Possible confusion regarding application of Kirchoff's Laws?

In summary, Kirchoff's rules state that the sum of the currents in a circuit is zero, and that the voltage across a resistor is the same whether you are swimming upstream (downstream) or not.
  • #1
Enharmonics
29
2

Homework Statement



Determine Currents ##I_{1}, I_{2}, I_{3}##

Homework Equations



Kirchoff's Rules:

$$\sum_{i=1}^n I_{i} = 0$$

$$\sum_{i=1}^n V_{i} = 0$$

The Attempt at a Solution



zJe4NbGl.jpg


Basically, I'm not sure if I'm applying the Loop Law correctly. For example, in Loop A, going clockwise from the upper leftmost corner of the circuit, it's easy enough to see that I'd have ##-\epsilon_{1}## and ## +I_{1}R_{1}## based on the direction I'm "tracing" the circuit in, but when I come to the first junction, ##J_{1}##, I'm not sure what to do.

Applying Kirchoff's Junction rule to it based on the entirety of the circuit shows that

$$\sum_{i=1}^n I_{i} = I_{2} + I_{3} - I_{1} = 0 \rightarrow I_{1} = I_{2} + I_{3}$$

But ##I_{3}## isn't in Loop A... so then

$$\sum_{i=1}^n I_{i} = I_{2} - I_{1} = 0 \rightarrow I_{1} = I_{2}$$

I used a similar argument to set up my equation for Loop B.

Is the way I have my system of equations set up (in the image above) correct? If not, would I have to calculate the voltage drop using ##I_{2} = I_{1} - I_{3}##?

Using my current system of equations, my values for the currents are

##I_{1} = \frac{935}{626} A , I_{2} = \frac{205}{313} A, I_{3} = \frac{525}{626} A##

Is that correct?

Thanks in advance.
 
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  • #2
It looks correct to me.

The metaphor I use is about swimming up or downstream. Imagine that the stream is flowing in the direction of the arrow that indicates the direction of each current ##I_k##. Start at one point in the loop and go around the loop, adding a positive or negative term for every item we encounter. For a resistor, we add (subtract) the term ##I_kR_j## if we are swimming upstream (downstream) because that is how much the potential increases as we pass through the resistor from the downstream end to the upstream end. For a power cell, we add (subtract) the voltage of the cell if we enter at the - (+) terminal and leave at the + (-) terminal.

EDIT: When I say it looks correct I'm referring only to the image of the hand-written calcs. As per @LemmeThink's post below, I cannot follow your reasoning in the bit he quoted, which is not in the hand-written calcs. Nor do you need that bit. You already have one equation from the Kirchoff Junction rule, and on the handwritten page you get two more equations from the loop rules. So you have three equations and three unknowns, which can be solved.
 
Last edited:
  • #3
Enharmonics said:
But I3I3I_{3} isn't in Loop A... so then

n∑i=1Ii=I2−I1=0→I1=I2​
Hi!
Your equations seem to have been setup correctly. However, the above quoted text doesn't seem to make sense. How did you get this?
 

1. What is Kirchoff's Laws and how are they applied?

Kirchoff's Laws are a set of fundamental principles in circuit analysis that govern the behavior of electronic circuits. These laws are used to determine the voltage, current, and resistance in a circuit. They can be applied to both DC and AC circuits.

2. What are the two laws that make up Kirchoff's Laws?

Kirchoff's Laws consist of two main principles: Kirchoff's Current Law (KCL) and Kirchoff's Voltage Law (KVL). KCL states that the sum of all currents entering a node in a circuit must equal the sum of all currents leaving that node. KVL states that the sum of all voltage drops in a closed loop must equal the sum of all voltage rises in that loop.

3. How do Kirchoff's Laws apply to complex circuits?

Kirchoff's Laws can be applied to any type of circuit, including complex circuits with multiple batteries, resistors, and other components. The laws are used to determine the values of unknown currents and voltages in the circuit by setting up a system of equations based on KCL and KVL.

4. Can Kirchoff's Laws be used in circuits with non-linear elements?

Yes, Kirchoff's Laws can be applied to circuits with non-linear elements such as diodes and transistors. However, in these cases, the equations used may be more complex and require the use of advanced mathematical techniques.

5. What are some common mistakes made when applying Kirchoff's Laws?

Some common mistakes include not properly labeling the direction of currents and voltage drops, not considering the direction of current flow in parallel branches, and neglecting to include all components in the circuit when setting up the equations. It's important to carefully analyze the circuit and double-check all calculations to ensure accurate results.

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