Electric phenomena depend on charge.

• There are two kinds of charge, called positive and negative.

• Electrons and protons—the constituents of atoms—are the basic charges of ordinary matter.

• Charging is the transfer of electrons from one object to another.

  

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Charges have well-established behaviors:

• Two charges of the same kind repel; two opposite charges attract.

• Small neutral objects are attracted to a charge of either sign.

• Charge can be transferred from one object to another.

• Charge is conserved.

  

There are two classes of materials with very different electric properties:

• Conductors are materials through or along which charge moves easily.

• Insulators are materials on or in which charge is immobile.

  

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Coulomb’s law is the fundamental law for the electric force between two charged particles. Coulomb’s law, like Newton’s law of gravity, is an inverse-square law: The electric force is inversely proportional to the square of the distance between charges.

《 LOOKING BACK Sections 3.2, 3.3, and 3.4 Vector addition

《 LOOKING BACK Sections 13.2, 13.3, and 13.4 Gravity

How is a long-range force transmitted from one charge to another? We’ll develop the idea that charges create an electric field, and the electric field of one charge is the agent that exerts a force on another charge. That is, charges interact via electric fields. The electric field is present at all points in space.

Computers, cell phones, and optical fiber communications may seem to have little in common with the fact that you can get a shock when you touch a doorknob after walking across a carpet. But the physics of electric charges—how objects get charged and how charges interact with each other—is the foundation for all modern electronic devices and communications technology. Electricity and magnetism is a very large and very important topic, and it starts with simple observations of electric charges and forces.