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Second law of thermodynamics problem

  1. Feb 19, 2013 #1
    The second law of thermodynamics states, (in simple terms) that heat always dissipates towards cold (correct me if I am wrong). Thus, the heat in an object should radiate or convect or conduct towards the cold.

    So if this is true, can someone explain how icicles have a tendency to move towards warmer objects? For instance, when I have a campfire in a hole of snow (so that snow surrounds the fire, but doesn't smother it), the snow does it's usual thing, melting, but some of it also does not melt but moves towards the fire against gravity (sideways icicles!).

    Can someone please explain how this works while conserving the theory, or do we have to throw out thermodynamics :eek: (joking, don't get out your tinfoil hats yet).
     
  2. jcsd
  3. Feb 19, 2013 #2
    I have no idea if that is true. It might be an optical illusion that the snow around a particular point is melting faster than the melting rate at that point, which results from anisotropies in the heat source.
     
  4. Feb 20, 2013 #3
    Well, would have thought that too, but then there are images like these which really threw me off.

    https://sphotos-a.xx.fbcdn.net/hphotos-snc7/387027_10200356985756012_360218913_n.jpg [Broken]

    I made the first note of the idea when I saw this photo (I've been there as well and seen the icicles themselves), which is why I first made the conclusion that it was the same effect in smaller form (that is, the sideways icicles from said fire, which I observed while camping). I tried to see if what you said was true, but there really isn't a way to find that out when I am camping.

    I do know that the effect was uniform to all the ice and snow around the fire which were close enough to be melted, I also know that whenever I flattened the icicles they came back (again, these are tiny sideways icicles, so its hard to tell whether they 'grew' or surrounding snow shrunk).
     
    Last edited by a moderator: May 6, 2017
  5. Feb 20, 2013 #4
    I'm not convinced the assumptions you attach to your observations are completely sound, but I'll look at the mechanisms involved.

    First take note that 'warmer' air (not necessarily above freezing) is less dense than colder air. This is why a helium balloon will migrate toward a fireplace. Second, just because heat migrates from warm to cold doesn't mean heat is a substance in itself that pushes other objects in the same direction. Hence, for objects such as a helium balloon and icicles for the sake of argument, the increase in density of colder air will tend to create a pressure gradient drawing objects toward the heat source.

    In the campfire case you have another force at work. Since warmer are is light than colder air the warmer air will tend to go straight up, and this causes colder air to be drawn into space the warm air evacuates. This is by far a stronger effect than the conduction you are implying by way of the second law, and has even been used to produce ice in the desert. This effect is so strong it can create a noticeable breeze around a campfire. In fact it is even necessary to keep the fire supplied with oxygen. Otherwise the oxygen around the fire would get used quickly and the fire go out.

    Even if the above explanation is valid for camp fires it's badly inadequate to explain the picture you linked. This is almost certainly the effect of a mostly sustained breeze. I have observed this in the process of occurring on many occasions. It doesn't take much wind push a single droplet of water off center from straight down, and as each drop freezes it exaggerates the next drop.
     
  6. Feb 20, 2013 #5
    I'm actually implying the opposite of the second law (because cold went towards hot? lol).

    Anyway, your explanation makes plenty of sense. :) Anyone else have an opinion on this?
     
  7. Feb 20, 2013 #6

    DrClaude

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    Staff: Mentor

    Saying that a cold icicle growing towards a hot source is a violation of the 2nd law makes no sense. The second law says that, when two objects can exchange heat, in the absence of any external work heat will flow from the hot object to the cold one. In the examples you give, there is no heat going from the icicle to the hot source.

    The growing of an icicle is a complicated process, involving cycles of melting and freezing. My guess is that the side of the icicle that is closer to the warm source will grow faster, and this is what results in an asymmetric icicle.
     
  8. Feb 20, 2013 #7
    I think what you are seeing in that photo is the snow on the roof is slowly flowing off the edge of the roof as the snow load increases. As the snow flows off the roof edge, the sheet of snow slowly bends down as it loses support of the roof below. That tilts the icicles formed along the perimeter of that sheet towards the wall of the house. Before the sheet moved, they were vertical.

    As the icicles continue to form, they try to form vertically, so the tip of each icicle is curved with the top end being farther from the wall, the middle portion angled toward the house and the bottom end turning vertical.
     
    Last edited by a moderator: May 6, 2017
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