helium, pt. 3

god, why can’t I let this go?

okay, so I thought I had finally settled my problems with helium balloons. But now, I’ve come to a brick wall.

Helium balloon gets pushed upwards by slight gradient in pressure, which is due to gravity pulling air down towards earth. Got it.

But what if the balloon was a cube, and I placed it on the floor? I feel that it would still rise. Would it? And if so, by what mechanism?

later,
jb

It would still rise, though probably a bit slower. There are still spaces for air to creep in around the edges of the balloon.

If it was really a perfect cube, on a perfect surface, I dunno what would happen.

I thought balloons rose because the amount of air they displaced weighed more than the baloon, and because of this they rose. I imagine this would be independant of shape (except for the perfect cube thingy)

It would sit there, because there is no air under it to buoy it up. Eventually, a random motion caused by thermal energy would bounce it up enough that an air molecule could get under it. This would increase the chance that another molecule could get in, and eventually you’ll have thousands of molecules under it, then a gap of several thousandths of a centimeter, and then you’ve got air pressure under it and up it goes.

You can see this effect in your bath tub. If you find an object that very closely fits to the tub floor, when you carefully put water in the tub it can stay submerged even if it would otherwise float. Nudging it or just waiting will get it to pop to the surface.

A cubic helium balloon would rise as high, if perhaps more slowly as any other shape with the same density. If the flatness of the top were a major aerodynamic factor, it would tilt, and rise with one corner upwards. If the fit with the floor was perfect, and able to seal the edges from leakage the balloon would remain on the floor until disturbed because of the partial vacuum pressure underneath it. Once it was above the floor by even a tiny amount, it would continue to rise.

Air is a fluid. It expresses it’s pressure in all directions, including up on the bottom of the balloon. Since the balloon weighs less than the amount of air it displaces it is out of equilibrium with the air around it, and it rises, until it reaches equilibrium, or bursts.

where does the reverse gravitational potential energy helium has come from?

is a slight amount of the energy of alpha emission diverted into giving the helium is potential energy?
thanks,

jb

The key concept is buoyancy. A helium balloon has potential energy when it is at ground level because it displaces air upward. The air has gone up, so its potential energy has increased. When you let go of a balloon, the air rushes in under it, allowing the balloon to go up. The air ends up lower than it was, releasing the potential energy.

This has absolutely nothing to do with alpha emission, in fact alpha emission is something helium cannot do (helium is alphas)

Think of a cork at the bottom of a swimming pool. If you let go of it, it floats up. Where did it’s “reverse potential energy” come from? It came from displacing water. The water, being more dense than the cork, flows down around it, pushing it upward.

Helium isn’t special, it’s just lighter than air. There is no magic helium property without which stuff can’t be lighter than air, it’s just a density thing.