# Where am I going wrong on this power question?

#### thatsmessedup

Joined Feb 15, 2018
36
My instructor says the answer is 8mW but I cannot reach that conclusion. Where is my logic flawed?
Thanks for any help.

question c:

#### WBahn

Joined Mar 31, 2012
26,398
I didn't spot anything wrong with your five set up equations and I'm pretty sure they are correct (other than extremely sloppy lack of respect for units).

But since your result for Vcs just appears out of thin air, I have no way to even guess what you did wrong.

The fact that you DID do something wrong can be detected by checking your answer.

Assume you are correct and then work the problem backwards. Do the results make sense?

You might try solving the problem in the sequence they recommended -- although you should be able to solve it using the approach you did. Perhaps if you show the actual work you did to go from your equations to the value of Vcs, we can spot where you went wrong.

#### The Electrician

Joined Oct 9, 2007
2,846
Did you do parts a) and b)? What did you get for the current Ix when each independent source is made active , one at a time?
What did you get for part b)?

Are these statements equivalent?
1. The is2 source delivers -X watts.
2. The is2 source absorbs X watts.

Did your instructor say "The is2 source delivers 8 miliwatts"? Or did he just say "The answer is 8 milliwatts."?

#### WBahn

Joined Mar 31, 2012
26,398
Did your instructor say "The is2 source delivers 8 miliwatts"? Or did he just say "The answer is 8 milliwatts."?
What's the difference?

If he said that the answer to part (c) is 8 mW, then doesn't that mean that the is2 source delivers 8 mW since part (c) explicitly asks for the power delivered by the is2 source.

#### thatsmessedup

Joined Feb 15, 2018
36
You guys are always thinking the way I wish to think. Thank you for all your inputs as they will surely help to resolve future problems. The issue this time was my input to the calculator. I had punched a number wrong (multiple times) which lead me to believe that my equations were wrong somehow. Thanks again!

#### The Electrician

Joined Oct 9, 2007
2,846
What did you get for part b)?

#### The Electrician

Joined Oct 9, 2007
2,846
What's the difference?

If he said that the answer to part (c) is 8 mW, then doesn't that mean that the is2 source delivers 8 mW since part (c) explicitly asks for the power delivered by the is2 source.
As you know. sometimes people are sloppy with signs, as they are also sloppy with units. I was considering the possibility that a sign was being ignored by saying the answer is 8 mw. Adding the word "delivered" provides some redundancy as a partial antidote for sloppiness. Since there is a dependent source in this circuit, there was the possibility that negative resistance might show up in the solution, which might lead to mistakes in signs.

#### The Electrician

Joined Oct 9, 2007
2,846
thatsmessedup, since you have already got all your equations set up, try setting the 12 volt source to 10 volts and calculate the power delivered by is2.

Did you actually do part a) of this problem? Your method of solving the equations lends itself to a really easy to way do a solution by superposition; do you know what it is?

What calculator are you using to do the row reduce?

#### WBahn

Joined Mar 31, 2012
26,398
You guys are always thinking the way I wish to think. Thank you for all your inputs as they will surely help to resolve future problems. The issue this time was my input to the calculator. I had punched a number wrong (multiple times) which lead me to believe that my equations were wrong somehow. Thanks again!
This is why I recommend making a clear demarcation between the setup and the math handle cranking.

Set up the equations as closely as possible to the annotated schematic. This greatly reduces the likelihood that you will make a mistake in the first place and also makes it a much easier and surer thing to verify that your setup equations are correct.

Remember that these setup equations represent the schematic and the problem. ALL of the EE stuff happens here. Everything after this is just mathematical gyrations. Don't split your attention by trying to do any simplifications at this stage -- keep your attention focused like a laser on applying tried and true analysis concepts to transform the problem represented in the schematic to a set of equivalent mathematical equations.

Once that is done, the you can largely forget the EE and focus on doing the math correctly. This is not to say that you shouldn't still be asking if the results you are producing along the way make sense -- that is a highly valuable tool to catch many of the math errors you WILL make from time to time.