Why Do Styrofoam Pieces Dance and Stick on a Charged Plastic Surface?

[billbillington]

Styrofoam "dancing" question.

Homework Statement

A large piece of plastic is placed on top of a metal cafeteria tray so that there is a small space of air between the bottom of the metal tray and the large plastic piece. In this space are small Styrofoam pieces. When the plastic covering is rubbed with paper, the small Styrofoam pieces "dance" At some point, the Styrofoam pieces stick to the underside of the plastic covering. If I touch the plastic covering right above the Styrofoam piece, the Styrofoam piece falls and may dance some more.

1) Explain why the Styrofoam pieces "dance around"
2) Explain why the Styrofoam pieces fall when I touch the plastic right above them.

2. Homework Equations [/b]
No equations needed.

The Attempt at a Solution

1) The plastic has excess electrons which causes the neutral styrofoam to attract...? I'm not sure what causes it to repel from it after though...
2) When I touch the plastic right above them, i am grounded so i discharge the excess electrons. This causes the attraction to stop since the charge of the plastic becomes neutral... I don't know after that.

 

[andrevdh]

According to my knowledge the sheet should charge up negative. Since the plastic is not conductive the charge on it cannot wander around. This means that some regions might get charged up more than others and some may receive no charge at all. The electric field generated by the sheet is therefore most likely nonuniform. This electric field will induce regions of irregular charge distribution on the tray (think of the charge on the tray as being kept there [frozen] by the electric field produced by the sheet).

Question 1: "Will this charge be of the same (negative) or opposite charge as than on the sheet?"

Question 2: "How will this charge on the tray influence the stryrofoam pieces resting on it considering the fact that styrofoam is also nonconductive?"

Question 4: "What effect will the irregular charge distribution on the tray have on the stryrofoam pieces?"

Question 5: "Can the electric field induce an irregular charge distribution on the styrofoam pieces since some parts of the stryrofoam is closer to the sheet than others?"

Question 6: "Under what circumstances do you think that two nonconductive objects can exert electrostatic forces on each other?"

 

[m2003]

I would explain the phenomenon of Styrofoam "dancing" in the following way:

1) The rubbing of the plastic covering with paper creates a static charge on the surface of the plastic. This static charge attracts the neutral Styrofoam pieces due to the phenomenon of electrostatic induction. The Styrofoam pieces are made of a material that is easily polarized, meaning that its electrons can easily shift within the material. When the plastic is rubbed, the excess electrons on the surface of the plastic cause the Styrofoam pieces to become polarized, with one side becoming positively charged and the other negatively charged. This creates an electrostatic force between the plastic and the Styrofoam pieces, causing them to move towards the plastic.

2) When you touch the plastic right above the Styrofoam pieces, you provide a conductive path for the excess electrons on the surface of the plastic to flow to the ground. This neutralizes the charge on the plastic, removing the electrostatic force between the plastic and the Styrofoam pieces. As a result, the Styrofoam pieces are no longer attracted to the plastic and fall down due to the force of gravity.