Inflating a Balloon (elastic balloon vs. paper balloon)

[lily001]

Homework Statement

When you blow up a paper balloon it still stays inflated but with a real elastic balloon it doesn't and we have to Tie it. Why is the different?

Relevant Equations

Not Available.

When you blow up a paper balloon it still stays inflated but with a real elastic balloon it doesn't and we have to Tie it. Why is the different?

 

[berkeman]

What's a paper balloon?

 

[kuruman]

What's a paper balloon?

https://www.amazon.com/dp/B00BT5CV4I/?tag=pfamazon01-20

 

[haruspex]

Homework Statement: When you blow up a paper balloon it still stays inflated but with a real elastic balloon it doesn't and we have to Tie it. Why is the different?
Relevant Equations: Not Available.

When you blow up a paper balloon it still stays inflated but with a real elastic balloon it doesn't and we have to Tie it. Why is the different?

The clue is in the name: blowing up an elastic balloon deforms it elastically, which means it will tend to contract again when the pressure differential is removed.
Inflating a paper balloon deforms it plastically; the paper is not stretched. Even so, the question arises as to why it does not largely collapse again under gravity. It is not clear from the image linked in post #3, but maybe it has a valve?

 

[phinds]

. It is not clear from the image linked in post #3, but maybe it has a valve?

The article explicitly states that the "valve" where you blow the air in stays open. As you said, the paper balloon deforms plastically, meaning it has no back-pressure that would deflate it.

 

[haruspex]

The article explicitly states that the "valve" where you blow the air in stays open. As you said, the paper balloon deforms plastically, meaning it has no back-pressure that would deflate it.

One of the comments mentioned a Physics Today article, https://pubs.aip.org/physicstoday/a...5/Kamifusen-the-self-inflating-Japanese-paper.
The explanation offered is feasible but far from proven.

It reminds me a sleight of hand trick. The conjurer has concealed in her hand the crumpled up, dried membrane of an egg, the skin just inside the shell. It has, of course, a small hole whence the egg contents were blown out. How the shell is removed I forget.
The conjurer waves a fan about, held horizontally, secretly releasing the membrane onto it. As it is batted about, the membrane inflates, giving the appearance of a whole egg.

 

[Orodruin]

Inflating a paper balloon deforms it plastically; the paper is not stretched.

Just to mention that plastic deformation does not necessarily imply that there is no stretching involved.

 

[haruspex]

Just to mention that plastic deformation does not necessarily imply that there is no stretching involved.

Yes, I meant not stretched in a way that will lead to rebound. Well, not much.

 

[Tom.G]

Hi @lily001,


Let's see if this helps. It does take a little imagination though - and it is somewhat far from the real world.

For the paper bag, you can consider it as a whole bunch of tiny sticks that are held together with chewing gum. You can crumple it into a ball, or lay it out flat on the table. What you can NOT do is stretch it out so it is bigger, it tears if you try that.

The Rubber balloon is a whole bunch of crinkled fibers (actually long molecules) that are tangled up with each other - sort of like yarn used for knitting a sweater.

Here are some photos and a description from another post, about yarn:
(https://www.physicsforums.com/posts/6484592/)


Here are some photomicrographs of 2-strand 2-ply Peruvian Highland Wool yarn. It is unused and still in a ball. The fine marks on the ruler are 1/100 inch apart.

The first image is the relaxed yarn just unwound from the ball. Note that the individual fibers (hair) are crinkled, not straight, and rather tangled.

The second image is the yarn stretched by pulling on it. Here the fibers are straighter and closer together and the yarn diameter is smaller.

The third image is the yarn again relaxed. You can see it has regained much of its diameter and can probably detect that individual strands are again not straight.
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It is the tendency of the hair to recover its shape (crinkled) that causes the finished product to at least partially recover the original shape after being stretched. Each fiber acts somewhat like a spring.


When you blow up the balloon, you are pulling on and stretching the long molecules just like when the yarn is stretched. When you release the pull on the rubber by letting the air out of the balloon, the rubber returns (mostly) to its starting size/shape.

Hope this helps!

Cheers,
Tom