How does the particle theory explain balloons rising up?

Answer 1

Let's take the example of the balloon, which is maybe the easiest to visualize. An uninflated rubber balloon can contain a certain volume of gas at atmospheric pressure. You can see roughly what that volume is by pouring water into the balloon, then putting it under water, with the open end up, and letting the pressures equalize.

When you start to blow up the balloon, three things happen almost simultaneously.

1. You force more molecules into the available space, so a unit area (say 1 cm^2) of the balloon's surface will undergo increased bombardment by the gas molecules: this means the pressure on the surface increases. If it were a rigid container, that would be the end of the story: more molecules - more pressure. If the molecules pushed harder on the container, the container would push back harder on the molecules.

2. But a balloon isn't rigid: it's elastic, so it deforms when a force is applied to it. Since the force of the extra molecules is pushing in all directions, the balloon expands.

3. But that doesn't continue for long. As the balloon expands, and the rubber stretches, the forces in the walls of the balloon increase. After a short time, the forces tending to pull the balloon back to its original unstretched size balance out the increased forces exerted by the extra molecules, and everything reaches equilibrium. The pressure's higher, the balloon's bigger, and the rubber's stretched a bit.

And that continues until the balloon can't stretch any more. Same with tires and balls.

The subject of the actual pressure changes when blowing up a balloon is quite fascinating. Why does that floppy bit on the neck never expand? Why does a long balloon start to inflate near to your mouth instead of at the far end? I can't locate it right now, but if you search the Scientific American 'Amateur Scientist' archives you may be able to find an article published years ago on exactly these topics and many more.

I hope this is some assistance.

Answer 2

The particle theory explains balloons rising up by the principle of buoyancy. The helium or hydrogen gas inside the balloon is less dense than the surrounding air, causing it to float upwards. This is because the buoyant force acting on the less dense gas is greater than the gravitational force pulling it down, allowing the balloon to rise.