Combination circuit help

[kwired]

I am not changing any thing, because I did not set that question paper.

I am just trying to bring to your kind notice the self-contradictory nature of the problem statement.

:? Like I said, I haven't done any of the math needed here, but from what I see there is nothing wrong with the presented problem. They gave enough information that one should be able to find all missing values off what was given. Based on others calculations though, the correct answer apparently is not in the answer choices given.

 

[Sahib]

:? Like I said, I haven't done any of the math needed here, but from what I see there is nothing wrong with the presented problem. They gave enough information that one should be able to find all missing values off what was given. Based on others calculations though, the correct answer apparently is not in the answer choices given.

Then I invite you to do some math. Find the voltages and currents based on 22mA current through 242ohm resistor. Sum up the voltages. Compare the resulting voltage with 50V source voltage. You will find that they are not equal. That is why I informed you that the problem statement is self-contradictory.

 

[kwired]

Then I invite you to do some math. Find the voltages and currents based on 22mA current through 242ohm resistor. Sum up the voltages. Compare the resulting voltage with 50V source voltage. You will find that they are not equal. That is why I informed you that the problem statement is self-contradictory.

You are correct that 50 volts applied to 242 ohms does not yield 22mA. That is not what we have in the problem though.
It doesn't work out to that value because there is another resistor in parallel with the one carrying 22mA, and those two parallel resistors are in series with another resistor. The 50 volts is not the volts across the 242 ohm resistor it is voltage applied to the entire circuit. Go back and read the OP again.

 

[Sahib]

You are correct that 50 volts applied to 242 ohms does not yield 22mA. That is not what we have in the problem though.
It doesn't work out to that value because there is another resistor in parallel with the one carrying 22mA, and those two parallel resistors are in series with another resistor. The 50 volts is not the volts across the 242 ohm resistor it is voltage applied to the entire circuit. Go back and read the OP again.

You misunderstood. I meant the entire voltage. Calculate it and compare it with the source voltage. They will not be equal.

 

[kwired]

You misunderstood. I meant the entire voltage. Calculate it and compare it with the source voltage. They will not be equal.

Ok, I finally solved this problem myself and come up with 29.5 milliamps as the answer to the question asked, as did many other responders.

Source voltage is not needed to solve this problem so why do you reference it? All that is needed to answer what has been asked is resistance values of 242 and 180 for the parallel components and the fact that the current through the 242 ohm resistor is 22mA. Forget all other information given and solve for current flowing in the 180 ohm resistor. 50 volt source and the other 420 ohm resistor mean nothing to solve what was asked for.

 

[n1ist]

The original problem as given (with both the 50V source and 22mA current as givens) violates the laws of physics. Therefore, it is impossible to come up with a valid answer for the problem. As we have shown, if you specifically discard one of the contradictory pieces of information, you can come up with valid answers that satisfy both Ohm's and Kirchhoff's laws, but it sounds like none of those are ones that are choices in the (obviously error-filled) book that the OP is working from.

/mike

 

[Sahib]

Ok, I finally solved this problem myself and come up with 29.5 milliamps as the answer to the question asked, as did many other responders.

Source voltage is not needed to solve this problem so why do you reference it? All that is needed to answer what has been asked is resistance values of 242 and 180 for the parallel components and the fact that the current through the 242 ohm resistor is 22mA. Forget all other information given and solve for current flowing in the 180 ohm resistor. 50 volt source and the other 420 ohm resistor mean nothing to solve what was asked for.

Since my first post in this thread I did not try to answer the OP's question but tried to rectify the mistake in Hurk's line of reasoning and to point out the self-contradictory nature of the problem statement as the problem is really inconsistent with ALL ​ values of the circuit taken into account.

 

[kwired]

The original problem as given (with both the 50V source and 22mA current as givens) violates the laws of physics. Therefore, it is impossible to come up with a valid answer for the problem. As we have shown, if you specifically discard one of the contradictory pieces of information, you can come up with valid answers that satisfy both Ohm's and Kirchhoff's laws, but it sounds like none of those are ones that are choices in the (obviously error-filled) book that the OP is working from.

/mike

But 50 volts is the source applied to the entire circuit not the voltage across the resistor drawing 22mA.

The question is a good question, but none of the given answers match the question. One can look at the resistance and current of that one resistor and find the voltage across it. Same voltage will be present across the other resistor that is parallel to it, and you can then calculate current in that resistor. The 50 volts applied is irrelevant to the question that was asked and so is the resistance of the single resistor in series.

 

[Sahib]

kwired:
I understand your point in post#45. If 29.5mA is among the choices, it is wise to tick it and get the mark from the examiner.
But from the forum point of view it is good to analyze the circuit from all angles.

 

[Besoeker]

kwired:
I understand your point in post#45. If 29.5mA is among the choices,

Evidently not.
Read post #18.

 

[gar]

130724-0932 EDT

Sahib:

The complete circuit was never defined in the statement of the problem. You might imply that the three resistor network was connected across the 50 V mentioned, but that was never stated.

If you make that assumption, then the circuit and given values are in conflict.

Tests should be teaching tools, not something designed to trick or confuse a student.

