Finding the current through a resistor using superposition theorem.

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Discussion Overview

The discussion revolves around finding the current through a 3 ohm resistor in a circuit using the superposition theorem. Participants explore the application of the theorem in the context of a homework problem involving both current and voltage sources.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes their approach of shorting voltage sources and calculating the current through the 3 ohm resistor using only the current source, arriving at a value of 1.7A.
  • Another participant questions the validity of calculating the current without considering the voltage sources, suggesting that the current should differ when the voltage sources are included.
  • A participant clarifies their intent to re-evaluate the circuit with the voltage sources present while treating the current source as an open circuit, seeking confirmation on their initial current calculation.
  • One participant expresses skepticism about the approach being taken, suggesting it may be unnecessarily complicated, but refrains from making a definitive judgment.
  • A later reply encourages the original poster to calculate the currents due to the other sources and sum the results, implying a continuation of the superposition method.

Areas of Agreement / Disagreement

Participants express differing views on the appropriateness of the initial approach to the problem, with some questioning the method while others provide encouragement to continue with the calculations. No consensus is reached regarding the correctness of the initial calculations or the methodology employed.

Contextual Notes

There is uncertainty regarding the assumptions made when shorting voltage sources and how this affects the overall current calculation. The discussion does not resolve whether the initial current calculation is correct or if the approach is valid under the superposition theorem.

Pottymouth
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Homework Statement


We have this circuit.
P1cLu.png


The problem states to find i, or the current through the 3 ohm resistor.

Homework Equations


V = IR

The Attempt at a Solution



I've started by shorting out both voltage sources and leaving only the current source in the circuit. I paralleled the 4ohm resistor and the 3ohm resistor nearest the current source to get 12/7 ohms equivalent resistance. Multiplying that by 3A gives me 5.1 volts going through the 4 ohm resistor nearest the current source and 5.1 volts going through the 3 ohm resistor.

Dividing 5.1 volts by 3ohms gives me 1.7A going through the 3 ohm resistor from the current source alone. I'm not sure if did that right and need some help on what to do.
 
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Pottymouth said:
I've started by shorting out both voltage sources and leaving only the current source in the circuit.

So how do you reckon the current will be the same with and without the voltage sources? That is, you have been asked to find the current WITH the voltage sources in the circuit but you have solved for the current having replaced the voltage sources with wires. How do you figure you'll get the same answer?
 
phinds said:
So how do you reckon the current will be the same with and without the voltage sources? That is, you have been asked to find the current WITH the voltage sources in the circuit but you have solved for the current having replaced the voltage sources with wires. How do you figure you'll get the same answer?

Ahh I should have clarified that I was going to solve it again with the other sources still there while the current source becomes an open circuit. I guess my real question was if I got the right amount of current through the 3 ohm resistor using the current source alone, which that will be added to the other currents I get when using the separate voltage sources alone.
 
Pottymouth said:
Ahh I should have clarified that I was going to solve it again with the other sources still there while the current source becomes an open circuit. I guess my real question was if I got the right amount of current through the 3 ohm resistor using the current source alone, which that will be added to the other currents I get when using the separate voltage sources alone.

Sounds crazy to me, and it's certainly not anything I ever have, or would ever, try, but I'm too tired to figure out if it's wrong. What I CAN say is that you sure are going about this the hard way, seems to me.
 
You're doing okay. Now work out the currents due to the other individual sources and sum the results.
 

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