Electric Fields and Potentials

1. Electric Fields and Potentials Joey Multari, Shannon Burt, Katie Abbott

3. 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

5. Review • How many coulombs travel through a 100W light bulb in 3 seconds? • What type of behavior does charge have? A-Attracting B-repelling C-both D- Fast movement E- Slow movement

6. 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

7. 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

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

19. 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

20. 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

21. 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

22. 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.

24. 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.

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

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

27. capacitor capacitor

28. 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

29. 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.

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

33. 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

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

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

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

37. Electric Forces and Charges

38. Like Signs Repel

39. Unlike Signs Attract

40. Woman is touching negatively-charge sphere Electrical Force Electrical force is more powerful than gravity

41. Review • Which two signs attract? Which two repel? • A) Unlike signs attract, like signs repel • B) unlike signs repel, like signs attract • C) like signs repel and attract • D) vary depending on amount of charge • E) No charges attract, they only repel

42. Conservation of Charge

43. Neutron Proton Electron Energy Levels or Orbits Structure of the Atom

44. Charge • Electrons and protons have an attribute called charge • Electrons have a negativecharge • Protons have a positivecharge • 1800 times more massive than electrons • Neutrons have no charge • 1800 times more massive than electrons

45. Charge Conservation • Charge is neither created or destroyed. • What we call charging is either • Transfer of charges, or • Internal rearrangement of charge carrying units • Uncharged (neutral) objects have equal amounts of positive and negative charge • An object with unequal number of electrons and protons is electrically charged • Negative – Electrons > Protons • Positive– Protons > Electrons

46. Rubber scrapes electrons from fur atoms Removing Electrons from Atoms

47. Charge Quantization • Charge is always an integer multiple of a constant. • Six billion billion electrons is  - 1 Coulomb of charge • Six billion billion proton is + 1 Coulomb of charge • Q=Ne, where e is the unit electrical charge • Electrons have –e charge, protons have +e. • Millikan’s Oil Drop experiment

48. Coulomb’s Law • One Coulomb = 6.24 x 1018 electrons • Electrons have a negative charge • qe = -1.6 x 10-19 Coulomb • Protons have a positivecharge • qp = +1.6 x 10-19 Coulomb • Electrical Force can be positive or negative • Positive – repulsive force • Negative – attractive force

49. Example One pair of charges of 1 C each are 1 m apart F = kq1q2/ d2 F = (9 x 109 N m2/C2)(1 C)(1 C)/(1-m)2 F = 9 x 109 N m2/C2)(1 C2)/1-m2 F = 9 x 109 N (repulsive) 10 times the weight of a battleship

50. Review • The charge on an electron is 1.6 X 10^-19 C. How many electrons make a charge of 1C?