Why they don’t fall?

Flour is commonly found in paper bags and not levitating in the air at your home. So how possible that my flour just hang there? Heh … so … uhm …, well I’m going to try explain it somehow easily. Ugh! Before you proceed reading please prepare enough imagination. It will come in handy.

Imagine four parallel wires pointing out of vertices of the square on your monitor. Between them is located moving grain, for example as it is in the following figure (why is it multicolored we will discuss in a moment):

Sketch cross-section of the trap.

Forget now all physically important things and assume, that on the grain acts only electric force. Since we use source of alternating voltage, the force “pulls” the particle for a moment toward the wires out of trap itself, and in the sequel “pulls” it back inside trap. Grain will oscillate back and forth and back and forth and back and forth … It oscilate like a weight on a spring. This movement we’ve seen on video at the end of previous section “Life in a trap.”

Now lets have a look again on the picture above, especially on the ”path” of grains. We start in the center of the line, that means in the black dot, where the magnitude of velocity of particle is the biggest. Assume that it is just “going” outside. In the green area grain slows down at first and on the edge it stops completely and then accelerates back toward the center of the trap. During this whole movement the electrical force acts on the grain towards the center of trap. Similarly, in the red area the particle slows down, stops and goes back, it has to be “driven” by electrical force to the outside. The following video shows fourteen times slower “living” particle, where you can see the braking and accelerating with a bit of effort.

There is one more major thing I didn’t tell you yet. The force acting on the particle is varying depending on how far is the particle from the center of the trap. In the center it is vanishing, nearby wires in the contrary is the greatest. So in the green area is the force greater than in the red area. Let put it all together now. In green area force is acting towards the center of the trap and is slightly larger than in the red area, where he acts out. So if the force acting in is greater than the force acting out, the total force must act towards the center of the trap! And this force holds grains inside the trap.

Is that clear? Do you now know, why grains stay inside, what holds them there? If something is unclear, write me a comment. Thanks.

However particles aren’t trapped forever. For example, when we begin to reduce voltage, grains begin to slowly fall down. Check it for yourself.