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Electric Fields and Potentials

Electric Fields and Potentials. Electric Force. Electricity exerts a force similarly to gravity. F e = kq 1 q 2 r 2

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Electric Fields and Potentials

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  1. Electric Fields and Potentials

  2. Electric Force Electricity exerts a force similarly to gravity. Fe = kq1q2 r2 where q1 and q2 represent the amount of charge in Coulombs (6.24 x 1018), r is in meters and k is the electrical constant (9 x 109 Nm2 /C2) 1 Coulomb of electrons travels through a 100-W lightbulb in about one second

  3. Electric Fields Just like gravity field, charges have a force field (E) as well, measured in force per unit charge E = F = kQ q r2 where Q is a positive test charge Direction of fields – away from a positive charge, toward a negative charge

  4. Force Field Lines • Fields have strength and direction • Field is determined by the force and direction of motion of a positive test charge • Field is strongest where the force is the strongest – where the lines are the most concentrated

  5. Electric Shielding Electrons repel toward the outside of any conducting surface Net charge inside is zero Electrons flow outward evenly, but pile up on sharp corners Shielding is important in electronic devices such as televisions and computers

  6. Faraday Cage • The Faraday cage is an electrical apparatus designed to prevent the passage of electromagnetic waves, either containing them in or excluding them from its interior space • It is named for physicist Michael Faraday, who built the first one in 1836

  7. Faraday Cage • Faraday stated that the charge on a charged conductor resided only on its exterior • To demonstrate this fact he built a room coated with metal foil, and allowed high-voltage discharges from an electrostatic generator to strike the outside of the room • He used an electroscope to show that there was no excess electric charge on the inside of the room's walls.

  8. Faraday Cage • A more impressive demonstration of the Faraday cage effect is that of an aircraft being struck by lightning • This happens frequently, but does not harm the plane or passengers • The metal body of the aircraft protects the interior. • For the same reason, and if it were not for the highly flammable nature of petrol, a car would be a very safe place to be in a thunderstorm

  9. Person in a car hit by artificial lightning. The lightning strikes the car and jumps to the ground bypassing the front tire arcing from the axle to the ground.

  10. Electrical Potential Just like gravity—the potential (possibility) of falling to earth, charges have the potential to move toward or away from each other

  11. Electrical Potential • Force of attraction/repulsion causes the potential • Potential is energy divided by charge—since charge is usually small, potential can be relatively large—5000 volts on a charged balloon • A larger amount of charge makes larger potential

  12. Voltage – Electrical Potential Voltage = PE/Q PE in Joules and Q in Coulombs 100 Volts 0.000001-J/0.00000001-C 100-J/ 1-C 1,000,000-J/10,000-C

  13. Storing Charges Capacitors can store charges on plates which are separated — as in Franklin’s Leyden jars

  14. Storing Charges • A capacitor is a device that stores electric charge • A capacitor consists of two conductors separated by an insulator

  15. Capacitors and Capacitance A capacitor in a simple electric circuit. Charge Q stored: The stored charge Q is proportional to the potential difference V between the plates. The capacitance C is the constant of proportionality, measured in Farads. Farad = Coulomb / Volt

  16. Parallel-Plate Capacitor • A simple parallel-plate capacitor consists of two conducting plates of area A separated by a distance d. • Charge +Q is placed on one plate and –Q on the other plate. • An electric field E is created between the plates. +Q -Q +Q -Q

  17. Capacitor Applications • . • Computer RAM memory and keyboards. • Electronic flashes for cameras. • Electric power surge protectors. • Radios and electronic circuits. • Power supplies

  18. capacitor capacitor

  19. Van de Graaf Generator This machine is capable of producing very high electrostatic potential differences in the order of millions of volts It works by friction of the belt with the rollers and separates charges at combs which take the charges to the dome and picks them up from the ground at the base

  20. Van de Graff Generator http://demoroom.physics.ncsu.edu/movies.html

  21. Van de Graff Generator http://demoroom.physics.ncsu.edu/movies.html

  22. Van de Graff Generator http://demoroom.physics.ncsu.edu/movies.html

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