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Thread: Latent heat.

  1. #1
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    Latent heat.



    Does any one know why changing from a vapour to liquid with no temp change ( latent heat) uses more energy than sensible heat.

    any one know a Scientific answer ?

    And what does gas to "plasma" mean? whats ionization ?



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    Re: Latent heat.

    It's all on wiki
    http://en.wikipedia.org/wiki/Latent_heat

    http://en.wikipedia.org/wiki/Plasma_(physics)

    S
    ave's a lot of typing ,some good reading in there.

    Cheers
    Stu
    Tool's ? check ! Condom's ? check !
    If you can't fix it , f*ck it !!!

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    Re: Latent heat.

    I read the first one before i posted . didnt really tell me a reason that it uses more energy . mybe i missed something.

    So does gas to plasma have the same latent heat effect? could you make a heat pump heater the goes from gas to plasma and works similar to an air con on heating mode. . Im sure you cant because your putting in more energy to get this changed, and about 10 other reasons
    Last edited by Simeonx1; 04-03-2012 at 11:15 AM.

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    Re: Latent heat.

    Sublimation

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    Re: Latent heat.

    Hi Simonx1
    There are 5 states of mater that we know of at this time. We are generally only taught of 3 states of mater.
    1 Bose-Einstein condensate
    2 Solid
    3 Liquid
    4 Gas
    5 Plasma
    An example of plasma is the average flouro tube light. When a gas is heated or energy added it will change phase into plasma. If you could imagine an increase in energy to electron velocity (where density would be pressure) the atoms will be so excited they will emit light (photons).
    I spoke to a professor in the physics building a few weeks ago on this subject and he advised me that they use plasma cooling to cool small things down to absolute zero (Bose-Einstein Condensate) but this method of cooling is very inefficient and not practicable for commercial use in AC or refrigeration.
    One book I find helpful is called The Quest for Absolute Zero (1976) is an oldie but gets into the history of when, who and what work has been done in the quest for lower temperatures. Roy Dossat - Principles of Refrigeration will guide you through the processes of change of states and types of energy for calculation.
    Really it is quite a fascinating topic - if you think of a lightening bolt which is something like 50,000 degC inside it actually cools the surrounding air to minus 50degC or there abouts taking energy from its surroundings to feed the discharge arc.

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    Re: Latent heat.

    Thanks for the post telsa, Im familiar with bose-condesate ive watch a few documentary i didnt know it was counted as a state of matter. i guess no moving atoms at all is different than everything else.

    Cool that lighting thing makes sense. and good to know they do have a plasma cooling ,

    Would you happen to know why changing state uses more energy than just de/increasing temp or is it just one of those things that doesnt have an exact answer ?

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    Re: Latent heat.

    Maybe because your question makes no sense!

    When (pure) matter changes state there is no sensible heat, so sensible heat=0 but because matter changes state you are adding energy (latent heat>0) Therefore it follows that latent heat>sensible heat only in the vicinity of a change of state.

    You can always add enough energy to make sensible heat>>latent heat if you add enough energy to take your system to point far enough from the change of state process. (>>=much greater than).

    I am not sure I can explain the process of change of state well enough so that you may clarify your question, but…let's try!

    There are several forms of energy present in molecules of those two are important in any physical state change which are Internal Energy (U) and work (W).

    Molecules tend to store energy in their bonds as chemical energy, they vibrate and move around (mechanical energy) and we don’t need to go further than that. All these add up to the term “Internal Energy” which is energy the molecule is able to “store” and use when needed.

    Thermal energy (heat) is a type of energy molecules exchange with their surroundings (it is related to the number of collisions between molecules. At higher temperature more collisions.

    Work is another form of energy that molecules exchange and it may take two forms: volume change and pressure change.

    When I say “exchange” it means the energy is transferred from molecule to molecule until it reaches the frontier of mass you are analyzing or “system” and pass it on to molecules which are outside your system.

    When a substance changes from a liquid to a gas because you apply heat (thermal energy flowing into your system in exactly the amount called “latent heat”) all of the following happen:

    - Molecules store internal energy, vibrate more and start to move apart.

    - Molecules separate from each other absorbing enough energy to overcome some chemical bonds or electrical attraction.

    - Molecules move faster and collide more until the change in momentum from the collisions increase.

    - They interact with molecules not in your “system” doing work (changing volume).

    - It is only when the level of interaction between your molecules is such that the number of collisions increase that you begin to measure a change in temperature. All energy added to the system thereafter goes only to increase the kinetic energy of the molecules because they cannot “store” more internal energy in that state of matter.

    Hope I didn’t add fuel to the fire!

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    Re: Latent heat.

