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Cavendish's experiment was similar, but the attraction between the pairs of spheres was due to their gravitational attraction. This tiny force was measured by the twist in the wire.

This symmetry of what appears to be two unrelated forces provides a glimpse into a beauty of our world. Physicists remark on this beauty. It drives them to try to find out if there are other underlying understandings of the two forces because of that symmetry. This is what physicists are exploring when you hear about their work on unified theories.

torsion fiber

Reflecting on the Activity

Physics to go

Physics Words

Newton's Law of Gravitational Attraction: the relationship among gravitational force, masses, and the distance between the masses.

Reflecting on the Activity and the Challenge

You probably know that atoms are composed of electrons and other particles. These particles have charges. You will need to provide a description of the interaction between the charges when you provide a description of the atom. You may find a way to include in your exhibit the larger concepts of conservation of charge or the ability to actually calculate these forces of attraction and repulsion.

Physics To Go

1. Electrons are transferred from a rod to a piece of material.

Which object will become negatively charged?
Which object will become positively charged?

2. A rubber rod is negatively charged after being rubbed by wool. Explain how this happens.

3. Two identical spheres are mounted on insulated stands. The first sphere has a charge of -1. The second sphere has a charge of -3. After the spheres touch, what will the charge on each be?

4. One of two identical metal spheres has a charge of +1 and the other sphere has a charge of -5. Compare the total charge on the spheres before and after contact.