Basic Circuit Analysis: Labelling Currents & Assigning Polarity

In summary, the conversation discusses confusion about drawing circuits with LaTeX and labeling currents and polarities in circuits. They mention a Node Voltage method and how to choose the direction of currents. The conversation also touches on components in series and parallel and when currents change.
  • #1
FrogPad
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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:
Code:
            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)
Code:
. . . ----(- +)-----/\/\/------/\/\/------(- +)---- . . .
           V_0        R_0        R_1       V_1
 
Last edited:
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  • #2
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?
 
  • #3
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.
 
  • #4
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?
 
  • #5
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.
 

1. What is basic circuit analysis?

Basic circuit analysis is the process of analyzing an electrical circuit to determine its behavior and characteristics. It involves using mathematical equations and principles to calculate voltage, current, and resistance within a circuit.

2. What is the purpose of labelling currents in a circuit?

Labelling currents in a circuit helps to keep track of the direction and magnitude of the flow of electric charge. This is important for understanding how components in the circuit are connected and how they affect the overall behavior of the circuit.

3. How do you assign polarity in a circuit?

Polarity in a circuit refers to the direction of the current flow. To assign polarity, you must determine the positive and negative terminals of each component in the circuit. The current flows from the positive terminal to the negative terminal.

4. What is Kirchhoff's Current Law and how is it used in circuit analysis?

Kirchhoff's Current Law states that the sum of all currents entering a node (or junction) in a circuit must equal the sum of all currents leaving that node. This law is used in circuit analysis to calculate unknown currents at different points in a circuit.

5. How do you label currents in a series circuit versus a parallel circuit?

In a series circuit, the current is the same at all points and can be labelled with a single value. In a parallel circuit, the current splits and flows through multiple paths, so each branch of the circuit must be labelled with a different value. The total current in a parallel circuit is equal to the sum of the currents in each branch.

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