1 Chapter 6 - Electricity (& Magnetism) Electricity - deals with interactionsbetween electric charges* causes forces motion* two types of charges:+ positive proton- negative electronAncient Greeks - rub amber and it attractssmall objectselectron - from Greek for “amber”Law of Electric charges - basic law of interaction“opposites attract, likes repel”Where do charges come from?Atomic Theory - smallest particles of natureNeutral atom+-+nucleus - made of protons- fixed positions--electrons - tiny negatives- move quickly around nucleus- some move between atoms----remove electrons - add electronstransfer charges between objects
2 - - - - - - - - - - - - - - Charges are transferred between objects ions - charged atomsatom acquires extra electron - negative ionloses an electron (to another atom) - positive ionRub balloon on your hair-electrons transferred to balloon (friction)balloon acquires negative charge-+-noforces-+----++-+-+---+--+force from balloon chargeattracts +, repels - attracted to balloonInduced charge - uses law of electriccharges to separate chargeLAW OF CHARGE CONSERVATION - when onebody acquires a charge from another, the secondacquires an equal and opposite charge from the first-net charge in universe constant-charge neither created nor destroyedcharges don’t just appear out of nowhere!
3 Electrical properties of materials Two general behaviors of matter regarding electricity:how they act in the presence of chargeConductors - transmit charge readily+fixed nucleus++++e-strongly held e-++++conductorloosely held e-move from atom to atompath for e- to travelAlso conduct heat wellfrom motion of e-Example: wires - transport charge for use in circuitsInsulators - charge cannot freely move+no loose e-get stuck on surfacepoor heat conductors
4 - Some materials have both properties atmospheric air nitrogen water (humidity)OxygenCarbon dioxidePolar - act likeseparated charge+-GOODINSULATORGOODCONDUCTORdamp day - charges leak offwater molecules form chainsto drain e- to groundSEMICONDUCTORS - properties of bothnormally insulatorsadd energy loosely held statesenergy from light, heat, electricalused as switches - add energy for charge to flowElectrostatics - charge is confined to an object- charge assumed not moving- static electricity - accumulated charge at restlike charge on balloonor charge on your body from walking
5 Electroscope - early device used to measure charge add charge heremetal leaves (gold)spread apart when charged-likes repel-more charge, spread moreMethods to charge objects:conduction and induction (and friction)CONDUCTION – touch two charged objectstogether to transfer charge------sparkcharge transferredcharge sharedleaves move apartneutralelectroscopecharge becomes evenly distributed
6 now positively charged But still connected to ground Charge by INDUCTION – two objects never actually touchcharge by using electric forces (induced charge)NO DIRECT CONTACTbring charged rod close-pushes e- awayleaves separate-++++----e- try to get asfar away as possibleneutralelectroscopestill neutralsame number of + as --+connection to grounde- can get even furtherfrom charged rodleaves falle-(Earth) Ground –reservoir of electronscan accept or donateany number of e-w/ no resistancenow positively chargedBut still connected to ground----+-+Break connection w/ grounde- can’t go back++leaves try to get as faraway as possibleSeparate because likes repel – like hair in Van de Graaf demoRemove the charged rod+ redistributeleaves separate for goodNET POSITIVE CHARGE
7 Coulomb’s Law - forces on charges ELECTRIC forces between chargesCHARGE – physical quantity; described by the CoulombSI UNIT : for charge (Q,q) Coulomb (C)actually very large charge, 10-6 C on a balloon (mC, nC)FUNDAMENTAL CHARGEelectron (e-) charge = 1.6 x Ccannot transfer less than 1 e- to charge objectsall charge in multiples of an electron – fundamentalcharge not continuousCoulomb’s Law - forces on chargesempirical-brute forceF=k q1q2 / d 2stiffwireFfrom calibsimpler modelq1q2q1ddq2F = force (in N)q1, q2 – charges (in C)d - separation between charges (m)k = 9x109 Nm2/C2Coulomb constantCoulomb actually measured!
8 } } Force is a vector – direction important F=k q1q2 / d 2 + and + or force acts along a linejoining two chargesF=k q1q2 / d 2}+ and +or_ and _positive forcecharges repel}negative forcecharges attract+ and -or just remember “opposites attract, likes repel”Example: What is the electric force between anelectron and proton in a hydrogen atom,spaced about 0.53 A apart?1 A = mmodelqp= +1.6x10-19 Cproton–positive chargeequal to magnitude of e-+-q e = - 1.6x10-19 Cd=5.3x10-11 mAnother example: A balloon charged to 3.4x10-5 C islocated 2.6 m from a can charged at -5.6x10-5 C. Whatis the direction and magnitude of the force between them?
