# Mesh current problem

Discussion in 'Homework Help' started by netwillie, Sep 25, 2010.

1. ### netwillie Thread Starter New Member

Aug 10, 2010
11
3
Hi

In the attched mesh current example from the DC notes i have a problem with the negative sign in the second mesh equation. No matter how i look at this the second equation should read " I2R3 + (I1 -I2)R2 -B2 = 0 " and not " I2R3 - (I1 -I2)R2 -B2 = 0" as shown. I know that I'm wrong but I cannot figure out why. Can any one maybe explain. Your assistance will be greatly appreciated.

Many Thanks

Willie

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2. ### Ghar Active Member

Mar 8, 2010
655
73
What they did was they wrote down the voltage across R2 and used it in both loop equations. Since they started with loop 1, they wrote it as (I1-I2)R2 with positive at the bottom. Since I1 enters this resistor from the bottom it appears as +(I1-I2)R2 in the loop 1 equation, but I2 enters from the top so it appears as -(I1-I2)R2 in the loop 2 equation.

Personally I don't like that because it requires you to write it down and try to remember...
The way I do is if I'm in loop 1, I keep I1 positive. If I'm in loop 2, I keep I2 positive.

So, loop 1 is:

I1R1 - B1 + (I1 - I2)R2 = 0

And loop 2:
I2R3 + (I2 - I1) R2 - B2 = 0

Notice that is equal to what they wrote... since +(I2 - I1) = -(I1 - I2)

netwillie likes this.
3. ### netwillie Thread Starter New Member

Aug 10, 2010
11
3
Thank you. I have been sitting on this for hours.

Have a great day.