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A question about Joule's experiment

  1. Apr 11, 2010 #1
    In the famous experiment of James Joule,
    he used eight movable paddles and four fixed ones to prevent water circulation
    so why did he want to prevent water circulation?
    Thank you
     
  2. jcsd
  3. Apr 11, 2010 #2

    Andy Resnick

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    I'm unaware of that level of detail regarding his apparatus, but if true, a likely reason is that he did not want the water to have any energy available to do work, which would be the case if the water within the tank was rotating uniformly. There would, for example, be a transfer of energy from the rotating water back onto the (moving) paddles.
     
  4. Apr 12, 2010 #3
    I don't really understand this .
    but i think if the water rotated uniformly - without using the fixed vanes- the molecules of water wouldn't rub together , consequently no rise in temperature would occur

    i don't know whether this is true or not but do u have better explanation?
     
  5. Apr 12, 2010 #4

    Andy Resnick

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    Let me back up- what was the result of Joule's experiment, and how is this conclusion supported by the data?
     
  6. Apr 12, 2010 #5
    Joules experiments to determine the mechanical equivalent of heat were actually a series of experiments, the paddle wheel being just one.

    Essentially a known amount of energy was put into a known quantity of water and the resultant temperature rise measured.

    Since the input agitated the water paddles were used to still the water so all the energy input ended up as heat, not motion.

    In one case paddles were also used to agitate the water. These were driven by a falling weight, so defining the mechanical work.

    There were four experiments in all with the following resulting numbers of foot-pounds of work required to raise 1 pound of water one degree Farenheit.

    Paddles (fluid turbulence) 773
    Compressed Gas (PV work) 795
    Rubbing two iron blocks (Friction) 775
    Electric current (resistive heating) 838

    This lead to his conculsion that the mechanical equivalent of heat was 4.16 Joules/calorie. (CF today's value 4.184)
     
  7. Apr 12, 2010 #6

    sophiecentaur

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    I never did the Joule paddle experiment but I do remember doing the Callender and Barnes experiment with water flowing over an electrical element. I was 'well chuffed' (as you can be at 17) by the idea of making sure the hot outlet temperature and cold inlet temperature were kept symmetrical about room temperature to eliminate / reduce the effects of spurious heat loss.
    I can't remember what answer we got but I can guarantee that we would have calculated it to an inappropriately large number of sig figs.
     
  8. Apr 12, 2010 #7
    but this is not the answer to my question
     
  9. Apr 12, 2010 #8

    Integral

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    Without the paddles most of the energy would go into moving the water rather then heating the water. To heat the water you need to induce lots of collisions between water molecules so each molecule needs to have a motion which is not the same as its neighbors. If they all have the same velocity as in nice uniformly stirred water which moves with the exact same velocity of the moving paddles then you will not see much change in temperature. The idea is to put the paddles energy into RANDOM motion of the water, not uniform motion. The stationary paddles do not allow the water to move uniformly around the container.
     
  10. Apr 12, 2010 #9
    Actually, Misr I bothered to look up the original experiment and read it properly.

    I believed you had done so when you reported there were two sets of paddles, but there was only one set.

    You may have been confusing vanes which were fixed to the inside of the casing to assist mixing and friction. There was a narrow gap between the fixed vanes and the moveable paddles. They were more like tubine blades or a kitchen blender than paddle steamer wheels.

     
    Last edited: Apr 12, 2010
  11. Apr 12, 2010 #10

    sophiecentaur

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    "but this is not the answer to my question "

    The answer/s is /are clear, I think. If the water is still moving then that proportion of the input Energy is not yet transferred to Heat - it's still coherent KE.
    The longer the experiment takes, the more heat will 'get out' into the surroundings and add inaccuracy, so get the transfer to happen as fast as poss. Also, I suppose that you need good mixing so that the measured temperature really does represent the temperature of all the water.
     
  12. Apr 13, 2010 #11
    That's it!!
    Thanks for all of you
     
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