## Illustration 22.4: Charging Objects and Static Cling

These animations model materials with charges (red = positive and blue = negative). The arrows show the forces between particles. Restart.

There are a number of ways to charge objects. You may be familiar with what happens when you rub a balloon against your sweater. Due to the rubbing, the balloon becomes negatively charged. It will stick to the wall or the ceiling, but the wall and ceiling are neutral. Why will it stick to neutral objects? Run the balloon animation to see. The model shows the negatively charged balloon near a neutral ceiling. While neutral, there are charges in the ceiling. The ceiling is not chargeless; it just has an equal number of positive and negative charges. What happens to the neutral ceiling? This effect is called polarization (when the charges in an atom get slightly distorted due to other nearby charges).

Another way to charge an object is by induction (to induce it). First, look at the case where the left plate is positively charged and the right plate is neutral (equal number of positive and negative charges) as in this animation. Why do the charges separate as they do in the right plate? Charges move according to the forces they experience (like charges attract and opposite charges repel). Suppose we give the charges on the right a place to go (specifically ground) as shown in this animation? What happens? Why?; Neutral pairs of positive and negative charges separate from each other due to the nearby positive charge. Then the positive charges on the right leak off the grounded plate.

When an object is charged (like a computer screen), other objects can stick to it. We call this "static cling." Consider charged particles near a charged screen as shown in this animation. What happens to the positively charged particles? How about negatively charged particles? Now consider neutral particles as in this animation. What happens to the neutral particle between two charged screens? It gets polarized and then attracted to the screen. Notice that a charged screen can therefore attract both charged particles and neutral particles. This explains why your computer and television screens (which are charged) collect dust so easily.

Illustration authored by Anne J. Cox.
Script authored by Morten Brydensholt.
Applet authored by Vojko Valencic.

Physlets were developed at Davidson College and converted from Java to JavaScript using the SwingJS system developed at St. Olaf College.