# KVL Exercise - Basic

by Ammar w
Tags: basic, exercise
 P: 28 1. The problem statement, all variables and given/known data Click on the link https://www.diigo.com/item/image/2sb3i/yjim 2. Relevant equations KVL Law Similar Example : https://www.diigo.com/item/image/2sb3i/orwa 3. 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.
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 Quote by Ammar w 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 "+ -".
 P: 28 thank you but I see that there is no difference if it was "+8" instead of "-8" ???
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KVL Exercise - Basic

 Quote by Ammar w 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
 P: 28 ok, how the solution of (a) will be if : https://www.diigo.com/item/image/2sb3i/y0us
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 Quote by Ammar w 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...
 P: 28 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.
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P: 11,675
 Quote by 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.
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.
 P: 28 aha thank you Mr. gneill

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