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Thread: Why does temperature change with pressure?

  1. #1 Why does temperature change with pressure? 
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    According to Gay-Lussac's law, pressure and temperature are directly proportional. I understand why pressure changes with temperature but I don't understand why temperature changes with change in pressure.
    Same question applies for Charles' Law: Why does temperature change with volume?


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    Hm, I'm not greatly brushed up on this but since the topic hasn't had a response I'll give it a bash:

    If there's a change in pressure then there's a change in energy. Generally, to increase pressure an increase in energy is required to cause that pressure increase. As with all energetic reactions there are waste "products" and one of those is temperature. Thus as you apply more pressure to something, and therefore more energy, there will also be an increase of waste energy and thus temperature.


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    Gay-Lussac's law requires volume and density to remain constant. The only way to change the pressure is by changing the temperature. I don't really see how you would do it the other way around.

    Anyway: temperature, pressure and volume are only macroscopic properties. They are related with the average speed of particles and the number of collisions of those particles (with a wall or with each other).
    Since a higher speed speed causes more collisions, temperature and pressure are related. A bigger volume results in a lower particle density and less collisions, thus an inverse relationship between volume and pressure. Lastly if the number of collisions remains constant, a higher speed of the particles relates to a larger volume and a lower density.
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    Actually, using changes in pressure to force changes in temperature is basically how both your refrigerator and your air conditioner work. They force coolant through coils with areas of both high and low pressure. In the high pressure areas, the temperature rises, and heat moves out of the liquid. In the areas of low pressure, it cools and heat moves in. Put these two areas in the right places (one inside, one out) and you can move heat around to do neat things like keep your soda cold.

    (BTW, for the more general equation, look up the ideal gas law.)
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  6. #5  
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    Quote Originally Posted by MagiMaster
    Actually, using changes in pressure to force changes in temperature is basically how both your refrigerator and your air conditioner work. They force coolant through coils with areas of both high and low pressure. In the high pressure areas, the temperature rises, and heat moves out of the liquid. In the areas of low pressure, it cools and heat moves in. Put these two areas in the right places (one inside, one out) and you can move heat around to do neat things like keep your soda cold.

    (BTW, for the more general equation, look up the ideal gas law.)
    In that case, the volume doesn't stay constant and Gay-Lussac's law isn't applicable.
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    Hmm... Not that I'm disagreeing with you, but I don't really understand. The coil has a constant volume, as does the hot section and the cold section. Is the movement through the expansion and compression valves what makes the volume non-constant?
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    Quote Originally Posted by MagiMaster
    Hmm... Not that I'm disagreeing with you, but I don't really understand. The coil has a constant volume, as does the hot section and the cold section. Is the movement through the expansion and compression valves what makes the volume non-constant?
    Yes, the fluid goes from higher density to lower density during expansion and back to higher density during compression. The specific volume (the inverse of density) changes in the reference frame of the fluid.
    In the reference frame of the coil, and in a (dynamic) equilibrium, the volume of the coil doesn't change and the specific volume of any point in the coil doesn't change, but neither does the pressure or the temperature (in this case, you're looking at different particles flowing through a specific point, while in the first case you're looking at specific particles flowing through different points. I'm guessing you mixed those up)
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    Quote Originally Posted by MagiMaster
    Actually, using changes in pressure to force changes in temperature is basically how both your refrigerator and your air conditioner work. They force coolant through coils with areas of both high and low pressure. In the high pressure areas, the temperature rises, and heat moves out of the liquid. In the areas of low pressure, it cools and heat moves in. Put these two areas in the right places (one inside, one out) and you can move heat around to do neat things like keep your soda cold.

    (BTW, for the more general equation, look up the ideal gas law.)
    I think you meant that in high pressure areas heat moves into the fluid and vice versa.

    Notice I used the word fluid. If I'm not mistaken refrigerators and AC units compress a gas into a liquid which then decompresses back into a gas in the coils. This is done because evaporation is endothermic (i.e. the liquid absorbs heat from its surrounding as it turns into a gas). It's why sweating cools our bodies - as it evaporates it cools our skin.
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    Quote Originally Posted by ScienceWizard
    I think you meant that in high pressure areas heat moves into the fluid and vice versa.
    In the high pressure (liquid) areas, the temperature is higher than the environment, and the heat moves out, like MagiMaster wrote.
    Otherwise your explanation is right.
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  11. #10  
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    Ah yeah, fluid would be a better word. But, on your other point, I thought that when it's compressed, it gets hotter, so that it's then hotter than the environment and energy escapes; then it expands and gets colder, so that it's then cooler than its environment and absorbs energy?
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    What's all this got to do with the original question
    You are adressing principals of refrigeration- heat of vaporization, heat of condensation, heat capacity etc
    All interesting but what does it have to do with the gas ??????
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  13. #12  
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    Quote Originally Posted by MagiMaster
    Ah yeah, fluid would be a better word. But, on your other point, I thought that when it's compressed, it gets hotter, so that it's then hotter than the environment and energy escapes; then it expands and gets colder, so that it's then cooler than its environment and absorbs energy?
    Yeah, makes sense. After compression the fluid moves to the outside coils where it condenses into a liquid (exothermic), then through a pressure dropper (a valve) and then into the inside coils where it evaporates (endothermic) and back to the compressor.