I think this question has been a disservice to Eddy Current, but possibly thru this thread the badly designed question in combination with the incorrect possible answers may have taught Eddy Current how to spot this type of error.

.

 

[Besoeker]

130724-0932 EDT
I think this question has been a disservice to Eddy Current, but possibly thru this thread the badly designed question in combination with the incorrect possible answers may have taught Eddy Current how to spot this type of error.

Totally agree, gar.

 

[Sahib]

130724-0932 EDT

Sahib:

The complete circuit was never defined in the statement of the problem. You might imply that the three resistor network was connected across the 50 V mentioned, but that was never stated.

.

No, it was stated ''.....the combination in a 50 volt circuit.''

A 242 is in parallel with a 180 resistor, and a 420 resistor is in series with the combination in a 50 volt circuit.

 

[ggunn]

What about the 50V source? It is also given in the problem. You have not taken into account. Have you?

It is a non sequitur. The problem statement says that it is a 50V circuit but it does not say that there is nothing else in the circuit in series with the resistors in the problem. This is typical misdirection that is in many testing situations (the NABCEP solar professional exam I took was full of this kind of thing); the real test is to figure out which information is relevant and which is misdirection, while the calculation is secondary. If you infer things that are not explicitly stated it can send you down a rabbit hole.

 

[ggunn]

Tests should be teaching tools, not something designed to trick or confuse a student.

I think this question has been a disservice to Eddy Current...

I disagree. The real test posed by this question is to see if the testee can separate the information needed to solve the problem from extraneous irrelevant (and misleading) data. The calculation is trivial by comparison. The real world is like that.

The NABCEP Solar Professional exam I took (and passed) was chock full of this sort of thing. Sometimes there was a full page problem description with reams of data in it but there was only a number or two in all of that that had anything to do with deriving the solution to the problem.

 

[Sahib]

It is a non sequitur. The problem statement says that it is a 50V circuit but it does not say that there is nothing else in the circuit in series with the resistors in the problem. This is typical misdirection that is in many testing situations (the NABCEP solar professional exam I took was full of this kind of thing); the real test is to figure out which information is relevant and which is misdirection, while the calculation is secondary. If you infer things that are not explicitly stated it can send you down a rabbit hole.

Agreed subject to the point in

kwired:
I understand your point in post#45. If 29.5mA is among the choices, it is wise to tick it and get the mark from the examiner.
But from the forum point of view it is good to analyze the circuit from all angles.

 

[Besoeker]

Agreed subject to the point in

A point that has been shown to b.e irrelevant.
See post #18.

 

[ggunn]

A 242 is in parallel with a 180 resistor, and a 420 resistor is in series with the combination in a 50 volt circuit. A current of 22mA flows throw the 242 resistor. The current through the 180 resistor is?


Do i have drawn it out but do you not calculate everything in the parallel part of the circuit first?

Is that a verbatim quote of the entire problem statement? As stated there is no way any of the answers in post #18 can be correct, even assuming that 50V is across the network.

 

[Sahib]

A point that has been shown to b.e irrelevant.
See post #18.

My point is, from the forum point of view the circuit is to be analyzed from all angles and not merely to help the OP to gain more marks from the exam without understanding the theory behind.

 

[kwired]

130724-0932 EDT

Sahib:

The complete circuit was never defined in the statement of the problem. You might imply that the three resistor network was connected across the 50 V mentioned, but that was never stated.

If you make that assumption, then the circuit and given values are in conflict.

Tests should be teaching tools, not something designed to trick or confuse a student.

I think this question has been a disservice to Eddy Current, but possibly thru this thread the badly designed question in combination with the incorrect possible answers may have taught Eddy Current how to spot this type of error.

.

I don't necessarily see that there was any unnecessary trickery in the question, there is nothing wrong with providing information in a problem that is irrelevant to solving the problem. A little better written version would have been nice. Maybe one of the possible answers was the correct answer but the wording of the problem is wrong with the intent of the problem? If this is just practice questions one needs to confront the source of the material, if it is a true testing situation one must confront the examiner either directly or by making notes on the exam about the situation. Of course this is easier for us with years of experience to say than it is for someone with little experience to do.

No, it was stated ''.....the combination in a 50 volt circuit.''

The question could have been better worded to clarify what the 50 volts actually was, most of us have assumed it is the supply voltage and the three resistors are the only loads.

I disagree. The real test posed by this question is to see if the testee can separate the information needed to solve the problem from extraneous irrelevant (and misleading) data. The calculation is trivial by comparison. The real world is like that.

The NABCEP Solar Professional exam I took (and passed) was chock full of this sort of thing. Sometimes there was a full page problem description with reams of data in it but there was only a number or two in all of that that had anything to do with deriving the solution to the problem.

Agree, real world problems have a lot of information available that may not be relevant to solving the problem, you need to know what information you need to use to solve the problem.

My point is, from the forum point of view the circuit is to be analyzed from all angles and not merely to help the OP to gain more marks from the exam without understanding the theory behind.

We are good at analyzing things from every angle here even when it is not necessary. This just happened to be one of those cases where the given answers did not match the problem and additional analysis is all we can do and even then the result may just be speculation to what the original writers intent was.