Inflated balloon is put in a refrigerator and it shrinks. How does the first law of thermodynamics apply?

Answer 1

Good question!

First law of thermodynamics states that the internal energy change ΔE of a system is equal to (Q - W). Where Q is the heat absorbed by the system and W is the work done BY the system on the surroundings.

Now, as you already might be knowing that work can also be expressed in terms of pressure and volume as-

W = PΔV (where P is pressure of the gas inside the system and ΔV is the change of volume)

[Here: "system" = balloon; and "surroundings" = refrigerator]

And as the change in volume (ΔV) in the case of balloon stated above is negative i.e it deflates, work must be negative i.e (minus W).

Now, as Q is also negative (heat is given off by the balloon), the equation of First Law of thermodynamics reads-

ΔE = -Q - (-W)
ΔE = W - Q

Now, if the balloon was large in volume, the change in internal energy ΔE would be positive and the balloon will either burst or move frantically in the refrigerator shelf.

But, if the balloon was a small one, it'd slowly become limp and deflate as the Q will overpower W (PΔV) in the equation and the internal energy ΔE will decrease and become negative.

Please note that I have made a lot of assumptions here that perhaps, might not be so easy to apply practically.

The points you'd have to consider are:-

1-How tightly the balloon is wound so that air doesn't leak out of it.
2- Whether or not there's a leakage in the balloon.
3- The temperature of the balloon.
4- The material of the balloon.

Strictly speaking, the first law isn't even applicable in this case because the condition in the law is that the system (balloon) shouldn't be allowed to move. Its kinetic and potential energies should both be zero which clearly is not the case with our experiment.

I hope this answer helps!

Good luck!

Answer 2

If you think of the inside of the balloon as the system. The gas inside looses heat to the fridge. The atoms inside the balloon in return become closer together. They have less kinetic and electrical energy to keep them apart.