Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
The Voltage Difference Created by a Set of Parallel Plates Since the E-field between two plates is a constant, it’s easy to find the work necessary to move a charge against that field (just like moving a mass against a constant local g) for a distance Dd: |DW| = |DUE| = |qEDd| So the potential difference (also called the voltage difference) magnitude is |DV| = |DUE/q| = |E||Dd| But you are moving a displacement Dd against the field to increase voltage: DV = DUE/q = –|E|Dd So in general (not just between plates): |E| = –dV/ds, where ds is an incremental distance moved along the direction of E. 4/28/17 Oregon State University PH 213, Class #12 1 Which electric potential graph below describes this electric field? 4/28/17 Oregon State University PH 213, Class #12 2 Which electric potential graph below describes this electric field? 4/28/17 Oregon State University PH 213, Class #12 3 The Strength of the Electric Potential Field (Voltage) Created by a Point Charge Q: It’s all well and good to look at simple, uniform electrical fields between parallel charged plates, but what about when the fields are not uniform—such as when point charges create them? What is the electric potential (voltage) at any point in space due to a nearby point charge, q? A: V = kq/r, where r is the distance from q to the point in question. This is a result of integrating Again, the sign of the charge again indicates how the potential changes in the space around it. Note where we have selected V = zero only at an infinite distance from q. (The analogy to gravity goes on.) 4/28/17 Oregon State University PH 213, Class #12 4 Rank in order, from largest to smallest, the potential energies Ua to Ud of these four pairs of charges. Each + symbol represents the same amount of charge. A. B. C. D. E. 4/28/17 U a = Ub > Uc = Ud U b = Ud > Ua = Uc U a = Uc > Ub = Ud U d > Uc > Ub > Ua U d > Ub = Uc > Ua Oregon State University PH 213, Class #12 5 Rank in order, from largest to smallest, the potential energies Ua to Ud of these four pairs of charges. Each + symbol represents the same amount of charge. A. B. C. D. E. 4/28/17 U a = Ub > Uc = Ud U b = Ud > Ua = Uc U a = Uc > Ub = Ud U d > Uc > Ub > Ua U d > Ub = Uc > Ua Oregon State University PH 213, Class #12 6 Rank in order, from largest to smallest, the electric potentials (“voltages”) of these four collections of charge [measured at the midpoint between them]. Each + symbol represents the same amount of charge. [Voltage is a field—a description of a certain property—of a point in space, so we must specify that point.] 4/28/17 Oregon State University PH 213, Class #12 7 Summary of Electrostatics Field “payload” E ·q Effect = E = –dV/ds V 4/28/17 F F = –dUE/ds ·q = UE Oregon State University PH 213, Class #12 8