am assuming doors are closed.

    Assume that is how heat is transferred. Then if the freezer was at the bottom, the fridge would have a heavy load of warm air in the upper porportion resulting in uneven temperture in the fridge - not good. As you pointed out.

    If the freezer was at the top than warmer air in the fridge would touch the layer separating them and cool down again.

    Although we still have the problem of the freezer having an uneven temperture distribution when at the top. It would have a more even temperture distibution being at the bottom. But heavier emphasis is put on the fridge having an even temperture distribution so that is why the freezer shoudl be at the top?
     
  14. Jan 2, 2007 #13
    I checked my own fridge and saw that the evaporator was installed near the top of the fridge. This makes sense since it would mean warm air rises and gets cooled again so a stable convection mechanism can happen. However, I checked my freezer and didn't see the evaporator anywhere. Where could it be? What would it look like? In the fridge, it was a rectangular board/plate with the evaoparation probably at the back of that board.
     
  15. Jan 4, 2007 #14

    daniel_i_l

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    As i was reading this i thought of the following question:
    Lets say that you start with hot air on the bottom and cold air on top. after a while they change places - my question is - is the hot air which is now on top the same air [= same particles] that were previously on the bottom or are the particles in basically the same places just with different velocities? in other words, do the particles themselves move or does the "heat" move via colliding particles?
    Thanks.
     
  16. Jan 4, 2007 #15

    Gokul43201

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    Both, but often in fluids, it's more of the former - the molecules themselves moving (this is called convection, as opposed to the latter case, which describes conduction).
     
  17. Jan 4, 2007 #16
    since daniel_i_l was talking about air hence gas that are moving than convection is even more prevalent and conduction is negligible.

    The physics behind warm air and fluids (that are of the same chemical element) rising compared to their colder counterpart is due to Archemides principle. The key to this principle is that the particles are immiscible and particles can move through oneanother. The latter is satisfied by air and fluid while the former is satisfied because of the temperture differences in the particles.
     
    Last edited: Jan 4, 2007
  18. Jan 5, 2007 #17

    DaveC426913

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    Yes. There's more convection than conduction. This why we have weather - masses of moving air.

    It is possible, and often happens in the summer, to have an "temperature inversion", where a layer of warm air gets trapped under a layer of cold air. The warm air is unable to rise. (Think of how hard it is to get ketchup out of the bottle if you hold it directly upsidedown)
     
  19. Jan 5, 2007 #18

    russ_watters

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    Yes! And this is related to another recent thread (not sure if it was started by you) about buoyancy in nonmixing fluids. Much of the convection in the atmosphere invovles descrete "bubbles" of relatively uniform temperature/density regions that percolate up through the atmosphere in a way that if you could see it would look a lot like a lava lamp.

    Sometimes, though, you can see it - the white puffy cumulous clouds you see developing in spring and early summer are updrafts - bubbles of warm air - that have risen, expanded, started to cool, and thus condensed out the water vapor in them. So now you can explain rain.... If the air mass is even warmer and even more moist, that creates a cumulonimbus cloud - a thunderstorm. Next time you see one, take a closer look (with binoculars, even). If it is still well-defined (puffy, not wispy), it is growing. You can actually see it grow, gain altitude, and expand, not too unlike a bubble in a lava lamp.
     
  20. Jan 5, 2007 #19
    Why is the warm air unable to rise?
     
  21. Jan 6, 2007 #20

    DaveC426913

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    Because there's a cold layer of air pressing down on top of it. The interface is so flat that the warmer air has no chance to "bubble up" through the cool air layer. That's why I compared it to ketchup coming out of the bottle.
     
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