Can someone explain me this problem? (Circuit analysis using superposition)

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Discussion Overview

The discussion revolves around a circuit analysis problem using the superposition theorem, focusing on the inclusion or exclusion of specific resistors and the behavior of voltage and current sources in the circuit. Participants seek clarification on the steps involved in the analysis and express concerns about the exercise's validity in teaching circuit concepts.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested, Homework-related

Main Points Raised

  • Some participants question why the 7Ω resistor is excluded in the first step of the analysis, suggesting it does not affect the current sought due to the fixed potential difference set by the voltage source.
  • Others argue that the 7Ω resistor can be ignored in step 1 because the voltage remains constant at 18V, regardless of its presence.
  • In step 2, participants explain that the 18V and 24V sources are shorted to analyze the effects of the current source independently.
  • One participant expresses confusion about the exercise, stating that a current source cannot have voltage and questioning the practicality of the example used for teaching circuit analysis.
  • Another participant counters that a current source can have any voltage across it, emphasizing the concept of infinite impedance and referencing Thevenin and Norton equivalent circuits.

Areas of Agreement / Disagreement

Participants express differing views on the validity of the exercise and the treatment of current sources, indicating that multiple competing perspectives remain unresolved.

Contextual Notes

There are unresolved assumptions regarding the behavior of current sources and the implications of circuit components in the analysis steps. The discussion reflects varying interpretations of circuit theory principles.

Alexmanh
I do not really understand why we don't include 7Ω resistor in step 1, and I need some explanation with step 2.
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In 1 it is irrelevant for the sought current (the potential difference is set by the source. In the other two it is parallel with a zero resistance (again, known potential difference - zero).
 
In step 1 the 7 ohm resistor does not change the effects of the 18V voltage source on the rest of the circuit so it can be ignored. The voltage is 18V regardless of the value of the (7ohm) resistor.. You are computing the effects of the 18V source by itself.
In step 2 you are shorting the 18 and 24V voltage sources and computing the effects of the current source by itself.
Step 3, same idea as step 1 except this time the 4 ohm resistor is in series with the shorted 24V source so it must be included.
Note the fact that all three sources are mutually independent.
 
Last edited:
This is a strange exercise! Who did it?
The voltage over the 4 ohm resistor must be 24 V.
So the the current -I2 is 24/4=6 A.
Why is that? A current source cannot have voltage, i.e it is zero.
In practical life this is not possible.
If the target is to teach circuit analysis, I don't know if it is a good idea
with such examples.
 
jocke said:
This is a strange exercise! Who did it?
The voltage over the 4 ohm resistor must be 24 V.
So the the current -I2 is 24/4=6 A.
Why is that? A current source cannot have voltage, i.e it is zero.
In practical life this is not possible.
If the target is to teach circuit analysis, I don't know if it is a good idea
with such examples.
A current source can have any voltage you choose around it. A current source has infinite impedance.
There is nothing "strange" about the circuit. Learn about Thevenin and Norton equivalent circuits.
 

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