The validity of the interconnection of ideal sources

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Discussion Overview

The discussion revolves around the validity of interconnecting ideal sources in an electrical circuit, specifically addressing a homework problem involving a circuit with a -30V ideal voltage source, a +10V ideal voltage source, and an -8V ideal current source. Participants explore the implications of these connections on power absorption and development, as well as the conditions for circuit validity.

Discussion Character

  • Homework-related
  • Debate/contested
  • Exploratory

Main Points Raised

  • One participant questions the validity of the circuit due to the apparent contradiction in voltage values according to Kirchhoff's Voltage Law (KVL).
  • Another participant suggests that a circuit is valid if voltage sources can carry the current supplied and the current source can handle the voltage drop.
  • There is confusion regarding the labeling of the ideal current source, with some participants noting it is labeled in volts rather than amps, leading to further discussion about the nature of current sources.
  • Participants discuss the implications of the current source's labeling, with one asserting that it should be 8A instead of 8V, indicating a potential typo or error in the problem statement.
  • Some participants clarify that voltage sources in series can be analyzed from any starting point in the circuit, while others express uncertainty about how to determine which sources are supplying or absorbing power.
  • One participant concludes that without a specified current, the circuit's configuration is nonsensical, suggesting it may be invalid.
  • Another participant expresses gratitude for the discussion, indicating that it helped clarify their understanding despite the circuit's potential errors.

Areas of Agreement / Disagreement

Participants express differing views on the validity of the circuit, with some suggesting it is invalid due to the labeling of the current source, while others focus on the power dynamics between the sources. No consensus is reached regarding the overall validity of the circuit.

Contextual Notes

Participants note the lack of specified current in the circuit, which complicates the analysis and raises questions about the circuit's validity. The discussion highlights the importance of accurate labeling and definitions in circuit analysis.

marsupial
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Homework Statement


Sorry in advance for such a basic question, just starting my first electrical engineering course. The question asks if the interconnection of ideal sources is valid.

The next part of the question asks us to verify that the total power developed in the circuit equals the total power absorbed.

The diagram given is, from the left going clockwise: a -30V ideal voltage source, +10V ideal voltage source, -8V ideal current source.

edit: image added as requested:

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https://ibb.co/eGkEev
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Homework Equations



p = vi

The Attempt at a Solution


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I gave the sign of each voltage above in lieu of a diagram (negative sign a voltage rise, positive a voltage drop). The 30V and 8V source are developing power, the 10V consuming it. I am not sure how this is a valid circuit given that we have -38V + 10V -> According to KVL shouldn't that equal zero?

However, it confuses me as to how this could be valid. According to the answer to a similar question, a circuit is valid when the voltage sources can carry the Amp of current supplied, and the current source can carry the voltage drop required by the interconnection. I don't understand how to calculate whether this is true of a circuit. In the similar question I referenced, which was valid, you first had one 50V source, then a 10V and 40V source, so I suppose you could equate 50 and 40+10 (there was also a 5A source) (the directions in at question were -50, -10, +40, so I wasn't adding them up, just trying to work out why it was considered valid (and I don't know if that is how you go about it anyway).

Regarding power absorbed, we are not given the current, so I am unsure how to proceed there.
 
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Can you just show us the circuit diagram ?
 
Nidum said:
Can you just show us the circuit diagram ?
I added a link to the diagram. It's not very clear, sorry, best I could do.
 
circuit.png


That's the best that the image enhancer could do . It's good enough to see what the problem is about anyway .
 
Hint: What kind of source is the one marked "8V" ?
 
CWatters said:
Hint: What kind of source is the one marked "8V" ?
It's an ideal current source, but I'm not sure how that helps.
 
marsupial said:
It's an ideal current source,
Yes, and how is it lebelled? What is the unit of current?
 
cnh1995 said:
Yes, and how is it lebelled? What is the unit of current?
It's labelled in volts. So 8V goes into the 10V source.
 
marsupial said:
It's labelled in volts. So 8V goes into the 10V source.
That dosn't make sense. How can 8V go into 10V source? It should be 8A. Current sources provide constant current.
It is either a typo or they did it intentionally and want you to spot it.
 
  • #10
cnh1995 said:
That dosn't make sense. How can 8V go into 10V source? It should be 8A. Current sources provide constant current.
It is either a typo or they did it intentionally and want you to spot it.
That's what I thought (that they provide constant current), but they also supply some voltage too don't they? That's why it was confusing.Okay, so assuming it is a typo, how can 30V go into a 10V source?
 
  • #11
marsupial said:
how can 30V go into a 10V source?
The two voltage sources are in series. I don't understand what you mean by "go into" 10V source.
 
  • #12
cnh1995 said:
The two voltage sources are in series. I don't understand what you mean by "go into" 10V source.
Sorry, I just thought we started from the left. So do we start from the ideal current source and go round the circuit in that direction? In which case the 10V is developing power and the 30V absorbing it. And to make it balance the 8A current would have to develop 20V.
 
  • #13
marsupial said:
Sorry, I just thought we started from the left. So do we start from the ideal current source and go round the circuit in that direction? In which case the 10V is developing power and the 30V absorbing it. And to make it balance the 8A current would have to develop 20V.
You can start from anywhere, it doesn't matter. I think what you want to find is who is supplying power and who is absorbing power.
marsupial said:
In which case the 10V is developing power and the 30V absorbing it. And to make it balance the 8A current would have to develop 20V.
Yes. So what is the 8A source doing? Supplying or absorbing?
 
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  • #14
marsupial said:
That's what I thought (that they provide constant current), but they also supply some voltage too don't they?

They produce whatever voltage is necessary to make the specified current flow. However no current has been specified. Specifying a voltage 8V is nonsense so the circuit has an error or is invalid. As a result further analysis of the circuit is rather pointless.
 
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  • #15
cnh1995 said:
You can start from anywhere, it doesn't matter. I think what you want to find is who is supplying power and who is absorbing power.

Yes. So what is the 8A source doing? Supplying or absorbing?
It would be supplying power. Thank you so much for your help, it makes sense now.
 
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  • #16
CWatters said:
They produce whatever voltage is necessary to make the specified current flow. However no current has been specified. Specifying a voltage 8V is nonsense so the circuit has an error or is invalid. As a result further analysis of the circuit is rather pointless.
Thank you. I think given the rest of the question it was an error. Going through it helped clear up some of my misconceptions and helped with my general understanding though, so I am grateful for the responses. I really appreciate this forum!
 
  • #17
marsupial said:
It would be supplying power. Thank you so much for your help, it makes sense now.
You're welcome!
 

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