Download Muddiest Points Week 2

Muddiest Points Week 2:
1) How to set up KVL/KCL equations (what is + and what is -) ?
Questions: The most confusing part for me was what direction to put the + and - for the current arrows. I
asked you in class today and you really helped me figure it out, as far as it is originally a guess and that it
could be wrong and that will be shown with our KVL calculations.
The most confusing concept is not what Kirchhoff's law is, but rather how to apply it. The best way to use
Kirchhoff's laws to solve for circuit measurements is to use the cook book. The other challenging part of
this process for me personally is finding which way the currents are moving in the node. To combat this I
need to label my diagrams well and pay attention to the location of the power source.
Answer: Label currents + at the tail, - at the tip. Go around a loop, picking up the first sign of each
element you come to. Loops cannot go through current sources.
2) How to solve KVl/KCL equations once you have them (‘the math’)
Question: The muddiest point of the week for me was the KCL and KVL section. Once you set
up the equations, how do you know what to substitute where?
Answer: You read the question and see what it asks you to find. If we have multiple unknowns,
for example: I2, I3, I4, and questions wants us to find I2, we solve the problem focusing on I2
only. Once we get the equations for the sum of the node, we solve for one of the unknowns,
excluding the one we are focusing on, and substitute from there.
Answer: Let's say you have 4 equations with 4 unknowns. Start with equation 4, and solve for unknown 4.
Plug that into equation 3,and solve for unknown 3. Plug that into equation 2 and solve for unknown 2.
Plug into 1 and solve for 1. Ok, it isn't QUITE that easy, but close.
Soon, I'll show you how to put your equations into a matrix. Then we will use our calculators or Matlab to
solve it.
But suppose you did want to do it by hand, here is how you could do it (this is how your calculator is doing
it). This video (for my numerical electromagnetics class) shows how to solve N equations for N unknowns.
I think they cover this in Calc II, but often you don't realize this is how you can use it. I'll link the video
here... (no you don't need to be able to do this, but since you asked, maybe you'll find it interesting!)
https://www.youtube.com/watch?v=P2abN3P32cY
3) Voltage and Current Sources
Question: The most confusing concept for me this week was the independent current sources. How does
the current from the current source work and when does it change when solving circuits.
Answer. A voltage source has a fixed amount of voltage, but the current coming from the voltage source
varies depending on what you connect it up to. A current source gives a constant current, but the voltage
may change depending on what you connect it up to. The current from a current source is the same
anywhere it goes even through a voltage source or a resistor until it hits an extraordinary node and can
split.
How to build a constant current source. This is a bit beyond where we are in the class today, so I do not
expect you to understand all of what is described here. But, perhaps it will help to know that there is in
fact a logical way to create a source that can supply constant current:
https://en.wikibooks.org/wiki/Circuit_Idea/Simplest_Transistor_Current_Source