Basic Circuit Analysis: Labelling Currents & Assigning Polarity

[FrogPad]

I'm confused with something really basic, and it is bothering me. I'm not sure if there is a way to draw circuits with LaTeX on this forum (or even how to do it if there is), so please excuse the ASCII art. (If there is a way to draw them, I'll be more than happy to learn. Anyways...


Lets say you have a circuit as such:

            R_1
  ----------\/\/\----------
 |            |            |
 |            |            \
(+) V_0      (-) V_1       /  R_2
(-)          (+)           \
 |            |            |
 |            |            |
  ----------\/\/\----------

How do you know how to label the currents? The professor just throws in currents, and I don't understand why, or how he is doing it. Is there an algorithmic way to do it? I always seem to be adding more currents than what is necessary. If someone could just point me to a website, explain it, or give any insight I would appreciate it. I don't want to get far into this course and not understand it. Thanks.

Also another question. How do you know how to assign polarity to circuit elements.

ex)

. . . ----(- +)-----/\/\/------/\/\/------(- +)---- . . .
           V_0        R_0        R_1       V_1

 

[FrogPad]

I just read this:
"Name the remaining n-1 nodes and label a current through each passive element and each current source"

It is referencing the Node Voltage method (something I have not covered yet), but this seems pretty straightforward. So then how do you know which direction to choose?

 

[berkeman]

Just throw the currents in clockwise in each loop. When you do the calculation, that is when you find out the actual vaule of each current and voltage. If some of the turn out to be negative, that doesn't matter, it's just part of the solution.

 

[FrogPad]

Ok. Cool.

Now I have a few more questions about when to change to a different current. Like when do I change from i1, to i2.

Is this correct?
1) As long as elements are connected in series then the current is the same.

for example, if we had the following branch, then the voltage would be the same from NODE to NODE?

NODE---R-----R----INDUCTOR----VOLTAGE_SOURCE-----NODE

2) When a node is reached then the current changes?

 

[berkeman]

All components hooked in parallel will have the same voltage across them. All components hooked in series will have the same current flowing through them. When you have, say, 3 resistors hooked in series, then the current is the same through all of them, and the voltage drop across each one is proportional to the resistance of each.

I'm not sure what you're asking about changing to different currents. Just draw the circulating currents around each loop, and where there are shared segments that are between two loops, the net current in that leg is the difference between the two loop currents.