How to play Touch and Share
- Choose a test and read the starting charges on the card.
- Identical metal spheres share their total equally; spheres joined by a wire share it in proportion to their radii; plastic takes nothing.
- Type your prediction, with a sign if it is negative, and press Start. The spheres touch one pair at a time.
- Compare the orange mark with the charge bar on the right.
Conductors and insulators
In a metal, some electrons are not tied to any atom: they drift freely through the whole piece. That makes metals conductors. When two metal objects touch, these electrons flow from one to the other until both are at the same electric potential, and then stop. Identical spheres end with equal charges; a big sphere ends with more than a small one, because it can hold more charge at the same potential. In plastic, glass, rubber, and dry air, every electron stays with its atom: they are insulators, and charge put on them stays where it is. That is why charged spheres stand on plastic, why wires have plastic coats, and why touching a charged metal object with your hand, a conductor joined to the huge Earth, takes almost all its charge away.
A focused learning model
The spheres are identical except in the big-and-small test, and they stand on perfect insulators. Charge moves only at the moment of contact and spreads until both are at the same potential; the spheres then move apart without losing any. Spheres that actually touch share in a more complicated way when their sizes differ, so the big-and-small test joins them with a long thin wire, which leaves each as good as alone. Plastic takes no charge at all. The marks are about one per nC; the stands and spheres are not to scale, except the 3 : 1 radii. Answers within 2% are right.