Chapter 20 Static Electricity

1. 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

2. 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.

3. 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

4. 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

5. 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

6. How do objects obtain a charge? • Charging by Conduction -charging a neutral body by touching it with a charged body • 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.

7. Chapter Image Bank 20 Understanding Charging by Conduction

8. Con. • Charging by Induction - Charging an object without touching it a. 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.

9. Chapter Image Bank 20 Charging by Induction

10. 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.

11. Chapter Image Bank 20 A Negatively Charged Rod Induces a Separation of Charges in an Electroscope

12. Coulomb’s Law • Force depends upon 2 factors: • Distance- force varies inversely with the square of the distance between the centers of the spheres • 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.

13. Con. F=KqAqb r2 What is the unit of charge? Coulomb(C) 1 coulomb= 6.24x1018 electrons or protons 1 electron=1.60x10-19 C This is also called the elementary charge r is in meters K= 9.0x109 Nm2/C2

14. Examplep. 551 • Sphere A, with a charge of +6.0C 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. • What is the force of sphere B on A? • 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