KVL Exercise: Basic Homework Statement & Solution

[Ammar w]

Homework Statement

Click on the link
https://www.diigo.com/item/image/2sb3i/yjim

Homework Equations

KVL Law
Similar Example :
https://www.diigo.com/item/image/2sb3i/orwa

The Attempt at a Solution

(a)
In the left loop :
-8-12+VR2 = 0
VR2 = -20 v

(b)
In the big loop :
-8-12+7-9-V2-3+VR1 = 0
-8-12+7-9-V2-3+1 = 0
V2 = -24

Why is my solution wrong?

If I take the right loop for (b) using the right answer for (a) :
-4+7-9-V2-3+1 = 0
V2 = -8 which is true.

Is the wrong with the negative voltage (-8) ??

thanks.

 

[gneill]

Is the wrong with the negative voltage (-8) ??

Yes, it looks like whoever created the circuit diagram created confusion when they labeled the leftmost resistor's potential drop. As it is marked, strictly speaking there should be a -8V potential drop across that resistor proceeding from bottom to top, which translates into a +8V potential rise as you've properly interpreted it. However, it would appear that they (confusingly) intended the "+ -" indicators to show the actual direction of the potential change of 8V.

Bottom line: assume that the "-8V" is really "8V" oriented according to the "+ -".

 

[Ammar w]

thank you
but I see that there is no difference if it was "+8" instead of "-8" ?

 

[gneill]

thank you
but I see that there is no difference if it was "+8" instead of "-8" ?

Um, how can it make no difference if you change the sign of a potential change?

For part (a) your equation summing the potential drops would become: +8-12+VR2 = 0

 

[Ammar w]

ok, how the solution of (a) will be if :
https://www.diigo.com/item/image/2sb3i/y0us

 

[gneill]

ok, how the solution of (a) will be if :
https://www.diigo.com/item/image/2sb3i/y0us

Just solve your equation that I presented in my previous post...

 

[Ammar w]

I'm sorry
but can you tell me the equation of (a) if the potential change was "-8"?
I thought that the equation +8-12+VR2 = 0 is used when the potential change = -8.

 

[gneill]

I'm sorry
but can you tell me the equation of (a) if the potential change was "-8"?
I thought that the equation +8-12+VR2 = 0 is used when the potential change = -8.

Yes, that's right. The problem is not with your understanding, it's with the problem itself; the resistor is not labeled correctly for the given answers. The resistor should have its potential labeled as "8V", not "-8V", or alternatively, the + and - tags should be reversed.

 

[Ammar w]

aha
thank you Mr. gneill