Finding the current through a resistor using superposition theorem.

AI Thread Summary
The discussion focuses on using the superposition theorem to find the current through a 3-ohm resistor in a circuit with both current and voltage sources. The initial attempt involved shorting the voltage sources and calculating the current through the resistor using only the current source, resulting in 1.7A. There is confusion regarding whether this method accurately reflects the current when voltage sources are included. Participants suggest recalculating the current with the voltage sources active and then summing the results from each source. The consensus is to proceed with the calculations for each source separately and combine the findings for the final current value.
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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