Electric Fields 05/16/2008Lecture 17 1UCSDPhysics 10The Electric ForceThe Electric ForceElectric ChargeElectric ChargeElectric FieldsElectric FieldsElectron BeamsElectron BeamsSpring 2008 2UCSDPhysics 10Electric ChargeElectric Charge••Recall that fundamental particles carry somethingRecall that fundamental particles carry somethingcalled electric chargecalled electric charge– protons have exactly one unit of positive charge– electrons have exactly one unit of negative charge••Electromagnetic force is one of the basicElectromagnetic force is one of the basicinteractions in natureinteractions in nature– like charges experience repulsive force– opposite charges attracted to each other (like gravity)••Electrical current is flow of charge (electrons)Electrical current is flow of charge (electrons)Spring 2008 3UCSDPhysics 10Charge BalanceCharge Balance••Neutral atoms are made of equal quantities ofNeutral atoms are made of equal quantities ofpositive and negative chargespositive and negative charges– Neutral carbon has 6 protons, 6 electrons, (& neutrons)••Electrons can be stripped off of atomsElectrons can be stripped off of atoms– Electrons occupy the vulnerable outskirts of atoms••Usually charge flows in such a way as to maintainUsually charge flows in such a way as to maintainneutralityneutrality– Excess positive charge attracts excess negative charge– Your body has 51028 positive charges and 51028negative charges, balanced within millions or billionsSpring 2008 4UCSDPhysics 10Charge SeparationCharge Separation••Can separate charges by rubbing:Can separate charges by rubbing:– feet on carpet– atmosphere across ground– silk on glass– balloon on hair!••Insulators keep charges where they are (no flow)Insulators keep charges where they are (no flow)••Conductors distribute charge equally on surfaceConductors distribute charge equally on surface– charge is free to “move about the cabin”– why do the charges collect on the surface?Electric Fields 05/16/2008Lecture 17 2Spring 2008 5UCSDPhysics 10Induced ChargeInduced Charge••Charge can also be coaxed to redistribute itselfCharge can also be coaxed to redistribute itselfwithin an objectwithin an object––––––––––––––+––––––+++++Charged rod approaches sphere+ charge attracted to – charge in rod– charge repelled by rodResult: Attraction!Spring 2008 6UCSDPhysics 10Static ElectricityStatic Electricity••Rubbing action redistributes charge (unbalanced)Rubbing action redistributes charge (unbalanced)••If enough charge builds up, we get If enough charge builds up, we get dischargedischarge••Air spark is actually due to Air spark is actually due to ““breakdownbreakdown”” of air of air– neutral air molecules separate into ions (electrons arestripped away)– current can then flow through the “plasma-field” air– In essence, air becomes a “wire” for a short bit– this happens at 3 million volts per meter• 1 cm spark then at 30,000 volts• typical finger-spark may involve a few billion electrons• hold onto key to reduce pain of sparkSpring 2008 7UCSDPhysics 10LightningLightning••Lightning is an unbelievably hugeLightning is an unbelievably hugedischargedischarge••Clouds get charged through air frictionClouds get charged through air friction••1 kilometer strike means 3 billion volts!1 kilometer strike means 3 billion volts!••Main path forms temporary Main path forms temporary ““wirewire””along which charge equalizesalong which charge equalizes– often bounces a few times before equal••Thunder is bang produced by theThunder is bang produced by theextreme pressure variations induced byextreme pressure variations induced bythe formation and collapse of thethe formation and collapse of theplasma conduitplasma conduit••www.www.stormchasingstormchasing..nl/lightningnl/lightning.html.htmlSpring 2008 8UCSDPhysics 10Lightning RodsLightning Rods••Perform two functionsPerform two functions– provide safe conduit for lightning away from house– diffuse situation via “coronal discharge”Charges are attracted to tip of rod, and“electric field” is highly concentratedthere.Charges “leak” away, diffusing chargein what is sometimes called “St. Elmo’sFire”, or “coronal discharge”Electric Fields 05/16/2008Lecture 17 3Spring 2008 9UCSDPhysics 10““ElectrostaticElectrostatic”” Force Force••Two charges, Two charges, QQ11 and and QQ22, separated by distance , separated by distance rrexert a force on each other:exert a force on each other:FF = ( = (kk··QQ11··QQ22) / ) / rr22••kk is a constant (9 is a constant (9101099), ), QQ is in Coulombs, is in Coulombs, rr in meters in meters– One unit of charge (proton) has Q = 1.610-19 Coulombs••Looks a lot like NewtonLooks a lot like Newton’’s gravitation in forms gravitation in form••Electron and proton attract each other 10Electron and proton attract each other 104040 times timesstronger electrically than gravitationally!stronger electrically than gravitationally!– Good thing charge is usually balanced!Spring 2008 10UCSDPhysics 10Coulomb Law IllustratedCoulomb Law Illustrated••Like charges Like charges repelrepel••Unlike charges Unlike charges attractattract– ++ +rIf charges are of same magnitude (and same separation),all the forces will be the same magnitude, with differentdirections.––Spring 2008 11UCSDPhysics 10Coulomb Force Law, QualitativelyCoulomb Force Law, Qualitatively••Double one of the chargesDouble one of the charges– force doubles••Change sign of one of the chargesChange sign of one of the charges– force changes direction••Change sign of Change sign of bothboth charges charges– force stays the same••Double the distance between chargesDouble the distance between charges– force four times weaker••Double both chargesDouble both charges– force four times strongerSpring 2008 12UCSDPhysics 10Electric Force a lot like GravityElectric Force a lot like Gravity••Same 1/Same 1/rr22 dependence; charge takes place of mass. dependence; charge takes place of mass.••Does this mean electricity is product of geometry,Does this mean electricity is product of geometry,just like gravity (general relativity)?just like gravity (general relativity)?– No, because gravity as geometry accounts for the factthat all masses accelerate the