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The Laws of Circuit - you can learn and practice by just reading

Copyright. Charles Kim 2006

Oh! Hail Mary! (2)
The OHM (Oh! Hail Mary) and Hail Mary Pass also apply to capacitor and inductor (even though Ohm's law is only for resistor). In a node voltage method applied to, especially, op amp circuit problem, students forget this Hail Mary pass and goof at it. Look the capacitor below left. The current through through the capacitor is, of course, I = dV/dt. No second thought. Now look at the capacitor in the op amp circuit below right. Applying node voltage method at node n, (Vn-10)/1.25=(0-10)/1.25= -10/1.26 (since Vp=0=Vn) is the current flowing out from node n to the left. Then, how do you write your equation for the current flowing through the capacitor? Just dV/dt? What V is this then? Remember in node voltage method, all voltages are node voltages. In other words, the voltages in equation must be reference to ground. The voltage in dV/dt must be the voltage across the capacitor (Hail Mary Pass!). Then what is the voltage across? Since current is assumed to flow from left to right, applying passive convention, the voltage across is (left side of voltage) minus (right side of voltage). That means, V = Vn-Vo. Therefore, finally, the current flowing from node n toward node o is: d(Vn - Vo)/dt = - dVo/dt.