Further Reading


Polarization

Prerequisites

Clipped from PhET sim
by E. Redish, 2011

As we discussed in the page on Electric Forces, many of us have experience with what we identify as "static electricity." Mostly this seems like stuff sticking to other stuff and shocks.  Shocks involve electric currents, which we will deal with later in the class. But the "sticking to other stuff" must have something to do with our basic understanding of Charge and the structure of matter — that matter is made up of a balance of two kinds of charges that mostly cancel, but that can in principle move around and become unbalanced. We also have been told -- and could demonstrate it for ourselves with two pieces of Scotch Magic TapeTM. (See the Interactive Demonstration, "Making a model -- Thinking about electric force.") If you don't have any Scotch Magic Tape, you might try rubbing a balloon on your sweater as shown in the picture on the right and seeing where you can stick it. 

If you don't have a sweater — or a balloon — try the PhET simulation of the situation by clicking on the picture of the balloon and sweater.  Try it first with the radio button for "Show no charges" clicked. Rub the balloon on the sweater and see what it sticks to. The sweater? The wall?

The structure of matter

What's going on here? Why does the rubbed balloon go towards the sweater unless the balloon is very close to the wall, at which point it will stick to the wall? Our model of the charged structure of matter — electrons and ions — helps us make sense of this. When the balloon is rubbed on the sweater some electrons rub off from one or the other so the one that gets electrons will wind up with a negative charge and the one that loses electrons will wind up with a positive charge. The basics of the electric force (see Coulomb's Law) tell us that opposite charges attract, like charges repel, and the force between them gets weaker as they get farther apart. So the sweater should attract the balloon.  But why is the balloon attracted to the wall? The wall wasn't rubbed and should be neutral. In the page Electric Forces, why did the rubbed comb pick up little pieces of paper even if the paper wasn't charged in any way?

Insulators and conductors

All matter is made up of positive and negative charges approximately equally balanced, but different kinds of matter behave differently electrically. In some matter, the positive and negative charges are attached to each other and can't move very far. This happens when the matter is made up of molecules that are neutral and where electrons are not easily liberated to move from one molecule or another. Matter like this is called an (electrical) insulator.  (There are also thermal insulators, which is something different.)

In some kinds of matter, electrons can move more easily. In metals, electrons are shared over many, many atoms and can move back and forth in a metallic crystal rather easily. In ionic fluids, some electrons have moved from one atom or molecule to another, creating a balance of positive and negative ions, which may move around freely in the fluid in response to electric forces. Such materials are called conductors.

For now, we will limit our discussion to insulators, but in biological systems, the motion of ions plays a critical role in the processes of life and we will return to a consideration of them later in the class.

Any charge attracts neutral matter

We discover that our rubbed balloon will stick to almost anything. Why is that? We can understand this from a few basic qualitative ideas about electricity — without the full power of the Coulomb force law. All we need to know are the following:

  • All matter is made of a balance of positive and negative charges.
  • Any charge exerts a force on every other charge — attractive for charges of opposite sign, repulsive for charges of the same sign.
  • The force between any two charges gets weaker as the charges get farther apart.
  • The balanced charges in matter can move in response to electric forces, but in insulators, they may be able to move only a very small distance.
Clipped from PhET sim
by E. Redish, 2011

We can now see what's happening to make the balloon stick to the wall. The effect is shown in the PhET simulation when you click the radio button "Show all charges" and move the balloon to the wall. The negative charge on the balloon exerts forces on the positive and negative charges in the wall, pulling the positive charges closer and pushing the negative charges away. (This effect is hugely exaggerated in the drawing.) As a result, the positive charges are closer to the negative charges on the balloon than the negative charges in the wall. So the attractive force between the negative charges on the balloon and the positive charges in the wall is greater than the repulsive force between the negative charges on the balloon and the negative charges in the wall. Although the effect on each charge is small, the effect is the same on every molecule so it adds up to an observable effect.

The result is the striking (and confusing) observation:

Any charge is attracted to neutral matter.

You can imagine how confusing this was to the scientists in the 18th century trying to figure out what electricity was all about and who didn't know about charges!

Workout: Polarization

Follow-on

Article 386
Last Modified: February 4, 2019