9 Application: Lightning – electric discharge from clouds Ben Franklin – first to experimentwith lightningF=k q1q2 / d 2++++Large distance buthuge charge – big F------+++++-----water evaporatesionized by highvelocity motion+++Induces charge on objectsPuts force on cloud chargesgreatest force for highest objects (d smaller)Gigantic discharge – great amount of charge in cloudcauses destructive damage becauseof energy storedground to cloud, or cloud to ground (depends on – charge)lightning rod – sticks above buildings to attract chargethick wire connects to groundbypasses building to grounddestructive energy goes directly to groundHeat lightning – lightning between clouds from a distance
10 - - Electric Batteries - galvanic cells + History - Galvani and Volta observed frog leg twitch inpresence of dissimilar metalsGalvani: “animal electricity”stored electricity released whentissue touches metalVolta: dissimilar metals in contact througha solution produce a current(flow of electrons)Led to idea ofgalvanic cell - batteryproduceselectric currentCZnpositive terminalnegative terminal+--stores charge-Hook up to useelectrons can flowdischarge-deadmetals used upZn+2Zn+2Zn+2e-Zn+2Electrolyte- conducting solution
11 } Chemical work-energy to move e- from + to - terminal + - provides energyfor electrical work- light bulb heatse-+-e- uses energy as it goes from- terminal to terminalbattery used up when metal used upRECHARGABLE - able to reverse chemical processlithium ion, NiCad, wet/dry cell, fuel cells, solarPOTENTIAL DIFFERENCE - “voltage”Describes amount of chemical energy available to chargeV = Work/q work per charge J/C SI Volt (V)how much work a charge is able to dorelated to chemical work (potential)PE or Work W=qVIncrease battery :voltage (potential)add more galvanic cellswires - no energy lost by e-}3X voltageof a single cell+-+-+-Connected in series
12 - FORCE FIELDS - visual representation of invisible“action-at-a-distance”interactions-shows lines of force - extends all thru space- force on object in direction of lines- measure with test particle (field map)test massExample: gravityMass feels forcefrom touching fieldField points IN-attractive force-mass follows linemassELECTRIC FIELD - positive test charge to measurelong distance force of chargesPositive charge will go:outwardrepulsiveinwardattractiveForce along field lines-+
13 Magnetism - acts between moving charges - current ANCIENT GREEKSlodestone-natural magnet like magnetiteattracts small pieces of ironMagnetic fields different from other forces1. Field not in direction of forceforce perpendicular to field2. NO MAGNETIC MONOPOLES-cannot isolate polesNorth and South poles always pairedField lines form closed loops!point from N. Pole to S. PoleSNCANNOT SPLIT POLESSNBreak apart -get 2 magnetsboth have N & SSIMILARITIES:Like poles repel, opposite poles attract
14 EARTH’S Magnetic Field Motion of molten iron coreEARTHNNSCompassS. Pole of compassmagnet points toN. Pole of EarthSfor navigationEarth North PoleDeflects solar wind - high energyparticles ejected from Sun
15 Magnetism from electricity What causes magnetism?OerstedA current (electron flow) causes a force ona compass needleSI UNITCurrent I = Q / t (C/s=Ampere = 1 A)how fast electrons are flowing in a wireNSI (current)NSNSForce perpendicular toboth magnet and currentNSCompass needle points around incircle surrounding wiremagnetic field formscircle around wireA current exerts a force on apermanent magnet!
16 Ampere - two currents exert forces on each other no permanent magnets involved!Magnetism-has to do withmoving chargesI 2two wires are attractedI 1If currents opposite repelAlso invented solenoid – electromagnet (wire coiled on bolt)loop of wire producesfield through centerCoil intensifiesthe magnetic field at the center:Looks like bar magnetmagnetic domainsPermanent magnets:Electrons in atoms move – electric currentsproduce fieldAtomic magnets line up inmagnetic materials:iron, nickel, cobalt, etc.domainboundaries
17 Electricity from magnetism Faraday : can magnetism produce electricity?-built on Oersted’s & Ampere’s resultsCoil and galvanometermagnetic sitting in field - no currenttake out - current flowsput in - current flowsFaraday’s Law of Inductioninduced voltage and current produced bychanging magnetic field or circuit motionin field electromagnetic inductionDynamo - electric generatoruses mechanical energyto produce electricityturbine turns circuit in magnetwater wheel, steam. NuclearProduces current- electricityforce electrons through a circuit
18 Applications of Electromagnetism Electric meter - detects flowing currents“galvanometer”-coil wound on on pointer needle-force when current flows in magnet-force bigger when current largeruse to measure I, V, and RElectromagnetic Switch (Relay)-small switch closes toproduce small current insolenoid-solenoid produces magneticfield to pull in metal contactso larger current can flow
19 -receiver - carbon granules compress with diaphragm Telephone-receiver - carbon granulescompress with diaphragmchanging resistance-changes current which istransmittedSpeaker-current changes in magneticfield-force on coil moves coneElectric Motor-converts electrical energy to mechanical energy-rotating electromagnet spins in stationary magnetic field-electromagnet current changes direction to maintain rotation (always repels in magnet)-armature and commutator change current-generator in reverse
20 Electric currents provide electrical work +-Electric current - flow of chargefrom induced current (generator) or batteryI = charge passing a certain pointtime= Q / t = J/s (Ampere)Historically: Ben Franklin(first to experiment with electricity)Wrongly assumed + charges moveconventional current -still used todayActually - charges move in typical circuits - + fixedcurrent is flow of electrons in wireElectric field in wires forces e- to go from - to + .Does work on electrons - gives them energyPOTENTIAL DIFFERENCE - energy/charge availableto electrons - “voltage”V=work/charge = (Work Energy) / qSI: J/C = Volt (V)provides energy to circuits!
