# (easy) Mesh analysis problem on mit ocw

#### inkyvoyd

Joined Dec 6, 2011
25
Hi, I am trying to learn as much as I can about DC circuits for an upcoming Science Olympiad competition about circuits - I found out about node analysis, but I also started learning about mesh analysis. I did a quick google search and came up with some nice practice problems, and did the first few ones, but had a little trouble with a specific problem.

http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-071j-introduction-to-electronics-signals-and-measurement-spring-2006/lecture-notes/nodal_mesh_methd.pdf

I'm curious about the setup of the equations of the first problem on page 17.
my equations are
6i_a+4(i_a-i_b)=12
2(i_b-i_c)+4(i_b-i_a)=6
2(i_c-i_b)+2i_c=2
which simplifies down to

10i_a-4i_b=12
-2i_a+6i_b-2i_c=6
-2i_b+4i_c=2
The only problem is that this gives incorrect answers.
(i_a=44/21, i_b=47/21, and i_c=34/21)
Note - i_1=i_a,i_3=i_b,i_5=i_c

#### The Electrician

Joined Oct 9, 2007
2,751
You've got a sign error in your second equation. It should be:

2(i_b-i_c)+4(i_b-i_a)=-6

Also, the second equation in the second group of equations should be:

-4i_a+6i_b-2i_c=-6

• inkyvoyd

#### inkyvoyd

Joined Dec 6, 2011
25
You've got a sign error in your second equation. It should be:

2(i_b-i_c)+4(i_b-i_a)=-6

Also, the second equation in the second group of equations should be:

-4i_a+6i_b-2i_c=-6
That. Is so helpful - thank you very much 