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S5_mud

S5_mud - Lecture S5 Muddiest Points General Comments Today...

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Lecture S5 Muddiest Points General Comments Today, we finished up one PRS question (the first one on light bulbs), and did another similar one. The point of these questions is to get you to think about circuits in terms of potentials. Many students found these questions challenging, which is good — I need to challenge you preconceptions in order to change them. Responses to Muddiest-Part-of-the-Lecture Cards (45 cards) 1. How do we know if a battery is absorbing or supplying power? (1) How does the 12 V battery in the PRS question absorb power? (1) For any circuit element with current i and voltage v , the element is dissipating or absorbing power if iv > 0; the element is supplying power if iv < 0. But to make this work, you must use the passive sign convention, which requires that you label i as going into the + terminal (or out the terminal). 2. I can’t do the KVL equation for complex loops. (1) There is plenty of practice on the homework. Please see me at oﬃce hours if there is a problem. 3. In the PRS question, you sad that the battery forces the node voltage to be 12 V. What do you mena by “force”? (1) The constitutive law of the battery (a voltage source, really) is that the voltage across its terminals must be exactly the source strength, in this case 12 V. Since the terminal is at ground, the + terminal is at exactly 12 V. When the switch is closed, the middle node is at the same potential as the + terminal, so it must be at 12 V. Using the word “force” is a bit of colorful language to make this point. 4. Muddy point: How to deal with split currents in KVL in the last concept question. (1) Not sure I understand the question please ask again, or see me at oﬃce hours or recitation. 5. In the first PRS question, how do you apply KCL at the junction between the two bulbs? (1) The sum of currents out of the node sum to zero. Since the current into the node through bulb A is the same as the current out

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