S13Phys2BaLec27B

# But it wont affect our final answer basically when we

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But it won’t affect our final answer. Basically, when we chose our current directions, we chose a coordinate system, I 1 happened to go in the negative direction. We now know the true direction of I 1 .

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RC Circuits Up until now, you have only had capacitors or resistors as our circuit elements, but what would happen if we had both? The simplest such circuit is: What will happen when the switch is closed? A current will start to flow through the resistor and the capacitor will begin to charge. Charges from one plate of the capacitor will flow to the other.
RC Circuits Will the current be constant? No, as more and more charge build up, the current will slow down until it becomes zero. This means that the current, I, is time dependent. The charge, Q, on the capacitor plates will also be time dependent. As time progresses, the charge will increase until it reaches some final value, Q f .

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RC Circuits Perform a Kirchoff loop over the circuit to find Q f . When the capacitor is fully charged, the current, I, will become zero.
RC Circuits But if we would like to find the charge on the capacitor plate at any time, t, we have to perform some math. This is a differential equation, the solution is (to find Q for a charging capacitor): The capacitor charges exponentially as a function of time. ! ! dQ dt R ! Q C = 0

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For Next Time (FNT) Start reading Chapter 28 Finish working on the homework for Chapter 27
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