Helium balloons don't float all the time

If you'd like to make a point about the physics of helium balloons, just stick one in a vacuum.


One of the earliest physics demonstrations that I ever witnessed was a simple one explaining the motion of a helium balloon in Earth's atmosphere. It involved a clear plastic bucket full of unpopped popcorn kernels - I think it was an emptied-out red vine container - and a ping pong ball. The teacher pushed the ping pong ball to the bottom of the container. Then he started shaking the container back and forth, making the kernels shift. Soon the ping pong ball popped out at the top of the kernels.

The ping pong ball was less dense than the kernels, so they naturally nudged it upwards as they settled, with gravity, towards the bottom of the container. The same thing happens in the atmosphere, when a gas like helium competes for space with denser gases.

We naturally think of helium balloons as "flying," or "hovering" but they don't. They get pushed upwards by the denser air around them. We can see this the moment we remove the air. Put a helium balloon in a vacuum and it can't float any more than a rock, or for that matter, a person.

Most balloon-in-vacuum experiments focus on the way balloons expand, due to their inner air pressure, as the outer pressure on them diminishes. I like this one better because it shows that, the moment you take away the air, the helium balloon sinks like a rubber duck after the plug has been taken out of a tub. It's a nice reminder that we're not moving through "nothing." We're moving through a sea of air, and some things can float to the top of that sea. Remove the sea, and they all come down.



Something related to this happens when you put a helium balloon in a car. As you accelerate, brake and turn it creates pressure gradients in the air in the car, causing the balloon to move opposite the motion of ordinary objects, e.g. leaning into a turn.