15 Kirchoff's Laws

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Unformatted text preview: voltage drop that you meet its (+) sign first. Otherwise you take it as negative. For example in the above circuit you can write (cw): VAA V1 V2 V3 V4 0 you don't need to write this Now you know how to apply KVL and why it is true (in lumped‐element circuits). • How to apply KCL and KVL to solve a circuit. OK so what do you do if they give you a circuit and ask you to find its voltages and currents? We will answer this question completely later on but one general procedure is the following: 1. Assume a current that passes through each circuit element (including a voltage source). 2. Assume a voltage drop across each element (including a current source). 3. Apply KCL and KVL to solve for the currents and voltages 15.17 Notes: The directions of the currents and the polarity of the voltages can be arbitrary. However we usually assign them according to the passive sign connections for passive circuits and the active sign connection for sources. This procedure leads to a solution always! However you usually end up getting a large number of linear equations that need to be solved simultaneously. Computer programs can do this very well and this is how it is done in practice. However, many times we want to do simple and quick back‐to‐the‐envelope calculations. We will learn some “tricks” to do this in the next lecture. There are systematic ways to write KCL and KVL to solve a circuit. You will see few of them in 201H. The most successful of these methods are currently implemented in commercial computer programs to solve circuits (e.g. SPICE) Example 1. Find all of the currents and voltages in the f...
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