lect12 - Physics 227: Lecture 12 RC Circuits, Magnetism...

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Physics 227: Lecture 12 RC Circuits, Magnetism Lecture 11 review: Resistors in series add: R eq = R 1 + R 2 + . .. Resistors in parallel add inversely: 1/R eq = 1/R 1 + 1/R 2 + . .. KCL: sum of currents into node (junction) = 0. KVL: sum of voltages around loop = 0. Ammeters, Voltmeters, and all that. When is north south? When it is the earth’s magnetic pole. Monday, October 17, 2011
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Changing current in a capacitor Two lectures ago we watched the current through a bulb change as we charged and discharged a capacitor. How does this work? For a capacitor, Q = CV. But the current fow is a charge per unit time, so I = dQ/dt. Thus: I = dQ/dt = d(CV)/dt = C dV/dt. Monday, October 17, 2011
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Changing current in a capacitor I = C dV/dt. At time t=0, close the switch. The voltage across the capacitor cannot ``jump’’. dV/dt would be , so there would be power in the resistor! Instead, V ab = ξ , I= ξ /R, and V bc =0 at t=0 + . The physics: the current fows charging the capacitor until V bc = ξ , at which point I = 0. No power is dissipated in the resistor. Monday, October 17, 2011
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Changing current in a capacitor The physics: the current fows charging the capacitor until V bc = ξ , at which point I = 0. No power is dissipated in the resistor. The math: From KCL: I
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lect12 - Physics 227: Lecture 12 RC Circuits, Magnetism...

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