Static Electricity

Physics

Mrs. Coyle

http://www.uwec.edu/jolhm/EH3/Group2/Pictures/lightning.jpg

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 10-19 C (Coulombs)

• Mass of Electron:

9.11 x 10-31 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 10-31 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)

GlassWoolFur (cat)LeadSilkHuman skin, AluminumCottonWoodAmberNickel, Copper, Brass, GoldRubberSulfur

- 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.

http://www.ap.smu.ca/demonstrations/index.php?option=com_content&view=article&id=106&Itemid=85

Charging by Induction

• Objects ends up with opposite charge.• Involves grounding.

http://www.physics.sjsu.edu/becker/physics51/elec_charge.htm

Electroscopes are used to test the charge of an object.

When a charged object is brought near the electroscope,

its leaves spread apart.

http://upload.wikimedia.org/wikipedia/commons/e/ec/Electroscope_showing_induction.png

When a charged object touches an electroscope, the

electroscope is now charged.

What was the charge of the object that touched this

electroscope?

Polarization

http://www.csiro.au/helix/sciencemail/activities/WaterBend.html

Part II

• Coulomb’s Law

Coulomb’s Law

• 1785,Charles Augustin Coulomb (French scientist)

F = k q1q2

______

d2

• k = 9 x 109 Nm2/C2

• 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

http://web.ncf.ca/ch865/graphics/Coulomb.jpeg

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 2C is 0.5 m from a charge of 3C. 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