    My mistake was in assuming that the compressor compresses the gas into a liquid.
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    Quote Originally Posted by fizzlooney
    What's all this got to do with the original question
    You are adressing principals of refrigeration- heat of vaporization, heat of condensation, heat capacity etc
    All interesting but what does it have to do with the gas ??????
    Well, the original question was asking why pressure changes with temperature and why temperature changes with pressure, so I thought refrigeration was an example of this. As pointed out, this example isn't directly applicable to the original question, which was my mistake.
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  15. #14 Re: Why does temperature change with pressure? 
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    Quote Originally Posted by yara92
    According to Gay-Lussac's law, pressure and temperature are directly proportional. I understand why pressure changes with temperature but I don't understand why temperature changes with change in pressure.
    Same question applies for Charles' Law: Why does temperature change with volume?
    Why not focus attention on the fundamental equation of state for an ideal gas, Boyle's Law which really implies both Charles Law and Gay-Lussac's Law: PV=nRT.

    Now thing about a cylinder of gass with a piston. You increase the pressure by pushing on the piston, compressing the gas. It takes force to move that piston, and you exert that force over some distance, thereby performing work. Work is energy and that energy must go somewhere. It goes into producing thermal energy, which is reflected by an increase in the temperature of the gas.

    That same experiment will explain why the temperature increases when the volume is decreased, with an increase in pressure.

    Similarly if you let the gas expand against the piston to increase the volume the gas does work in lifting the piston, therefore energy is lost from the gas, realized by work done on the piston, and the source of the energy from the gas is the theremal energy, which is lost to the gas resulting in lower temperature.
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    Hang on a minute, Boyle's Law doesn't include temperature - it says pressure and volume are inversely proportional at constant temperature. You are quoting the ideal gas law, not Boyle's law.

    I think the original question was answered by Bender in the third post.
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    How about if it didn't we would have to throw away PV=nRT?
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  18. #17 Re: Why does temperature change with pressure? 
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    Quote Originally Posted by yara92
    According to Gay-Lussac's law, pressure and temperature are directly proportional. I understand why pressure changes with temperature but I don't understand why temperature changes with change in pressure.
    Same question applies for Charles' Law: Why does temperature change with volume?

    Say there is iron being heated, all of the little metal atoms are moving more and more about, and more collisions happen. There is more momentum with each collision as it heats. So the atoms get further apart and the metal expands. Now we see as the metal expands as it heats. But how easily can it expand? Thats how pressure and volume play a role, what if there is so much pressure that the distance between the atoms could not easily had, then it would require more energy to reach a higher temperature as pressure increases.
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  19. #18  
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    Quote Originally Posted by Bunbury
    Hang on a minute, Boyle's Law doesn't include temperature - it says pressure and volume are inversely proportional at constant temperature. You are quoting the ideal gas law, not Boyle's law.

    I think the original question was answered by Bender in the third post.
    Your terminology is correct. Mine was wrong. But the explanation remains valid.
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  20. #19 Re: Why does temperature change with pressure? 
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    Quote Originally Posted by Raymond K
    Say there is iron being heated, all of the little metal atoms are moving more and more about, and more collisions happen. There is more momentum with each collision as it heats. So the atoms get further apart and the metal expands. Now we see as the metal expands as it heats. But how easily can it expand? Thats how pressure and volume play a role, what if there is so much pressure that the distance between the atoms could not easily had, then it would require more energy to reach a higher temperature as pressure increases.
    In a solid, you can hardly say there are more collisions. It's a different mechanism than in a gas.

    If you look at this picture (potential energy of interparticle forces in function of the distance between two particles), imagine the energy level to be -50 [some unit]: the particles will vibrate between distances of 3.5 and 4.5 [some other unit], with an average around 4 (energy going from kinetic to potential and back as they go over the curve). If you increase the temperature, the energy level will increase, and because this curve is asymmetrical, the average distance between the particles is going to increase.

    (At high enough temperature, particles start "escaping" and shifting around, eventually becoming liquid)
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  21. #20  
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    Quote Originally Posted by yara92
    Why does temperature change with volume?
    If you ask me in terms of solution, specifically non ideal solutions which shows +ve and -ve deviation.
    -> In non ideal solution showing positive deviation. In this when two components are mixed, molecules move away from each other due to which the volume of solution increases.
    If forces are weak only then molecules move away from each other and hence there is an increase in volume. In this case the temprature of the solution decreases.
    -> In non ideal solution showing negative deviation when two components are mixed together the molecules come close to each other due to which the volume decreases. Due to more force of attraction energy is released. Hence delta H is negative which means that there is an increase in temprature.
    One more thing, when ever theres an increase in force of attraction energy is released.
    *Molecules will evaporate at a slower rate. Hence, vapour pressure of the solution decreases.
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  22. #21  
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    The person who asked the main question that is Yara92 is gone ??
    I just don't get it, that why people after asking question go away for some unknown time period ?????
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