Do dusters defy physics?

[bommy261]

same reason why your refrigerator can cool or why a fire extinguisher gets cooler as u use it. as a gas decompresses it cools the surroundings

 

[DrPizza]

What an appropriate username!

 

[mugs]

All I know is when I put a propane tank in a flaming box and shoot it, I get a big ball of fire and a cold propane tank.

Sometimes the tank takes off (with or without a treadmill).

 

[Lean L]

What an appropriate username!

Care to guess what the spoiler means? You in particular have a pattern of saying the same things over and over again, learn to think.

 

[TuxDave]

P1V1 / T1 = P2V2/T2

Volume remains constant, so with a reduction in pressure, temperature decreases.

But that equation isn't valid since the moles of gas has decreased too. But after some thought I'm still leaning towards the pvnrt equation since if you drop pressure around water (causing it to boil) I don't recall the water being very cold. But I really could've sworn that phase changes from liquid to gas was endothermic in this case. It's been too many years out of school

 

[artikk]

But that equation isn't valid since the moles of gas has decreased too. But after some thought I'm still leaning towards the pvnrt equation since if you drop pressure around water (causing it to boil) I don't recall the water being very cold. But I really could've sworn that phase changes from liquid to gas was endothermic in this case. It's been too many years out of school

I don't think the amount of moles decreased, since that would mean the amount of coolant molecules decreased as well. As per wikipedia, the liquid coolant releases heat when going through the phase transformation, lowering the overall temperature of the coolant accordingly.

 

[Kadarin]

NOTHING defies physics. If something appears to do so, then you need to modify your understanding of physics to explain the behavior you observe.

 

[DT4K]

NOTHING defies physics. If something appears to do so, then you need to modify your understanding of physics to explain the behavior you observe.

/thread

 

[Toastedlightly]

"The gases inside the can have been compressed to the point that they have become liquid, and they evaporate before leaving the can since the valve draws from the top."

Its driven by the phase change. From liquid to gas.

 

[Bignate603]

If you think something defies physics it means you don't understand the physics that governs it.

 

[TuxDave]

I don't think the amount of moles decreased, since that would mean the amount of coolant molecules decreased as well. As per wikipedia, the liquid coolant releases heat when going through the phase transformation, lowering the overall temperature of the coolant accordingly.

What's coolant have to do anything? When you use a duster, compressed air goes out, thus less molecules of whatever it was in the can.

 

[drinkmorejava]

What's coolant have to do anything? When you use a duster, compressed air goes out, thus less molecules of whatever it was in the can.

You have no concept of a control volume do you? The number of molecules inside the can + the number ejected from the nozzle is exactly equal to the number of molecules in the can before you did anything.

The molecules that left the can underwent a phase change, where the number of molecules undergoing the phase change is exactly equal to the number of molecules that left the can. Yes, it's circular, that's the point!

It's obvious something expanded in the process. Then all you do is follow PV=nrT --> PV=T


Of course, this isn't all exactly right because you have the partial pressure step in between. Assuming that the can is held upright, any gas leaving was actually a gas while it was in the can, and the volume that was displaced by the leaving gas was filled by liquid inside the can changing to gas. However, as long as there is still liquid left, the pressure in the can will remain exactly the same (for a given temperature) and the number of molecules leaving will be equal to the number undergoing a phase change. There are secondary effects caused by the decrease in liquid volume, thereby increasing the gas volume, that will cause more molecules to change state than actually leave, but we will ignore that for your first class.

 

[TuxDave]

You have no concept of a control volume do you? The number of molecules inside the can + the number ejected from the nozzle is exactly equal to the number of molecules in the can before you did anything.

Not really sure what I said to imply the above was false... but whatever.

The molecules that left the can underwent a phase change, where the number of molecules undergoing the phase change is exactly equal to the number of molecules that left the can. Yes, it's circular, that's the point!

It's obvious something expanded in the process. Then all you do is follow PV=nrT --> PV=T

Of course, this isn't all exactly right because you have the partial pressure step in between. Assuming that the can is held upright, any gas leaving was actually a gas while it was in the can, and the volume that was displaced by the leaving gas was filled by liquid inside the can changing to gas. However, as long as there is still liquid left, the pressure in the can will remain exactly the same (for a given temperature) and the number of molecules leaving will be equal to the number undergoing a phase change.

So pressure remains the same, moles of gas is slightly larger since volume has increased a little due to the reduction in the volume of liquid. But in the end, PV=nrT P,V,r are all roughly the same before and after and so it doesn't cause a large drop in T? Then are you in agreement with my original guess (scroll up) that the drop in temp is mostly due to the liquid to gas phase change in the chamber?

 

[sandorski]

It's the breath of Jesus. It does what it wants to do.

 

[drinkmorejava]

Edited in another line that you may have missed, but it's not relevant to your question:
"There are secondary effects caused by the decrease in liquid volume, thereby increasing the gas volume, that will cause more molecules to change state than actually leave, but we will ignore that for our first class"

To determine where most of the cooling is from you actually have to do all of the math out. There something called the enthalpy of phase change where when something changes state it either gains or loses energy from the environment. To determine the total energy change you must also know the partial pressures where the two phases can coexist (vapor pressure), from that pressure you can determine the delta_P to the atmospheric pressure and determine the temperature (energy) change from the gas expansion.

 

[TuxDave]

Edited in another line that you may have missed, but it's not relevant to your question:
"There are secondary effects caused by the decrease in liquid volume, thereby increasing the gas volume, that will cause more molecules to change state than actually leave, but we will ignore that for our first class"

To determine where most of the cooling is from you actually have to do all of the math out. There something called the enthalpy of phase change where when something changes state it either gains or loses energy from the environment. To determine the total energy change you must also know the partial pressures where the two phases can coexist (vapor pressure), from that pressure you can determine the delta_P to the atmospheric pressure and determine the temperature (energy) change from the gas expansion.

Makes sense. Thanks!

 

[silverpig]

It gets colder cause that's what it's supposed to do.