Thermodynamics

[collegelife101]

Hello everyone!

I just had a quick question on heat transfer and change in volume. One problem states that a rubber balloon moves from the bottom of a tank of fluid to the top. The temperature of the fluid remains constant. It asks what causes the decrease in temperature in the balloon as the balloon moves up?

I can see how the pressure is reduced, thus allowing the volume of the balloon to increase and perform work. However, I don't get why there isn't any heat exchange. One solution said that this is because of the uniform temperature of the water. I'm not sure I really understand what that means. Any help is appreciated!

Thanks!

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[premed1001]

What is the question number and what book?

 

[Chrisz]

This is because when balloon flows up, the forces around the balloon gets smaller, so the gases can expand. Expansion of gas is endothermic so the gas molecules absorbs heat from the space inside the balloon. No net exchange of heat with liquid surrounding the balloon has taken place yet. This causes decrease in temp within the balloon.

 

[premed1001]

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Thanks!

 

[Chrisz]

f=density*g*volume
pressure=density*g*height
pv=nrt

so going up height gets smaller, therefore pressure gets smaller
since pressure goes down, volume goes up
if volume goes up, then the force should go up also shouldnt it?

gas expansion endothermic, didnt know that thanks!
is there a formula for this?

First, pressure=density*g*height. this is delta pressure, which depends on delta height.
Think about that pressure is force per unit area. Think about this term. Also, pressure increases as you go deeper into water. So as the balloon rises, it feels less pressure, which means it experiences less force per unit area. if the pressure inside the balloon is greater than the surrounding pressure, it will expand in an attempt to equilibrate with the surrounding force per unit area on the balloon.

 

[premed1001]

Thanks!
Also is there another formula relating volume to temperature? Besides pv=nrt
I did not know expansion was endothermic or vice versa so thanks for that!

 

[Chrisz]

Thanks!
Also is there another formula relating volume to temperature? Besides pv=nrt
I did not know expansion was endothermic or vice versa so thanks for that!

pv=nrt describes the initial and final state of a reaction. It does not tells us what is exactly happening during the middle of a reaction. For example, if the balloon rises up to the furthest point it can reach, and the temperature equilibrate to the ambient temp. Then, pv=nrt would tell you the final state. I have no idea if there is a formula to describe what is happenning at each moment. For qualitative understanding, when gas expands, it has to overcome intermolecular force between the molecules, so energy must be absorbed to break the intermolecular forces. Therefore, endo

 

[Chrisz]

Gotta go back to study right now, have spent too much time on studentdoctor.net today. LOL

 

[premed1001]

pv=nrt describes the initial and final state of a reaction. It does not tells us what is exactly happening during the middle of a reaction. For example, if the balloon rises up to the furthest point it can reach, and the temperature equilibrate to the ambient temp. Then, pv=nrt would tell you the final state. I have no idea if there is a formula to describe what is happenning at each moment. For qualitative understanding, when gas expands, it has to overcome intermolecular force between the molecules, so energy must be absorbed to break the intermolecular forces. Therefore, endo

This is amazing! Thank you!!