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Static Electricity Physics Mrs. Coyle http://www.uwec.edu/jolhm/EH3/Group2/Pictures/lightning.jpg
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Part I History Charge and its Conservation Conductors, Semiconductors, Insulators Methods of Charging
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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.
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Ben Franklin’s Kite Experiment (1740’s)
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Leyden Jar
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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.
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Law of Conservation of Electrical Charge The net charge of an isolated system remains constant.
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Example: An object of +10C touched an identical object that was neutral. What is the charge of each object?
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Law of Charges Like charges repel Opposite charges attract
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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.
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JJ Thomson with the CRT
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Cathode Ray Tube
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Cathode Ray
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Electrical Charge Symbol: q Unit : Coulomb, C
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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.
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Positively charged objects lost electrons. Example: Rubbing a glass rod with silk. Rod becomes + (loses electrons) Silk becomes - (gains electrons).
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Negatively charged objects have gained electrons. Example: rubbing a rubber rod with fur. Rubber Rod: - charged Fur: + charged
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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.
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Types of Materials in terms of Electrical Conductivity Conductors (metals) Semiconductors (germanium, silicon) Insulators (wood, glass, rubber)
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Electrostatic Charging Methods Friction Conduction Induction
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Charging by Friction The two objects wind up with opposite charges.
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Triboelectric Series +Fur (rabbit) Glass Wool Fur (cat) Lead Silk Human skin, Aluminum Cotton Wood Amber Nickel, Copper, Brass, Gold Rubber Sulfur - Celluloid
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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
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Charging by Induction Objects ends up with opposite charge. Involves grounding. http://www.physics.sjsu.edu/becker/physics51/elec_charge.htm
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Electroscopes are used to test the charge of an object.
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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
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When a charged object touches an electroscope, the electroscope is now charged.
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What was the charge of the object that touched this electroscope?
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Polarization http://www.csiro.au/helix/sciencemail/activities/WaterBend.html
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Part II Coulomb’s Law
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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.
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The Direction of the Electrical Force
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Coulomb’s Law is an Inverse Square Law http://web.ncf.ca/ch865/graphics/Coulomb.jpeg
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The electrical force is one of the four fundamental forces.
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Comparison with Gravitational Force What are 3 differences between the electrical force and the gravitational force?
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Comparison with Gravitational Force What are 3 differences between the electrical force and the gravitational force?
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Example 1 A charge of 2 C is 0.5 m from a charge of 3 C. Find the electric force.
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Example 2 Three charges are positioned as shown. Find the force acting on the 2 C charge.
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Example 2 Three charges are positioned as shown. Find the force acting on the 2 C charge.
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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
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