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

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