Chapter 20Static Electricity

What are Electrostatics? - the study of electric charges that

can be collected and held in one place

Like charges repel Opposite charges attract Neutral –positive charge equals the

negative charge

Separation of Charge Why can two neutral objects become

charged when rubbed together? When a rubber rod is rubbed with a piece

of wool, electrons from the atoms on the wool are transferred to the rod. The rod has a net negative charge and the wool has a net positive charge. The combined total charge remains the same. Charge is neither created nor destroyed.

Conductors and Insulators Insulator- a material through which a

charge will not move easily Examples: glass, wood, plastics,

cloth, dry air Conductor- a material that allows

charges to move easily Examples: metals, graphite

Electric ForceForces on Charged Bodies

There are two kinds of electric charges: positive and negative

Charges exert forces on other charges at a distance

The force is stronger when the charges are closer together

Like charges repel; opposite charges attract

How can charges be detected?

Use a device called an electroscope detects charges Consists of a metal knob connected

by a stem to two thin, lightweight pieces of metal foil called leaves

How do objects obtain a charge?

1. Charging by Conduction-charging a neutral body by touching it with a charged body

2. Separation of charge on neutral objects-a charged object(tape) will induce a separation of charges within a neutral conductor(finger). It will result in an attractive force between the charged object and the neutral conductor.

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Understanding Charging by Conduction

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Con.

3. Charging by Induction- Charging an object without touching ita. Two neutral spheres are touching. A charged rod is brought near them which will separate the charges(one is pos. other is neg.) The spheres are separated with the rod nearby. Each sphere will have a charge that is equal in magnitude but opposite in sign.

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Charging by Induction

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Con.b. Grounding

- process of connecting a body to Earth to eliminate excess charge.A neg. charged rod induces a separation of charges. The electroscope is grounded(finger) and the neg. charges are pushed from the electroscope to the ground(finger). The ground is removed before the rod and it is left with a pos. charge.

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A Negatively Charged Rod Induces a Separationof Charges in an Electroscope

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Coulomb’s Law Force depends upon 2 factors:1. Distance- force varies inversely with the

square of the distance between the centers of the spheres

2. Charge(q)- force varies directly with the charge of the bodies

Coulomb’s Law- the magnitude of the force between charge qA and and charge qB separated by a distance r, is proportional to the magnitude of the charges and inversely proportional to the square of the distance between them.

Con.

F=K qAqb

r2

What is the unit of charge?Coulomb(C)1 coulomb= 6.24x1018 electrons or protons1 electron=1.60x10-19 CThis is also called the elementary charger is in metersK= 9.0x109 Nm2/C2

Examplep. 551

Sphere A, with a charge of +6.0C is located near another charged sphere, B. Sphere B has a charge of –3.0 C and is located 4.0 cm to the right of A.

a. What is the force of sphere B on A?b. A third sphere, C, with a charge of +1.5

µC is added to the configuration. If it is located 3.0 cm directly beneath A, what us the new net force on sphere A and at what angle?

C