S1_mud

# S1_mud - Lecture S1 Muddiest Points General Comments In...

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Lecture S1 Muddiest Points General Comments In this lecture, I began by explaining why active learning is important, and expectations for the year. I then began a review of resistive circuits. The most diﬃcult part of the lecture was probably the discussion of the passive sign convention. This was reﬂected in the muddy card questions. Please don’t worry about this too much — we will talk about this a lot in the coming days. I ended the lecture with a discussion of the diﬀerence between a battery and a voltage source. This also caused some confusion, which I hope will be cleared up in the next lecture. A few students commented that the pace was too slow, although others commented that it was a good review at a good pace. The pace will pick up, but I like to start the Frst lecture making sure everyone is on the same page. Responses to Muddiest-Part-of-the-Lecture Cards (69 cards) 1. General confusion about voltage, current, and power sign conventions. (12 students) We will cover this more in class. ±or now, it’s important to understand that a convention is “general agreement on or acceptance of certain practices or attitudes.” (See http://dictionary.reference.com/.) The Passive Sign Convention (PCS) allows experts to talk to one another without ambiguity as to the meaning of “the current through the resistor” or “the current through the voltage source.” The convention is probably the best possible one (of several possibilities), because it simpliFes the discussion of power. When using the PCS, the power dissipated (or in the case of an energy storage element, absorbed) is given by P = iv . When P is negative, power is supplied by the element. 2. What is the relevance to thermodynamics? (1) The ±irst Law requires that energy be conserved, always . That means that the sum of the power supplied by any sources in a circuit must equal the sum of the power dissipated by the resistors. Said another way, the power dissipated by passive elements in a circuit minus the power supplied by sources in the circuit equals zero. Since power dissipated is

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S1_mud - Lecture S1 Muddiest Points General Comments In...

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