21 Example : Car batteryA 12 V car battery is used to start a car. If 1x109electrons go from the negative terminal to thepositive terminal, then how much work is done?charge equivalent: 1 e- = 1.6x10-19 CV = W/q W = qVcurrent flow in wirese- make collisions w/ atomsin wire-does not accelerate-lose energy-move at a very smallspeed (drift velocity)e-EElectric field moves at speed of lightelectrons move very slowly (hours tofrom switch to light socket)Large number of charges (1015) producecurrent - drip out like full water hose
22 how current flows in conductors George Simon Ohm -how current flows in conductors+-Current depends onpotential difference (V)VAOHM’S LAWI=V/ RR - resistance to a flowof currenthow difficult it is to passa currentResistance (R) SI: Volt/Amp = ohms (W)how energy is lost - flow of electrons impededdepends on:- type of material (copper, gold, graphite)- length of wire - longer, more resistance- cross-sectional areathinner wire, more resistiveless charge can flow- temperaturelow T - no R!How current flows determines how circuits work!
23 Combinations of resistances most circuits are combinations and batteriesand wires-connections with no resistanceRV-+Two ways to combine resistors:SERIES COMBINATION - same currentthru each resistorR1 R R3ReqIVVequivalentcircuitEquivalent - Total - Combined Resistance:Req = Rtot = R1 + R2 + R3total biggerthan individuallooks like a longer resistor-each will resist currentCan analyze I-V characteristics of circuit with Ohm’s Law V = I ReqHow much Ibattery life
24 Parallel Combination of resistors Divided circuit in which the current cantravel in multiple pathssame potential differenceacross each componentR1R2ReqR3VequivalentcircuitVCombined Resistance:1/Req = 1/R1 + 1/R2 +1/ R3Total smallerthan individualsmust take reciprocal for Req“path of least resistance” - most of the divided currentwill go through resistor with the smallest resistanceFor parallel, current can bypassbroken circuit (burned out) elementsChristmas lights - will stay lit evenif one light burns outHome outlets wired in parallel
25 Example : light bulbs1. Three light bulbs with resistances of 5 W, 8 W,and 12 W are connected in parallel across a5 V battery.a) What is the total (combined, equivalent)resistance of the combination?b) How much current is drawn from the battery?REMEMBER for parallel : flip for resistance2. Three light bulbs are connected in series acrossa 20 V battery. The resistance values of the lightbulbs are all 5 W.a) What is the equivalent resistance of the combination?b) What is the current flowing thru the circuit?
26 Heat Power of Currents P= I2 R P = I2R = V2/ R = I V (V=IR) Collision of electrons with atoms- hit atoms- atoms vibrate (gain energy)-heats wire- JOULE HEATINGJOULE’S LAW - wires heat up as currentflowsVP= I2 R***remember power=(work energy)timeAmore current -e- make more collisionshigher resistance-more energy lost to atomsmaterial impedes flowJoule’sExperimentP = I2R= V2/ R= I Vmost generalCan rewrite withOhm’s Law(V=IR)Example: car revisitedHow much energy is used to start a car?The car uses 10 A for 4 second with a 12 V car battery.
27 Joule heating used in many electrical applications More examples:A radio uses 0.5 A through a resistance of 6 WDuring operation. How much power is consumed?A 3 W lightbulb is connected is connected to a 120 VSource of potential difference. How much poweris used?Joule heating used in manyelectrical applications-hair dryer-space-heater-toaster-stove-lightbulb - filament heated to > 2500oCHeat generated also a problemBroken cord: loose connectionhigh resistance heatShort circuit: bypasses loadlarge current heatP = I2RI=V/R
28 Power Stations provide current to homes Called power station because it providescurrent and voltageDon’t pay for powerPay for energy!kilowatt-hour meterE=PtSafety device to limit dangerous currentfuse- filament heats up too muchand will melt-connection to currentsource broken-circuit breaker similarI fromplantI tohouseLow melting point conductorVoltage lost as currenttravels along power linesJoule heatingTRANSFORMERsteps up the voltageBut at the expense of the currentConstant power device P=IVincrease V, decrease Iprimary coilChanges voltage bysecondary coil