# Aerosol can

1. May 12, 2004

### alexbib

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

2. May 12, 2004

### Allday

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 whats 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

3. May 12, 2004

### MiGUi

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.

4. May 12, 2004

### alexbib

alright, I'll look it up. thanks!

5. May 16, 2004

### Pierre

this is exactly refrigirator work!

6. May 16, 2004

### alexbib

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?

7. May 17, 2004

### alexbib

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?