# Help with Superposition theorem.

#### Mahmoud Salem

Joined May 6, 2017
6
Hello,
I am fairly familiar with how superposition theorem is applied and the ideas and steps behind it. But my problem is when circuits become abit confusing (Like the one attached below) I fairly get stuck .

How do i approach the following problem? This is for my upcoming exam in a week.

#### WBahn

Joined Mar 31, 2012
26,294
What's confusing about the circuit? If it makes it more familiar, redraw it with the two voltage sources shown explicitly and all the grounds connected by wires.

Beyond that, YOU need to show YOUR best attempt to solve the problem -- we'll provide feedback and point out where you are going wrong.

This is particularly important on this one because your exam is coming up -- you won't be able to ask someone on the internet how to work an exam problem that you don't quite understand. So treat this just like an exam problem and answer it to the same degree that you would an exam question. You will learn a lot more that way.

#### MrAl

Joined Jun 17, 2014
7,849
Hello there,

When using superposition with multiple sources, you 'kill' all the sources, then 'unkill' each source one at a time and calculate the response at the node in question for each, then add the responses together for the final calculation at that node.

You should also show some of this work you did yourself first.

Do you know how to 'kill' and 'unkill' a source?

#### LesJones

Joined Jan 8, 2017
2,726
I would start by simplifying the circuit using Thevenin's theorem.

Les.

#### MrAl

Joined Jun 17, 2014
7,849
I would start by simplifying the circuit using Thevenin's theorem.

Les.
Hi,

But he is being asked to do it using superposition, so that would be cheating wouldnt it?

#### Mahmoud Salem

Joined May 6, 2017
6
What's confusing about the circuit? If it makes it more familiar, redraw it with the two voltage sources shown explicitly and all the grounds connected by wires.

Beyond that, YOU need to show YOUR best attempt to solve the problem -- we'll provide feedback and point out where you are going wrong.

This is particularly important on this one because your exam is coming up -- you won't be able to ask someone on the internet how to work an exam problem that you don't quite understand. So treat this just like an exam problem and answer it to the same degree that you would an exam question. You will learn a lot more that way.

My problem lies exactly in redrawing such shapes as we never went over it in class or anything. I will be able to apply the theorem once i get the circuit right.

#### Jony130

Joined Feb 17, 2009
5,186
A hint, the voltage at the far right side is +8V with respect to a reference point (ground), hence you can put a voltage source (8V) from this point to ground. And you can do the same for far left node.

#### LesJones

Joined Jan 8, 2017
2,726
Hi Mr Al,
After the circuit is simplified superposition can be used. It will depend exactly how the question is worded as to weather my suggestion would be permissible.

Les.

#### shteii01

Joined Feb 19, 2010
4,644
The first thing you do is REDRAW the circuit so that you can actually understand what is going on there.

#### WBahn

Joined Mar 31, 2012
26,294
Hi Mr Al,
After the circuit is simplified superposition can be used. It will depend exactly how the question is worded as to weather my suggestion would be permissible.

Les.
Trying to play schoolhouse lawyer is generally not a very wise thing to do on an exam.

The purpose of the question is clearly to evaluate the ability of the student to apply superposition to the analysis of a circuit containing multiple sources with credit awarded based on the degree to which that ability is demonstrated. Any cute little games that are played to circumvent that is likely to do nothing except convince the grader that the student does not have that ability and/or has failed to demonstrate that ability with the consequent reduction in credit awarded.

Not a game worth playing, most of the time.

#### WBahn

Joined Mar 31, 2012
26,294
The first thing you do is REDRAW the circuit so that you can actually understand what is going on there.
Be sure to give him your cell number so that during his exam when he sees something that he doesn't know how to do he can give you a call to have you do it for him without him even having to make the attempt. I'm sure the exam proctor won't mind.

#### shteii01

Joined Feb 19, 2010
4,644
Be sure to give him your cell number so that during his exam when he sees something that he doesn't know how to do he can give you a call to have you do it for him without him even having to make the attempt. I'm sure the exam proctor won't mind.
Petty.

#### WBahn

Joined Mar 31, 2012
26,294
No. But a bit annoyed that, once again, you are violating the rules of this forum.

The TS was given hints by two people on how to proceed so that he could learn how to work through it on his own. But less than twenty minutes later you have to jump in and do the work for him so that he didn't get the change to even try. Maybe he learned something from that and maybe he didn't. It's quite likely that he didn't since he's probably seen problems like this worked for him more than once already, so how likely is it that he is going to suddenly grasp what he's been missing because you step in and do yet one more problem for him. Sadly, if he didn't get it as a result, he will likely think that he did and not realize that he really didn't until he needs to do it on the test -- and then where will he be when he needs you to do his work for him yet again?