Physics 2102 Jonathan Dowling Physics 2102 Lecture 08 FRI 30 JAN Electric Potential I Ch24 1 5 Danger Electric Potential Energy Electric Potential Energy U is Negative of the Work W to Bring Charges in From Infinity U W The Change in Potential Energy U Between an Initial and Final Configuration Is Negative the Work W Done by the Electrostatic Forces U Uf Ui W Q What is the potential energy of a single Q Q a charge What is the potential energy of a dipole A proton moves from point i to point f in a uniform electric field as shown Does the electric field do positive or negative work on the proton Does the electric potential energy of the proton increase or decrease Electric Potential Electric potential difference between two points work per unit charge needed to move a charge between the two points V Vf Vi W q U q dW F ds dW q0 E ds W dW q0 E ds f f i i W V V f Vi E ds q0 i f Electric Potential Energy Electric Potential Units Potential Energy U J Joules Electric Potential V U q J C Nm C V Volts Electric Field E N C V m Volts per meter Electron Volt 1eV Work Needed to Move an Electron Through a Potential Difference of 1V W q V e x 1V 1 60 10 19 C x 1J C 1 60 10 19 J Equipotential Surfaces W V V f Vi E ds q0 i f The Electric Field is Tangent to the Field Lines Equipotential Surfaces are Perpendicular to Field Lines Work Is Needed to Move a Charge Along a Field Line No Work Is Needed to Move a Charge Along an Equipotential Surface Electric Field Lines Always Point Towards Equipotential Surfaces With Lower Potential Electric Field Lines and Equipotential Surfaces Why am I smiling I m About to Be Struck by Lightning http www cco caltech edu phys1 java phys1 EField EField html Electric Potential and Electric Potential Energy The change in potential energy of a charge q moving from point i to point f is equal to the work done by the applied force which is equal to minus the work done by the electric field which is related to the difference in electric potential U U f U i Wapp W q V We move a proton from point i to point f in a uniform electric field as shown Does the electric field do positive or negative work on the proton Does the electric potential energy of the proton increase or decrease Does our force do positive or negative work Does the proton move to a higher or lower potential Example Consider a positive and a negative charge freely moving in a uniform 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 energy position d Negative charge moves to a lower potential energy position a b c d True False True True Q Q V 0 V Conservative Forces The potential difference between two points is independent of the path taken to calculate it electric forces are conservative W U V V f Vi E ds q0 q0 i f 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 from individual point charges We calculated the electric potential produced by a single charge 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 charge
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