Static Electricity Physics Mrs. Coyle

Part I History Charge and its Conservation Conductors, Semiconductors, Insulators Methods of Charging

History Electron means “amber” in Greek Thales of Miletos 600 BC discovered properties by Greek. He rubbed amber (mineral) with cat fur and attracted feathers.

Ben Franklin’s Kite Experiment (1740’s)

Leyden Jar

Benjamin Franklin 1740’s lightning experiment with kite, key and Leyden jar (stores static electricity). Franklin developed the lightning rod. Proposed conservation of charge. Saw electricity as a flowing fluid and called the flow direction positive.

Law of Conservation of Electrical Charge The net charge of an isolated system remains constant.

Example: An object of +10C touched an identical object that was neutral. What is the charge of each object?

Law of Charges Like charges repel Opposite charges attract

J(oseph) J(ohn) Thomson (1897, England) He discovered the electron. He found that the mass of the electron is about 1/1800 of the mass of a hydrogen atom. He won the Nobel Prize (1906) for his discovery of the electron.

JJ Thomson with the CRT

Cathode Ray Tube

Cathode Ray

Electrical Charge Symbol: q Unit : Coulomb, C

Charge and Mass of the Electron Charge of Electron: 1.6 x C (Coulombs) Mass of Electron: 9.11 x kg.

Positively charged objects lost electrons. Example: Rubbing a glass rod with silk. Rod becomes + (loses electrons) Silk becomes - (gains electrons).

Negatively charged objects have gained electrons. Example: rubbing a rubber rod with fur. Rubber Rod: - charged Fur: + charged

Note Negatively charged objects have more mass than an identical neutral object, since each extra electron has a mass of 9.11 x kg.

Types of Materials in terms of Electrical Conductivity Conductors (metals) Semiconductors (germanium, silicon) Insulators (wood, glass, rubber)

Electrostatic Charging Methods Friction Conduction Induction

Charging by Friction The two objects wind up with opposite charges.

Triboelectric Series +Fur (rabbit) Glass Wool Fur (cat) Lead Silk Human skin, Aluminum Cotton Wood Amber Nickel, Copper, Brass, Gold Rubber Sulfur - Celluloid

Charging by Conduction (direct contact) The objects end up with the same type charge. If the charges are equal in size, they share the charge equally.

Charging by Induction Objects ends up with opposite charge. Involves grounding.

Electroscopes are used to test the charge of an object.

When a charged object is brought near the electroscope, its leaves spread apart.

When a charged object touches an electroscope, the electroscope is now charged.

What was the charge of the object that touched this electroscope?

Polarization

Part II Coulomb’s Law

1785,Charles Augustin Coulomb (French scientist) F = k q 1 q 2 ______ d 2 k = 9 x 10 9 Nm 2 /C 2 d (or r): distance between the charges. q : charge of each object.

The Direction of the Electrical Force

Coulomb’s Law is an Inverse Square Law

The electrical force is one of the four fundamental forces.

Comparison with Gravitational Force What are 3 differences between the electrical force and the gravitational force?

Comparison with Gravitational Force What are 3 differences between the electrical force and the gravitational force?

Example 1 A charge of 2  C is 0.5 m from a charge of 3  C. Find the electric force.

Example 2 Three charges are positioned as shown. Find the force acting on the 2 C charge.

Example 2 Three charges are positioned as shown. Find the force acting on the 2 C charge.

Example 3 Two equal charges are located 1m from each other. The force acting between them is 2N. How many Coulombs is each charge? Answer: 15μC