-ELECTRIC CHARGE -CHARGING -COULOMB’S LAW AP Physics C Mrs. Coyle

Brief History Electron means “amber” in Greek Thales of Miletos 600 BC discovered properties when he rubbed amber (mineral) with cat fur and attracted feathers. Ben Franklin 1740’s lightning experiment with kite, key and Leyden jar (stores static electricity).

Benjamin Franklin Conservation of charge: The net charge of an isolated system remains constant. Saw electricity as a flowing fluid and called the flow direction positive. Franklin developed the lightning rod.

J(oseph) J(ohn) Thomson (1897, England) Discovered the electron using a cathode ray tube. 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.

Law of Charges: Like charges repel and opposite charges attract. Unit: Coulomb, C Symbol: q Charge of Electron, e : 1.6 x C (Coulombs) q = n e (charge is quantized, exists as multiples of e) Mass of Electron: 9.11 x kg.

Positively charged objects have 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

Coulomb’s Law 1785,Charles Augustin Coulomb (French scientist) F = k |q 1 | |q 2 | ______ r 2 k = 9 x 10 9 Nm 2 /C 2 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. What are 3 differences between the electrical force and the gravitational force?

Ex: 23.4 Find the magnitude of the charge on each sphere of mass 3x kg. Ans: 4.4 x10 -8 C

Ex Find x for the resultant force at q 3 to be zero. Ans: 0.775m q 1 = 15.0  C q 2 = 6.0  C