    Quote Originally Posted by Tesla View Post
    Hi Simonx1
    There are 5 states of mater that we know of at this time. We are generally only taught of 3 states of mater.
    1 Bose-Einstein condensate
    2 Solid
    3 Liquid
    4 Gas
    5 Plasma
    An example of plasma is the average flouro tube light. When a gas is heated or energy added it will change phase into plasma. If you could imagine an increase in energy to electron velocity (where density would be pressure) the atoms will be so excited they will emit light (photons).
    I spoke to a professor in the physics building a few weeks ago on this subject and he advised me that they use plasma cooling to cool small things down to absolute zero (Bose-Einstein Condensate) but this method of cooling is very inefficient and not practicable for commercial use in AC or refrigeration.
    One book I find helpful is called The Quest for Absolute Zero (1976) is an oldie but gets into the history of when, who and what work has been done in the quest for lower temperatures. Roy Dossat - Principles of Refrigeration will guide you through the processes of change of states and types of energy for calculation.
    Really it is quite a fascinating topic - if you think of a lightening bolt which is something like 50,000 degC inside it actually cools the surrounding air to minus 50degC or there abouts taking energy from its surroundings to feed the discharge arc.
    Tesla, I keep think that if i had to meet you somewhere, I would be looking for the person in your avatar....weird.

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    Re: Latent heat.

    Quote Originally Posted by MikeHolm View Post
    Tesla, I keep think that if i had to meet you somewhere, I would be looking for the person in your avatar....weird.
    Mike, Tesla is probably testing his swimming skills at the moment.Heavy rain and flooding down south
    To the optimist, the glass is half full. To the pessimist, the glass is half empty.

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    Re: Latent heat.

    Latent heat is the heat required to change State.

    A fluid in liquid state is at a lower internal energy condition than a fluid in vapor state. The vapor is loosely bound and highly energetic as well as intrusive. Thus vapor will try to fill whatever space is made available to it. Liquid is content to sit still. A low vapor pressure fluid will sit still almost indefinitely: consider mercury for example.

    For a given fluid: should you place a sample of liquid in an outsized evacuated volume, the fluid will try to generate vapor in order to fill the available space. In order to generate that vapor, the liquid phase will require the input of heat. That heat is the latent heat of vaporization.

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    Re: Latent heat.

    Hi Mike
    It is a bit weird for the Tesla Portrait as my avatar. I put it there because Tesla is responsible for many modern advances in refrigeration - electricity system, electric motor, control systems, flouro lighting, radio etc... But many of us are not taught this. He has a patent for a type of refrigeration compressor which I think has not been built. In reality I look nothing like Tesla, I look like a hippy 'borne out of time' with long hair. I thought it would be fitting with the alias of Tesla to have one of his most famous images as an avatar. We never know we may meet one day.

    Hi Mike
    I just missed the floods as I went north to Glenn Innis 7Hrs north and watched the full supermoon set and sun rise in the country. Had awesome weather for 5 days until I returned to Sydney. A building which flooded only a month ago flooded again, thankfully I was not there and another tech took the job of clean up in a plant room which flooded to over a meter, 2 chillers, mech board, pumps, boilers & AHUs full of water.

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    Re: Latent heat.

    The energy that is required to be "removed" from a vapour to make it a liquid (say steam to water) and then from a liquid to a solid (water to ice) is due to the change in state. Reverse your question and use a simple example (a kettle) and it is probably easier to understand... Take a container, with a thermometer, a heater and a block of ice at -20 degC (sub cooled)... Add some heat and the temperature will rise till it gets to 0 degC.... At this point the heat is going to be used to change the ice to water (latent heat).... It is going to excite the molecules till it becomes a liquid and in a perfect system the temperature will not rise until all the ice has changed to water (melted)... Once it is all water the temperature will now rise till it hits 100 degC and is again going to require added energy to again, change state, this time from a liquid to a vapour by exciting the molecules till they change from water to steam (boiling).... Again in a perfect system the temperature will not rise until all the water is steam (Latent heat again).... Then the temperature will again start to rise (super heat)... Replace the heater with a cooler and you can remove the energy (calm the molecules) to reverse the process...
    To answer your original question, you are reversing the last part and removing the energy from a vapour to form a liquid, this requires work to be done to change the state... It takes a lot more energy to change state than to just raise an objects temperature...

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    Re: Latent heat.

    Quote Originally Posted by keithice View Post
    ... It takes a lot more energy to change state than to just raise an objects temperature...
    Let's take 1k of R22 for example a known substance with a large latent heat (like ammonia)

    At -40°C the latent heat is 189 kJ

    Tot take this kilo from -40°C saturated vapour up to +250°C and 0.10133 MPa it takes 214 kJ (sensible heat)

    In this case the latent heat < sensible heat

    My point is sensible heat can be made as much larger than latent heat as you want, but of course you need to be far off saturated conditions, less further for other substances though.

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