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

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Presentation on theme: "Electric Fields and Potentials Joey Multari, Shannon Burt, Katie Abbott."— Presentation transcript:


2 Electric Fields and Potentials Joey Multari, Shannon Burt, Katie Abbott


4 Electric Force Electricity exerts a force similarly to gravity. F e = kq 1 q 2 r 2 where q 1 and q 2 represent the amount of charge in Coulombs (6.24 x 10 18 ), r is in meters and k is the electrical constant (9 x 10 9 Nm 2 /C 2 ) 1 Coulomb of electrons travels through a 100-W lightbulb in about one second


6 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

7 Electric Fields Just like gravity field, charges have a force field (E) as well, measured in force per unit charge E = F = kQ q r 2 where Q is a positive test charge Direction of fields – away from a positive charge, toward a negative charge

8 Force Field Lines Fields have strength and direction positiveField 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











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

20 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

21 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

22 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

23 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.


25 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.

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

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

28 capacitor

29 Capacitors and Capacitance 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 A capacitor in a simple electric circuit.

30 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.


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


34 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

35 Van de Graff Generator

36 Van de Graff Generator

37 Van de Graff Generator

38 Electric Forces and Charges

39 Like Signs Repel

40 Unlike Signs Attract

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

42 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

43 Conservation of Charge

44 Structure of the Atom Proton Neutron Electron Energy Levels or Orbits

45 Charge Electrons and protons have an attribute called charge –Electrons have a negative charge –Protons have a positive charge 1800 times more massive than electrons –Neutrons have no charge 1800 times more massive than electrons

46 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

47 Removing Electrons from Atoms Rubber scrapes electrons from fur atoms

48 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

49 Coulomb’s Law One Coulomb = 6.24 x 10 18 electrons Electrons have a negative charge –q e = -1.6 x 10 -19 Coulomb Protons have a positive charge –q p = +1.6 x 10 -19 Coulomb Electrical Force can be positive or negative –Positive – repulsive force –Negative – attractive force

50 Example One pair of charges of 1 C each are 1 m apart F = k q 1 q 2 / d 2 F = (9 x 10 9 N m 2 /C 2 )(1 C)(1 C)/(1-m) 2 F = (9 x 10 9 N m 2 /C 2 )(1 C)(1 C)/(1-m) 2 F = 9 x 10 9 N m 2 /C 2 )(1 C 2 )/1-m 2 F = 9 x 10 9 N m 2 /C 2 )(1 C 2 )/1-m 2 F = 9 x 10 9 N (repulsive) 10 times the weight of a battleship

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

52 What happens to the magnitude of the force as the charges get farther apart?

53 Conductors An electrical conductor is a substance through which electrical current flows with small resistance Metals are generally excellent electrical conductors The electrons in conductors lie in an ‘loose’ outer orbit – the so-called "valence band"


55 Valence Bonds In a periodic table the columns represent number of valence bonds Often times, the valence bonds in two combining elements will add up to eight with rare exceptions.


57 Insulators An electrical insulator is a substance with an extremely high resistance to the flow of charge Most nonmetals solids are generally excellent insulators Most atoms hold on to their electrons tightly and are insulators


59 Covalent Bonds In a tightly knit molecular bond the atoms are held together and therefore hang onto their electrons, creating excellent insulators.

60 Review 1. How much resistance does a conductor have? A- large amount B- small amount C- None D- Average amount E- infinite

61 Review Where are valence electrons represented on the periodic table? A- columns B-rows C- atomic number D- atomic mass E- grouping

62 Net Charge Neutral

63 Net Charge Positive

64 Net Charge Negative

65 Charging by Touching Charging by actually touching object


67 Charging by Induction Two charged objects placed on opposite sides create a charge in third object.


69 Polarization Electrons surrounding nucleus may be thought of as a cloud in which the total negative charged is smeared out.

70 Polarization Unpolarized atom Put negatively charged rod on the right side... Center of electron cloud shifts to the left.

71 Neutral Objects are Attracted to Charged Objects Charged comb attracts neutral bits of paper


73 Charge on Metals Metal Ball Charge Distributions Charge on Metal Points Lightning, lightning rods


75 Lightning As the negative charges collect at the bottom of the cloud it forces the negative charges in the ground to be forced away from the surface. This leaves the ground positive. A streamer of negative charges is repelled by the bottom of the cloud and attracted by the ground. As this streamer of negative charges approaches the ground, a streamer of positive charges is repelled by the ground and attracted to the negative streamer.

76 Lightning When the two streamers connect, they have created a fairly conductive path which allows a sudden down surge of electrons to jump to the ground. This is the lightning. The rapidly moving electrons excite the air along the path so much that it emits light. It also heats the air so intensely that it rapidly expands creating thunder. One thing to notice is that the positive charges that make up both the cloud and the ground do not move. Even the positive streamer launched by the ground is really only made up of positively charged air particles because the electron(s) left the particle.


78 Electroscope Insulator Metal Leaves Conductor


80 Bibliography The textbook

81 FIN

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