Volta's Hailstorm is all about charges and potential difference.
It is set up so that you may charge the bottom plate along with all of the little hail-styrofoam-thingies. Alternatively, you can just charge the top part. When charged, the pieces are repelled from the bottom (or attracted to the top, depending on the setup). Once they hit the top they deposit their charge on the top plate (or gain charge in the alternate scenario) and fall back down.