Superposition, Power, Thevenin equivalent and nodal analysis.

Thread Starter

Candyfloss_

Joined Aug 18, 2018
4
Hey,

Please help me with this assignment. I am struggling, especially, with question 1, 2(if you help me with a) I will be able to do b)), 3 and 5. I have attached the assignment.
 

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Thread Starter

Candyfloss_

Joined Aug 18, 2018
4
Hi Cf,
Welcome to AAC.
Could you please post your effort in solving the assignment, we can advise of any errors.
E
for question 2

V=IR
176=I*28
I=6 and 2/7

P=VI
=176*6 and 2/7
=1106 and 2/7

power in 24 ohm
P=V^2/R
=176^2/24
=1290 and 2/3

for question 1 I am stuck because of the Va-voltage controlled source

question 3 i got Rth as 10R/17

QUESTION 4
R1=5
R2=40
R3=25
R4=5

QUESTION 5 I am completely confused
 

shteii01

Joined Feb 19, 2010
4,647
QUESTION 5 I am completely confused
Yeah... whoever wrote question 5 is an idiot.

I am going to guess that 15V is 15 Volts.

V across 2 Ohm resistor is the unknown that you need to find. My advice is to give it some other name.

I am guessing 5v at the dependent voltage source is actually 5*voltage across 2 Ohm resistor.

Step 1. Assign reference node. I would use the bottom of the circuit for the reference node.
Step 2. Simplify the circuit. You have 20 Ohm and 40 Ohm resistors in parallel, find their equivalent resistance and replace them with it.
Step 3. Do the nodal analysis.
 

WBahn

Joined Mar 31, 2012
24,848
for question 2

V=IR
176=I*28
I=6 and 2/7

P=VI
=176*6 and 2/7
=1106 and 2/7

power in 24 ohm
P=V^2/R
=176^2/24
=1290 and 2/3
We need to see your work -- all of it -- otherwise you are forcing use to pull out a crystal ball and try to figure out what you are doing, or think that you are doing.

If you show us all the steps that you took, then we can quickly spot when and where you make your first mistake and get that corrected and move on.

As it is, when you say 176 = I * 28 I have to assume that the I you are talking about is the total current and that, somehow, 28 Ω is the total effective resistance of the circuit. Fortunately, in this case, this IS the total resistance. But what if this is where you had made your mistake? Then you would have had some other value in that equation and we would have no clue where it came from or where the mistake was that caused it. You need to show your work!

So now we agree that the total current supplied by the 176 V source is 6.38 A and that the power is 1106 W.

For the power in the 24 Ω resistor you are making one of the classic mistakes showing that you don't really yet understand the power equation you are throwing about. When you use P = V²/R you MUST use the voltage ACROSS that R. But 176 V is NOT the voltage across the 24 Ω resistor, it is the voltage across the entire resistor network. You need to work your way down to find either the voltage across THAT resistor or the current through THAT resistor.

But you know the voltage across the top/bottom nodes on the left side and you know the current flowing through the 16 Ω resistor. Can you use that information to find the voltage across the top/bottom nodes on the right side?

for question 1 I am stuck because of the Va-voltage controlled source
And we are NOT mind readers -- we have little chance of guessing why you stuck unless you show us your best attempt as far as you are able to get it so that we can see what you were trying to do and where you got stuck. Show your work!

question 3 i got Rth as 10R/17
This is wrong. And since you won't show us your work, it's awfully hard to even guess where you might have gone wrong, so I won't even try. Show your work!

QUESTION 4
R1=5
R2=40
R3=25
R4=5
You got some of them right, but at least one of them wrong. But since you only give answers, I would have to polish up that crystal ball and pull out by old mind-reading self-study course from years back to figure out where. But if you would have shown your work, I probably could have helped you find exactly where you went wrong. Show your work!

QUESTION 5 I am completely confused
What is causing the confusion? What parts seem contradictory? What information do you think is missing. Again -- we are NOT mind readers!

I have a pretty good guess what you are confused about -- but engineering is not about playing guessing games. YOU need to communicate your thoughts and concerns so that others can understand and address them properly.
 

Thread Starter

Candyfloss_

Joined Aug 18, 2018
4
Yeah... whoever wrote question 5 is an idiot.

I am going to guess that 15V is 15 Volts.

V across 2 Ohm resistor is the unknown that you need to find. My advice is to give it some other name.

I am guessing 5v at the dependent voltage source is actually 5*voltage across 2 Ohm resistor.

Step 1. Assign reference node. I would use the bottom of the circuit for the reference node.
Step 2. Simplify the circuit. You have 20 Ohm and 40 Ohm resistors in parallel, find their equivalent resistance and replace them with it.
Step 3. Do the nodal analysis.
Thank You it makes sense to me now.
I named it Vx
I got Vx as 5V
And power is supplied to the 15 volts, 30W
 

The Electrician

Joined Oct 9, 2007
2,750
Thank You it makes sense to me now.
I named it Vx
I got Vx as 5V
And power is supplied to the 15 volts, 30W
Your last assertion seems somewhat ambiguous to me.

A simple change of one word can drastically change the meaning of a sentence. Consider:

And power is supplied to the 15 volts, 30W. This would seem to mean that power is flowing into (being absorbed by) the 15 volt source.

And power is supplied by the 15 volts, 30W. This would seem to mean that power is flowing out of (being supplied by) the 15 volt source.

Which do you think is true? Is power absorbed by or supplied by the 15 volt source?
 

Thread Starter

Candyfloss_

Joined Aug 18, 2018
4
Your last assertion seems somewhat ambiguous to me.

A simple change of one word can drastically change the meaning of a sentence. Consider:

And power is supplied to the 15 volts, 30W. This would seem to mean that power is flowing into (being absorbed by) the 15 volt source.

And power is supplied by the 15 volts, 30W. This would seem to mean that power is flowing out of (being supplied by) the 15 volt source.

Which do you think is true? Is power absorbed by or supplied by the 15 volt source?
Sorry and thanks for bringing that up.
I meant supplied by the 15V.
 
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