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Unformatted text preview: FNT #10 Potential and voltage August 28, 2005 Question 1: A parallel plate capacitor A parallel plate capacitor is charged so that it has the voltage values shown below 20 V 30 V 10 V 1 mm a) Assuming the electric field between the plates is uni- form and constant, what is its value? Here we only need to know that the potential difference between the plates is 20 Volts. The magnitude of the electric field is then | E | = | V | d = 20 V 1 mm = 20 , 000 V/m The direction points in the way a positive charge would go, from the top plate to the bottom plate. b) Suppose an electron moved from the positive plate to the negative plate. What is the change in the electric potential energy of the electron? The electron starts on a potential of 30 Volts and ends on a potential of 10 Volts. The change in voltage is then V = V f- V i = 10 V- 30 V =- 20 V 1 What this means is that every (positive) Coulomb of charge moved from the positive plate to the negative plate will loose 20 Joules of energy. We are interested in how much the potential energy of an electron will change moving between the two plates. We have PE = q V = (- 1 . 6 10- 19 C)(- 20 V) = +3 . 2 10- 18 J This makes sense, as an electron wants to go toward the positive plate. By pushing it the way we do not want it to go, we are storing potential energy. Question 2: A vibrating molecule The Lenard-Jones potential of the electric potential energy versus separation distance r between a hydrogen and a fluorine atom in an HF molecule is shown below.shown below....
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