# Electric forces

#### Prerequisites

We've looked at one non-touching force - gravity. Since gravity is so weak we only need to worry about the gravity from the earth, and since we typically stay near the surface of the earth where gravity stays nearly constant, it was pretty straightforward.

Our next non-touching (action-at-a-distance) force is electricity and it's way trickier.  Of our three — gravity, electricity, and magnetism — it's probably the one with which you have the least physical experience. Everybody knows and feels gravity, and every kindergartener has played with magnets. Our direct experience with electrical forces are few and seem not very significant.

If I get a shirt fresh from the dryer it will often seem "clingy" — that is, the different parts of it will stick together.  Sometimes — even days later — I'll find a sock clinging to the inside of a shirt when I go to put it on.  One year, when my daughter was just a toddler continually clutching a woolen blanket, we had new nylon carpet put in on the stairs in our townhouse. She came down it the first time dragging her blanket down the stairs and when she touched the doorknob to go into the den, produced a two-inch spark, shocking her (literally) to tears. And most people know that if you rub a plastic comb with wool, you can pick up small pieces of paper, as shown in the figure at the right.

None of this seems very interesting and exciting. Amusing perhaps. The phenomenon was known to the Greeks nearly 2500 years ago. If you rubbed amber (Greek ηλεκτρον = electron) with wool, you got the "sparky" phenomenon I've just been describing. It took 2000 years for people to realize that something extremely important was going on here.  First, the comb — with just a bit of rubbing — is able to create a force on the bits of paper that is stronger than the entire earth pulling back on it with gravity! This indicates that there is another action-at-a-distance force taking place—- and one that is way stronger than gravity.

Second, where did the "electrical agents" — whatever they are — come from? The answer is that matter itself is highly electrical. It consists of extremely powerful counterbalancing electric agents; but the forces between them are so strong that they hold matter together electrically and we rarely see the powerful forces hidden inside.

This is a theme that will recur again and again as we study physics. The materials and objects that seem to us inert and ordinary are instead highly dynamic balances of immense energies and forces. Learning how to produce small shifts in those balances provide us with the powers of modern technology.

Joe Redish 10/9/11

Article 370