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Chapter 22 Electrostatics

Part V: Electricity & Magnetism. Chapter 22 Electrostatics. Electric Charges and Forces. There are two types of charge: positive charges and negative charges. REPEL. REPEL. ATTRACT. Like charges repel, opposite charges attract. Electrons, Protons, & Atoms.

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Chapter 22 Electrostatics

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  1. Part V: Electricity & Magnetism Chapter 22Electrostatics Physics 1 (Garcia) SJSU

  2. Electric Charges and Forces There are two types of charge: positive charges and negative charges REPEL REPEL ATTRACT Like charges repel, opposite charges attract. Physics 1 (Garcia) SJSU

  3. Electrons, Protons, & Atoms Electrons carry negative charge. Protons carry positive charge. Electrons are loosely bound to atoms Protons are tightly locked within the nucleus. Objects are usually neutral but can easily acquire or lose electrons to become charged. Helium atom Electrons Protons Physics 1 (Garcia) SJSU

  4. Charging by Friction/Contact Electrons move easily so an object can become charged by rubbing electrons off the object’s surface. • Brush your hair with a plastic comb. • Walk across a carpet with plastic-soled shoes. Electrons taken off of fur onto rubber rod Physics 1 (Garcia) SJSU

  5. Demo: Electroscope The “legs” of an electroscope separate when the electroscope is charged due to the repulsion of like charge on the two legs. Charged ebonite rod deposits negative charge (electrons) onto electroscope. “Legs” Physics 1 (Garcia) SJSU

  6. Demo: Dial Electroscope Alternative design for electroscope Neutral Charged Physics 1 (Garcia) SJSU

  7. Demo: Van de Graff Generator Van de Graff deposits large quantities of excess charge on its globe. A person with long hair can become a human electroscope. Physics 1 (Garcia) SJSU

  8. + + + + + Conservation of Charge Charge cannot be created nor destroyed. Objects become charged by transfer of charges. Electrons are rubbed off the hairs of a piece of fur, collecting and charging a plastic rod. Is the fur also charged? Positive or negative. Rod & Fur Neutral Physics 1 (Garcia) SJSU

  9. Conductors & Insulators Materials, such as metals, in which electrons flow freely are called electrical conductors. Materials, such as plastic and wood, in which electrons do not move easily are called electrical insulators. Materials, such as silicon, that can act as conductors or insulators under different conditions are called semiconductors. Physics 1 (Garcia) SJSU

  10. Plastic Insulators Plastic insulation on step of van de Graff and charge wand keep charge from leaving the metal balls. Plastic insulation on wire prevents charge to leave wire except at the ends, where the conducting metal is exposed. Physics 1 (Garcia) SJSU

  11. Demo: Tesla Coil Air is an insulator but at high voltages it can conduct electricity as lightning. Portable Tesla coil Physics 1 (Garcia) SJSU

  12. Tesla Coils Very high voltages achieved by large Tesla coils. Homemade, backyard model Fry’s Superstore, Fremont CA Physics 1 (Garcia) SJSU

  13. Coulomb’s Law Electric force has magnitude given by (Electric Force) = (k) x ( Charge of Object A ) x ( Charge of Object B) ( Distance ) x ( Distance ) DISTANCE Object A Object B Force Force Equal and opposite forces (Newton’s Third law) Physics 1 (Garcia) SJSU

  14. Demo: Induce the Can Charges separate in the can. Force of attraction is stronger since opposite charges are closer. Charged Object Can Rolls Metal Can (Neutral) + + + + Repulsion Force Attraction Force + + Physics 1 (Garcia) SJSU

  15. Charging by Induction Can separate charges by inducing them to opposite sides of a conducting object. Start Neutral Induce +’s to one side Allow –’s to Escape Finish with net + charge Physics 1 (Garcia) SJSU

  16. - - - - - - - - - - - - - - - - - - - - - - - - - - Demo: Electrophorus The electrophorus consists of a plastic plate and a metal disk with an insulated handle. Bring metal disk to plate; separate charge - - - - - - - - - - Charge plastic plate by rubbing it with fur • - - - - - - - - - - • + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - Remove electrons; plate positively charged Physics 1 (Garcia) SJSU

  17. + + + + + + Demo: Electroscope & Induction The legs of the electroscope separate when charged rod brought near the electroscope. Charge induction pushes electrons into the legs. Negatively charged rod repels electrons so they move as far away as possible. “Legs” Physics 1 (Garcia) SJSU

  18. Lightning Storms Charge separates inside of thunderclouds. The lower part of the cloud is negatively charged. This induces a positive charge on the ground. When the voltage difference is high enough, a lightning bolt can occur. Physics 1 (Garcia) SJSU

  19. Demo: Lightning Rod Charge flows readily to and from sharp metal points. Purpose of a lightning rod is to discharge a building before lightning occurs. Discharge Lightning Rods Physics 1 (Garcia) SJSU

  20. Demo: Electric Pinwheel Pinwheel turns as charge leaves the Van de Graff from the points at the tips of the spokes. Charged Air molecules Van de Graff Physics 1 (Garcia) SJSU

  21. Charge Polarization Charge polarization occurs when the positions of the electrons and protons in an atom are skewed. Polarized Atoms Polarized Neutral Charged Rod Physics 1 (Garcia) SJSU

  22. Demo: Static “Cling” Charged object attracts a neutral insulator by inducing charge polarization in the neutral object. Charged Balloon Polarization Polarization Charged Comb Neutral Paper Neutral Wall Physics 1 (Garcia) SJSU

  23. Check Yourself What causes a stream of water to be deflected when you bring a charged object next to it? Physics 1 (Garcia) SJSU

  24. Electric Field Lines Electric field lines indicate the direction of electric force if a positive charge was placed in the electric field. Physics 1 (Garcia) SJSU

  25. Demo: Streamers Paper streamers attached to the van de Graff show the direction of the electric field. Similar to hair-raising demo Physics 1 (Garcia) SJSU

  26. Lab: Electric Field Lines In lab you map out electric field lines and electric potential Physics 1 (Garcia) SJSU

  27. Electric Potential Energy Potential energy associated with electric force, similar to potential energy associated with gravitational force. Gravity Force Electric Force Physics 1 (Garcia) SJSU

  28. Electric Potential Electric potential is potential energy per charge, like temperature is internal energy per molecule. 5000 °F ? Check Yourself: Is this safe? Both situations are safe because the total amount of energy is small Physics 1 (Garcia) SJSU

  29. Capacitors Capacitors are used to store separated positive and negative electric charges. Industrial capacitor Basic design is pair of conducting plates insulated from each other. Physics 1 (Garcia) SJSU

  30. Demo: Discharging a Capacitor Charged capacitor holds enough charge to light a light bulb when discharged. Low voltage (low electric potential) but LARGE amount of charge – Dangerous! Physics 1 (Garcia) SJSU

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