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Energy flow

  1. Jan 15, 2004 #1
    Ok I am not especially knowledgeable in thermodynamics or physics so I am sure the answer to this is quite simple. However to me it is quite a mystery.

    This is my thought. You have a cylinder filled with water; above it on supports is a solid object the exact size of the cylinder. Through the water you have pipes containing liquid nitrogen or any very cold fluid. As the liquid flows through the pipes the energy in the water would flow towards the cold liquid. At some point the water would freeze. Because of the special and unique properties of water when it changes into a solid it expands. This expansion would raise the solid up above its current position. Assuming that the solid is then suspended at that spot it rises to I believe that its potential energy has increased. But nothing in this situation adds energy. But you have the heat energy flowing into the cold liquid or gas. In my mind by drawing energy from the cylinder you have in effect increased the potential energy of the solid that has been raised.

    I know that it is not possible that energy was created by taking it away so what are the processes that are taking place and what are the energy flows that are occurring
     
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  3. Jan 15, 2004 #2

    chroot

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    You're forgetting the fact that lower temperatures are necessary to make pressurized water freeze.

    - Warren
     
  4. Jan 15, 2004 #3
    the water is not under pressure until it expands. I have thought of this as well but energy flow is still towards the cold liquid flowing through the pipes. I am quite sure that liquid nitrogen would have the ability to absorb enough energy to allow water to freeze. I also am aware that as water expands to a point when it freezes and then starts to contract again as the temperature drops. Saw some formulas on this when I was trying to understand the mechanics of water freezing.
     
  5. Jan 15, 2004 #4

    NateTG

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    Consider that the net energy of the system has decreased, even if the potential energy stored in by the height of the weight has increased.

    Water/Ice needs to be cooled more for larger weights, and with sufficient pressure, the water will freeze with a different crystaline structure which does not increase it's volume.
     
  6. Jan 15, 2004 #5

    russ_watters

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    Freezing water expands as it cools (starting at about 4F iirc) and expands more as it freezes. So as soon as it starts to freeze, the expansion will lift your object off its supports, increasing the pressure in the cylinder and decreasing the freezing point of the remaining water. That extra energy removed from the water is the potential energy added to object.

    That's what warren was getting at.
    Didn't know that - any idea what pressure? It must be pretty high considering what ice will do to concrete or asphalt.
     
  7. Jan 15, 2004 #6

    NateTG

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  8. Jan 16, 2004 #7
    I wish I could understand this as what you say still does not make sense to me. The fact that water freezes at a lower temperature under pressure some how does not explain why the suspended object has an increase in potential energy. I did not say it was an especially big object as anything suspended right at the water survice would be lifted and therefore have a net increase in potential energy. I would suspect that the same is true if you could support all the ice that forms above the water surface. Of course this increase would be offset by a decrease in the remaining water/ice.

    Oh here is how I see it although I know my logic has to be flawed.

    Energy of object = X
    Energy of Water = Y
    Energy of Cooling substance = Z
    Energy needed to freeze water = F
    Total energy of all these = E

    So E = X + Y + Z

    So as Y and Z exchange energy I would see it as

    E = X + (Y-F) + (Z+F)

    But as the water freezes the potential energy of X would increase. So where is this come from. I just cannot see how taking energy away from water would increase its ability to do work (lifting object). Even if it take a lower temperature to freeze the flow of energy is away from the water. I am not questioning any of the answers above, I am just trying to understand what you are saying.
     
  9. Jan 16, 2004 #8

    russ_watters

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  10. Jan 16, 2004 #9

    russ_watters

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    We can start by defining the potential energy of the object to be zero when it is resting on its supports. The "X" would then be the change in potential energy of the object.

    If normally you need to take F amount of energy from the water to freeze it, adding the object on top of it changes the energy to F+X.
     
  11. Jan 16, 2004 #10
    I may be missing the point, but as far as I can see the greatest loss should be in the creation of the liquid nitrogen.

    That same energy could be put into a pully and motor system and probably raise the solid to a height higher than the freezing water could raise it.
     
  12. Jan 16, 2004 #11

    NateTG

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    My earlier post should probably have been clearer:

    There is more energy lost from the freezing water than is gained from moving the weight up. Thus the total change in energy is negative. That does not mean that there cannot be local increases in energy.
     
  13. Jan 16, 2004 #12
    Ok now this I finally understand. I guess I was not seeing the loss of energy from the water in the right way. So if the liquid was not water but something that contracts would the opposite be true and the liquid would freeze quicker when an object was placed over it if the liquid contracted when frozen.

    Oh the one comment about how expensive it would be to make liquid nitrogen well my point was not one of efficiency. It really was only something that I was not able to understand because appparently I was looking at cooling in the wrong way and I know that the people in this forum who have lots of knowledge could answer this quite easily. It however was not so easy for me to understand. But thanks to all who tried to explain it in terms that I could understand.
     
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