3Electric Force Electricity exerts a force similarly to gravity. Fe = kq1q2r2where 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
5ReviewHow many coulombs travel through a 100W light bulb in 3 seconds?What type of behavior does charge have?A-AttractingB-repellingC-bothD- Fast movementE- Slow movementAnswer to number one 3Answer to number two C
6Electric FieldsJust like gravity field, charges have a force field (E) as well, measured in force per unit chargeE = F = kQq r2where Q is a positive test chargeDirection of fields – away from a positive charge, toward a negative charge
7Force Field Lines Fields have strength and direction Field is determined by the force and direction of motion of a positive test chargeField is strongest where the force is the strongest – where the lines are the most concentrated
18Electrical PotentialJust like gravity—the potential (possibility) of falling to earth, charges have the potential to move toward or away from each other
19Electrical PotentialForce of attraction/repulsion causes the potentialPotential is energy divided by charge—since charge is usually small, potential can be relatively large—5000 volts on a charged balloonA larger amount of charge makes larger potential
20Voltage – Electrical Potential Voltage = PE/QPE in Joules and Q in Coulombs100 VoltsJ/ C100-J/ 1-C1,000,000-J/10,000-C
21Electric ShieldingElectrons repel toward the outside of any conducting surfaceNet charge inside is zeroElectrons flow outward evenly, but pile up on sharp cornersShielding is important in electronic devices such as televisions and computers
22Faraday CageFaraday stated that the charge on a charged conductor resided only on its exteriorTo 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 roomHe used an electroscope to show that there was no excess electric charge on the inside of the room's walls.
24Person in a car hit by artificial lightning 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.
25Storing ChargesCapacitors can store charges on plates which are separated — as in Franklin’s Leyden jars
26Storing Charges A capacitor is a device that stores electric charge A capacitor consists of two conductors separated by an insulator
28Capacitors and Capacitance A capacitor in a simpleelectric 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
29Parallel-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.
33Van de Graaf GeneratorThis machine is capable of producing very high electrostatic potential differences in the order of millions of voltsIt 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
40Electrical Force Woman is touching negatively-charge sphere Electrical force is more powerful than gravity
41Review Which two signs attract? Which two repel? A) Unlike signs attract, like signs repelB) unlike signs repel, like signs attractC) like signs repel and attractD) vary depending on amount of chargeE) No charges attract, they only repelA: Unlike signs attract, like signs repel
43Structure of the AtomNeutronProtonElectronEnergy Levelsor Orbits
44Charge Electrons and protons have an attribute called charge Electrons have a negative chargeProtons have a positive charge1800 times more massive than electronsNeutrons have no charge
45Charge Conservation Charge is neither created or destroyed. What we call charging is eitherTransfer of charges, orInternal rearrangement of charge carrying unitsUncharged (neutral) objects have equal amounts of positive and negative chargeAn object with unequal number of electrons and protons is electrically chargedNegative – Electrons > ProtonsPositive – Protons > Electrons
46Removing Electrons from Atoms Rubber scrapes electrons from fur atoms
47Charge QuantizationCharge is always an integer multiple of a constant.Six billion billion electrons is - 1 Coulomb of chargeSix billion billion proton is + 1 Coulomb of chargeQ=Ne, where e is the unit electrical chargeElectrons have –e charge, protons have +e.Millikan’s Oil Drop experiment
48Coulomb’s Law One Coulomb = 6.24 x 1018 electrons Electrons have a negative chargeqe = -1.6 x CoulombProtons have a positive chargeqp = +1.6 x CoulombElectrical Force can be positive or negativePositive – repulsive forceNegative – attractive force
49Example One pair of charges of 1 C each are 1 m apart F = k q1 q2 / d2 F = (9 x 109 N m2/C2)(1 C)(1 C)/(1-m)2F = 9 x 109 N m2/C2)(1 C2)/1-m2F = 9 x 109 N (repulsive)10 times the weight of a battleship
50ReviewThe charge on an electron is 1.6 X 10^-19 C. How many electrons make a charge of 1C?1/ (1.6 X10^-19) = 6.25 x 10^18
51What happens to the magnitude of the force as the charges get farther apart?
52ConductorsAn electrical conductor is a substance through which electrical current flows with small resistanceMetals are generally excellent electrical conductorsThe electrons in conductors lie in an ‘loose’ outer orbit – the so-called "valence band"
56InsulatorsAn electrical insulator is a substance with an extremely high resistance to the flow of chargeMost nonmetals solids are generally excellent insulatorsMost atoms hold on to their electrons tightly and are insulators
74LightningAs 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.
75LightningWhen 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.