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5.4 Electric energy, Electric Potential, and Electric Potential Difference Electric Potential Energy In Uniform Fields The Work-Energy theorem states that work done on an object is equal to the change in object’s energy. W = ΔEp This equation applies to all types of energy including gravitational potential energy and electric potential energy. In both cases lines of force show the direction that either a massive object or a charged object would move do to the gravitational or electric field. p. 187 - 188 5.4 Electric energy, Electric Potential, and Electric Potential Difference Electric Field, Energy and Charges: The positive charge would gain electric potential energy as it moves from position A to position B. Of course a negative charge would gain electric potential energy as it moved form position B to position A. When a positive charge spontaneously moves from position B to position A, it loses electric potential energy and gain kinetic energy (it will increase its speed) very much like a falling object. p. 189 5.4 Electric energy, Electric Potential, and Electric Potential Difference Electric Potential Energy of Multiple Charges – Non-Uniform Field Potential energy must be specified to a reference location. For electric potential energy we use distance set to infinity to set electric potential energy to zero at this point W = Fe x d And since : Fe = W = Ep = Ep = k Q1Q2 r2 k Q1Q2 xd (and d = r) r2 k Q1Q2 r p. 190 - 192 5.4 Electric energy, Electric Potential, and Electric Potential Difference Electric Potential Energy of Multiple Charges – Non-Uniform Field Determining the work done in moving a charge is accomplished by calculating the change in potential energy. For example :A charge Q1 is moved form d1 to d2 relative to charge Q2. d1 d2 Q1 Q2 To calculate the work done: W = ΔEp = k Q1Q2 d2 - k Q1Q2 d1 p. 192 5.4 Electric energy, Electric Potential, and Electric Potential Difference Electric Potential – It’s all about location Electric potential is defined as the amount of work (energy) required to move a unit charge to a point in an electric field. ΔEp V = Q Units: 1 Volt = 1 Joule/coulomb Electric potential is used to express the effect of a source’s electric field in terms of location within the electric field. Electric potential is a property of the location of a charge within an electric field, and not of the amount of charge. p. 193 5.4 Electric energy, Electric Potential, and Electric Potential Difference Electric Potential of Single Point Charges To calculate the electric potential at some distance from a point charge: ΔEp V = = Q k Q1Q2 P Qr r Cancelling out one of the charges leave you with: V = kQ r Q p. 194 5.4 Electric energy, Electric Potential, and Electric Potential Difference Electric Potential Difference To calculate the electric potential difference or Voltage as a charge moves in an electric field: VAB = ΔEpB ΔEpA Q Q WAB VAB = Q (WAB = ΔEpB – ΔEpA) The potential difference (V) between two points is defined as the amount of work required to move a unit of positive charge from the point that is lower potential to the point that is at the higher potential. p. 195 - 196 5.4 Electric energy, Electric Potential, and Electric Potential Difference Electron Volt On the atomic scale the Joule is a very large amount of energy. A smaller unit of energy, the electron volt (eV) is used. - + 1 eV is the energy gained when 1 electron is accelerated by a potential difference of 1 volt. electron 1 eV = 1.6 x 10-19 J p. 196 5.4 Electric energy, Electric Potential, and Electric Potential Difference Key Questions In this section, you should understand how to solve the following key questions. Page 189 – Quick Check #2 Page 191 – Practice Problem 5.4.1: #1 & 2 Page 195 – Quick Check #2 Page 196 – Practice Problem 5.4.2: #3 Page 197 – Practice Problem 5.4.3: #2 Page 198 - 199 – Review 5.4 #2,4,6,8, 12 & 13