static electricity physics mrs. coyle
TRANSCRIPT
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.
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.
Example:
• An object of +10C touched an identical object that was neutral. What is the charge of each object?
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.
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)
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
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
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.
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?