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Electric Charge

A long time ago, someone noticed that when amber is rubbed with a cloth it attracts small seeds or pieces of straw. No one knew why. (Amber is a soft ochre "stone" of hardened tree sap.) Many years and many experiments later, the following story has emerged as the best explanation:
Most material on Earth is composed of three particles, called protons, neutrons, and electrons. The protons and neutrons hold together in tight lumps, called nuclei. The much less massive electrons exist in a cloud around the nuclei, forming atoms. Sometimes the electrons hold the atoms together, forming molecules and so on. The electrons are sometimes quite mobile, and this mobility results in most of the changes we observe.

But that's chemistry. That is not our story.
This is our story: Electrons have a negative charge, and protons have a positive charge. Two charges of like sign exert a repulsive force on each other, while two charges of unlike sign exert an attractive force on each other (Figure 14-1). Most objects are neutral, that is, they have nearly the same number of protons as electrons. When our unknown predecessor rubbed the neutral amber it acquired a few extra electrons, giving it a net negative charge.

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Figure 14-1

Under normal circumstances, as in physical and chemical reactions, electrons, protons, and neutrons are permanent objects. They may move around, but they do not spontaneously disappear or appear. Thus we have the following:
Conservation of Charge
If a system is closed (no matter goes in or out), then the total charge of that system is conserved (stays constant as time passes).

Clearly this is true in chemical reactions, which are just rearrangements of electrons and nuclei, but it turns out to be true even in unusual circumstances, such as the radioactive decay of nuclei or reactions of exotic particles. (See Chapter 16.)

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