Rocket pulls a long strip of clear tape off the table and hangs it from the edge of a shelf.
He pulls a second strip the same way and hangs it beside the first. The two strips lean away from each other.
"They are pushing each other," he says. "But they are not touching. What is going on?"
Raven brings a tiny torn scrap of paper close to one strip. The scrap leaps up and sticks.
"What do you notice?" she asks. "Strip and strip push apart. Strip and paper pull together."
Nova hovers between the strips, her glow making them sway. "There are two kinds of something," she says. "Figure out which is which."
Rocket squints at the tape. "Two kinds. Like two poles?" Nova hums. "Close. Read on."
Static electricity has been known for a very long time. More than 500 years before the common era, Greeks noticed that polished amber attracted bits of straw.
A physics text explains it with a quantity called electric charge. There are only two types, called positive and negative.
Normal matter is made of atoms, and atoms contain positive and negative charges, usually in equal amounts. So most objects are neutral.
Rubbing two materials together can move some negative charge from one to the other. One ends up negative and the other positive.
No charge is created or destroyed. Existing charges are moved about. Scientists call this the conservation of charge.
Rubbing a balloon on your hair, a sock tumbling in a dryer, pulling tape off a table: each one separates charge.
The same text lists the basic rules. Like charges repel, whereas unlike charges attract. The force between charges decreases with distance.
When glass is rubbed with silk, the glass becomes positive and the silk negative. The two attract. Two rubbed glass rods repel.
Rocket's two tape strips were pulled off the same table the same way. They picked up the same kind of charge, so they repel.
A charged object can also attract a neutral scrap of paper, the way the charged comb pulled bits of paper on Monday.
| Pair | Charges | Result |
|---|---|---|
| Rubbed glass and the silk that rubbed it | positive and negative | attract |
| Two glass rods both rubbed with silk | positive and positive | repel |
| Two silk cloths that rubbed glass | negative and negative | repel |
| A charged comb and a neutral paper scrap | charged and neutral | attract |
| Statement | True or false? |
|---|---|
| Rubbing creates brand new charge. | ? |
| Atoms contain positive and negative charges, usually in equal amounts. | ? |
| The force between two charges gets stronger as they move apart. | ? |
| Polished amber attracting bits of straw was noticed more than 2,500 years ago. | ? |
Clear thinking. Tomorrow is Field Lab: you measure how fast a magnet's pull fades with distance.