Electric Potential IElectric Potential IPhysics 2102Jonathan DowlingPhysics 2102Physics 2102Lecture 5Lecture 5Electric potential energyElectric potential energyElectric potential energy of a system is equal to minus the work done byelectrostatic forces when building the system (assuming charges wereinitially infinitely separated) U= − W∞The change in potential energy between an initial and final configurationis equal to minus the work done by the electrostatic forces: ΔU= Uf − Ui= − W• What is the potential energy of a single charge?• What is the potential energy of a dipole?• A proton moves from point i to point f in a uniform electricfield, as shown.• Does the electric field do positive ornegative work on the proton?• Does the electric potential energy of theproton increase or decrease?+Q–Qa+QElectric potentialElectric potentialElectric potential difference between two points = work perunit charge needed to move a charge between the two points: ΔV = Vf–Vi = −W/q = ΔU/q000f fi iff iidW F dsdW q E dsW dW q E dsWV V V E dsq= •= •= = •! = " = " = " •# ##rrrrrrrrElectric potential energy,Electric potential energy,electric potentialelectric potentialUnits : [U] = [W]=Joules; [V]=[W/q] = Joules/C= Nm/C= Volts [E]= N/C = Vm1eV = work needed to move an electronthrough a potential difference of 1V:W=qΔV = e x 1V= 1.60 10–19 C x 1J/C = 1.60 10–19 JEquipotential Equipotential surfacessurfacesGiven a charged system, we can:• draw electric field lines: the electric field is tangent to the field lines• draw equipotential surfaces: the electric potential is constant on thesurface• Equipotential surfaces are perpendicularto electric field lines. Why??• No work is needed to move a chargealong an equipotential surface. Why??• Electric field lines always point towardsequipotential surfaces with lowerpotential. Why??0ff iiWV V V E dsq! = " = " = " •#rrElectric field lines and Electric field lines and equipotential equipotential surfacessurfaceshttp://www.cco.caltech.edu/~phys1/java/phys1/EField/EField.htmlElectric potential and electricElectric potential and electricpotential energypotential energyThe change in potential energy of a charge q moving from point ito point f is equal to the work done by the applied force, which isequal to minus the work done by the electric field, which is relatedto the difference in electric potential:f i appU U U W W q V! = " = = " = !We move a proton from point i to point f ina uniform electric field, as shown.• Does the electric field do positive or negative work on theproton?• Does the electric potential energy of the proton increase ordecrease?• Does our force do positive or negative work ?• Does the proton move to a higher or lower potential?ExampleExampleConsider a positive and a negative charge, freely moving in auniform electric field. True or false?(a) Positive charge moves to points with lower potential.(b) Negative charge moves to points with lower potential.(c) Positive charge moves to a lower potential energyposition.(d) Negative charge moves to a lower potential energyposition–Q +Q 0+V–V(a) True(b) False(c) True(d) TrueConservative forcesConservative forcesThe potential difference between two points is independent ofthe path taken to calculate it: electric forces are“conservative”.0 0ff iiW UV V V E dsq q!! = " = " = = " •#rrElectric Potential of a Point ChargeElectric Potential of a Point Charge2fPiRRV E ds E dskQ kQ kQdrr r R!!!= " # = " == " = + = +$ $$rrNote: if Q were anegative charge,V would be negativeElectric Potential of Many PointElectric Potential of Many PointChargesCharges• Electric potential is aSCALAR not a vector.• Just calculate the potentialdue to each individualpoint charge, and addtogether! (Make sure youget the SIGNS correct!)q1q5q4q3q2!=iiirqkVr1r2r3r4r5PElectric potential and electricElectric potential and electricpotential energypotential energyappU W q V! = = !+Q–QaWhat is the potential energy of a dipole?• First bring charge +Q: no work involved, no potential energy.• The charge +Q has created an electric potential everywhere, V(r)= kQ/r• The work needed to bring the charge –Q to a distance a from thecharge +Q isWapp=U = (−Q)V = (–Q)(+kQ/a) = −kQ2/a• The dipole has a negative potential energy equal to −kQ2/a: wehad to do negative work to build the dipole (and the electric fielddid positive work).Potential Energy of A System ofPotential Energy of A System ofChargesCharges• 4 point charges (each +Q and equalmass) are connected by strings,forming a square of side L• If all four strings suddenly snap,what is the kinetic energy of eachcharge when they are very farapart?• Use conservation of energy:– Final kinetic energy of all four charges= initial potential energy stored =energy required to assemble the systemof charges+Q+Q+Q+QDo this from scratch!Potential Energy of A System ofPotential Energy of A System ofCharges: SolutionCharges: Solution• No energy needed to bringin first charge: U1=022 1kQU QVL= =• Energy needed to bring in2nd charge:2 23 1 2( )2kQ kQU QV Q V VLL= = + = +• Energy needed to bring in3rd charge =• Energy needed to bring in4th charge =+Q+Q+Q+QTotal potential energy is sum ofall the individual terms shownon left hand side =2 24 1 2 32( )2kQ kQU QV Q V V VLL= = + + = +( )242+LkQSo, final kinetic energy of eachcharge =( )2442+LkQL 2LSummary:Summary:• Electric potential: work needed to bring +1C from infinity;units V = Volt• Electric potential uniquely defined for every point in space -- independent of path!• Electric potential is a scalar — add contributions fromindividual point charges• We calculated the electric potential produced by a singlecharge: V=kq/r, and by continuous charge distributions :V=∫ kdq/r• Electric potential energy: work used to build the system,charge by charge. Use W=qV for each
View Full Document
Unlocking...