# Expansion of balloon

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1. Dec 3, 2014

If I heat a rubber balloon filled with helium slowly and if the balloon is fully expandable and (the balloon) can be assumed to require no energy in its expansion,what type of thermodynamic process is taking place? Is it isobaric?
Since the balloon expands the pressure exerted by the gas on balloon should increase. Then how can it be isobaric?
It can't be isochoric nor isothermal.
It is either adiabatic or isobaric.

2. Dec 3, 2014

### proton007007

its called free expansion . work done by the balloon is zero and so is the change in heat . this results into no change in internal energy by the first law of thermodynamics .

3. Dec 3, 2014

It's given isobaric... This concept came in IIT
Exam.... The toughest exam in India for admission to B.tech. IIT IS THE MOST REPUTED COLLEGE IN MY COUNTRY.

4. Dec 3, 2014

### Staff: Mentor

Yes, it's isobaric. If the balloon is fully expandable (i.e., doesn't develop significant tension as it expands), the pressure inside the balloon is always nearly equal to the pressure outside the balloon. And, the pressure outside the balloon is 1 atm. (constant). So the pressure inside the balloon is constant at about 1 atm, and the process is isobaric. The key to all this is that the balloon membrane doesn't develop significant tension.

Chet

5. Dec 3, 2014

### mmeftahpour

If rubber tension is negligible it is isobaric otherwise we can consider P=f(V), and f depend on rubber specification and balloon dimensions.

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6. Dec 3, 2014

### Staff: Mentor

Actually, it would be the pressure difference across the rubber membrane ΔP that you can calculate from the rubber properties and the balloon shape.

Chet

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7. Dec 4, 2014

when you are heating a gas its randomness increases so it will collidewith the walls of the balloon more freaquently. hence pressure inside th balloon should increase right?

8. Dec 4, 2014

### Staff: Mentor

Not if volume increases so that the pressure matches the outside pressure.

Chet

9. Dec 4, 2014

so you mean that the area of contact increases and since P=F/A pressure increase gets countered by expansion of balloon.

10. Dec 4, 2014

### Staff: Mentor

No. Pressure stays the same. You're familiar with the ideal gas law, correct?

11. Dec 4, 2014

Pv=nRT. as volume temperature of balloon increases pressure should increase.
if pressure exerted by gas on balloon doesnt change, how does it expand.

12. Dec 4, 2014

### Staff: Mentor

From your understanding of mathematics, if the volume v increases in direct proportion to the temperature T, how does the pressure vary?
We already said the we are looking at a limiting case in which the stiffness of the balloon membrane is negligible. Under these circumstances, the pressure inside the balloon must match the pressure outside the balloon, and the pressure outside the balloon is constant at 1 atm.

Chet