Electric Charge and Coulomb’s Law

Electric Charge The effects of electric charge were first observed as static electricity: After being rubbed on a piece of fur, an amber rod acquires a charge and can attract small objects.

e = 1.60 x 10 -19 C e = proton charge -e = electron charge Particle Charges The fundamental electrical property to which the mutual attractions or repulsions between electrons and protons is attributed to charge. Any two charges feel a force between them. Electric force depends on the types of charges, the distance between the charges, and the amounts of the two charges. All electrons have exactly the same charge; the charge on the proton (in the atomic nucleus) has the same magnitude but the opposite sign.

Coulomb’s Law For charged objects, the force between the charges varies directly as the product of the charges and inversely as the square of the distance between them.

Coulomb’s Law The SI unit of charge is the coulomb, abbreviated C. A charge of 1 C is the charge of 6.24 × 1018 electrons. A coulomb represents the amount of charge that passes through a common 100-W light bulb in about one second.

Coulomb’s Law Newton’s law of gravitation for masses is similar to Coulomb’s law for electric charges. Whereas the gravitational force of attraction between a pair of one-kilogram masses is extremely small, the electrical force between a pair of one-coulomb charges is extremely large. The greatest difference between gravitation and electrical forces is that gravity only attracts but electrical forces may attract or repel.

Coulomb’s Law The forces on the two charges are action-reaction forces.

Insulators and Conductors Conductor: A material whose conduction electrons are free to move throughout. Most metals are conductors. Insulator: A material whose electrons seldom move from atom to atom. Most insulators are non-metals.

Insulators and Conductors If a conductor carries excess charge, the excess is distributed over the surface of the conductor.

Electric Charge Charging both amber and glass rods shows that there are two types of electric charge; like charges repel and opposites attract.

The Van de Graaff Generator In a Van de Graaff generator, a moving rubber belt carries electrons from the voltage source to a conducting sphere.

The Van de Graaff Generator The physics enthusiast and the dome of the Van de Graaff generator are charged to a high voltage. DEMO If you did not do so in Chapter 32, now is a good time to use the Van de Graaff generator to show the repulsion of like charges. Crank up the generator with a dozen 10-in. aluminum pie pans resting on top of the sphere. The weight of the pans above each pan is greater than the force of repulsion between the pans and so they remain on the sphere—all except the pan on top, which has no pans on top of it. The top pan “floats” off and the second pan becomes the top pan. It too floats off. This continues until all the pans have floated off one by one.