Aerosol Can Spray: Cooling Effects Explained

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Discussion Overview

The discussion revolves around the cooling effects observed when spray is released from an aerosol can, exploring the underlying physical principles, including the ideal gas law and the Joule-Kelvin effect. Participants examine the thermodynamic implications and entropy changes associated with this process.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Alex questions whether the cooling of the aerosol can upon spraying is true and seeks to understand the energy requirements for the gas to expand and escape.
  • Gabriel applies the ideal gas law to explain that as the pressure inside the can decreases when the gas is released, the temperature must also decrease, assuming a constant amount of gas and volume during a short burst.
  • MiGUi introduces the Joule-Kelvin effect, noting that it applies to real gases and describes how the temperature changes when a gas expands through a constriction without heat exchange.
  • Another participant mentions the similarity between this process and the operation of a refrigerator.
  • One participant raises a question about the change in entropy of the can, wondering whether it increases or decreases and how to evaluate this change given the limitations of the ideal gas law.
  • Another participant asserts that the overall entropy change is positive, but questions the specific entropy change of the can and its contents alone.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of the ideal gas law and the implications of the Joule-Kelvin effect. There is no consensus on the entropy changes associated with the process, indicating ongoing debate and exploration of these concepts.

Contextual Notes

Participants acknowledge limitations in applying the ideal gas law to this scenario, particularly regarding assumptions about gas behavior and the conditions under which the law holds. The discussion also highlights the complexity of evaluating entropy changes in this context.

alexbib
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I was told that when you release spray from an aerosol can, the can cools down. Is this true, and if so, why?

Does the gas in the can require outside energy to expand and escape the can?

Thanks,

Alex
 
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I think we can approach this problem with the good old ideal gas law. It states that P*V = n*R*T,

where P is pressure, V is volume, n represents the amount of gas, R is a constant, and T is temperature.

The gas in an aerosol can is under pressure. It wants to get out of the can and when you press the nozzle you provide the means for it to do so. Now were going to have to make some assumptions about what's going on when the nozzle is pressed and whatever gas is inside is sprayed out. I've only used this equation for gasses where n doesn't change. I think we can use a constant n as an approximation if we are considering a short burst. In this approximation the volume is going to remain constant as well (the can isn't changing shape) and R is defined as a constant. During the spray the pressure inside the can will go down, which for the above equation to be an equality, means the temperature has to go down.

Gabriel
 
That effect is known as Joule-Kelvin effect, and it only happens real gases! so the ideal gas don't work with this effect!

The Joule-Kelvin effect says that: If a real gas is expanding and it crosses a (i don't know the english word, but i want to mean that the section of the tube or so is lower than the section the gas was crossing before), without interchange of heat, then the temperature variates.

When you press the aerosol, the gas has to cross through the little hole, so the temperature of the recipient goes down.

MiGUi.
 
alright, I'll look it up. thanks!
 
this is exactly refrigirator work!
 
hey, anybody knows what happens to the entropy of the can? Does it increase or decrease? How could you evaluate the change in entropy, since pV=nRt is not true?
 
it is obvious that the overall entropy change is positive (compressed gas in a can is more ordered than when the pressure reaches an equilibrium, but what about the entropy of the can (and it's content) alone?
 

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