Perhaps without being aware of it, you used the concept of conservation of charge in your analysis of what you observed.
A few simple problems illustrate this conservation law. When two identical spheres touch, they will end up with identical charges. (If they did not end up with identical charges, then there would be a way to distinguish them. They would not be identical.) One sphere with a charge of -10, touches a neutral, identical sphere. After touching and separating, each sphere has a charge of -5. The charge was transferred, but the total charge remained the same.
Keep in mind that neutral matter already contains enormous amounts of pluses and minuses, but in equal amounts. Thus the "excess" and "deficiency" are relative to a baseline of starting out neutral.
Coulomb's Law and Newton's Law of Gravitational Attraction
You found you could actually calculate the force of attraction or repulsion of charges by using Coulomb's Law:
Here's an incredible part of physics that you did not examine in this activity. Coulomb's Law for electrostatic attraction and repulsion is very similar to Newton's Law of Gravitational Attraction! |
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where F is the force in newtons (N),
m1 and m2 are masses in kilograms (kg),
d is the distance between the centers of the masses in meters (m), and
G is the gravitational constant, always equal to 6.67 X 10-11 Nm2/kg2.
Look at the similarities:
- Both laws show forces that decrease with the square of the distance.
- Both laws show forces that depend on the product of the masses or charges.
- Both laws have constants.
Look at the differences:
- Electric forces are attractive and repulsive; gravitational forces are only attractive.
- Charges come in two varieties, + and -. Mass comes in one variety, +.
- The electric force constant is quite large, while the gravitational force constant is quite small.
If you look at the gravitational and electrical forces between two electrons, the gravitational force is much, much smaller. It is so small, you don't even take it into account when describing the behavior of the charges.
The experimental techniques to find the value of k and G are quite similar. In Coulomb's experiment, two spheres were attached to the ends of a rod and the rod was suspended by a wire. These spheres were charged, and similarly charged spheres were brought near the ends of the rods. The repulsive force caused the wire to twist. The twist was a measure of the force, and Coulomb was able to verify his law. |
