# Homework Help: Anybody Know The Currents And Equations For This Problem?

1. Feb 9, 2005

### skivail

Anybody Know The Currents And Equations For This Problem???

Please if you can, help me!! I am usually very good at circuit problems but this one has me baffled! here's the link: http://www.coloradoacademy.org/~phogan/apphys/quiz/rq2_8b.htm [Broken]

Thanks A Lot!

Last edited by a moderator: May 1, 2017
2. Feb 9, 2005

### learningphysics

Are you familiar with mesh current analysis?

Last edited by a moderator: May 1, 2017
3. Feb 9, 2005

Say What??

4. Feb 9, 2005

### skivail

sorry...that was rude of me. I am in 11th grade trying to redo this quiz for a final trimester test review, and i can't remember at all how it was done. If i knew mesh current analysis, i certainly have forgotten it. Is there anyway to do the prob without it? if not, can you explain this "mesh current" thing to me?

Thank you

5. Feb 9, 2005

### learningphysics

That's ok. I was just thinking of how to solve the problem without the mesh method (it's something you study in college electrical engineering to analyze circuits). It seems like a tough problem for high school.

You have V3... so getting the the current through R4,R5 and R6 is easy. I can't see the others.

There's probably some "trick" here. Otherwise it's just a huge problem.

You're probably not supposed to use mesh analysis if you're in high school.

If you're interested have a look at this site:
http://www.et.cpcc.cc.nc.us/Online/elc133/mesh.html [Broken]

I'll look at the problem some more and see if there's a trick.

Last edited by a moderator: May 1, 2017
6. Feb 9, 2005

### skivail

Thank you very much!

7. Feb 9, 2005

### learningphysics

You can certainly solve the problem "brute force"... you can do KVL around the different loops... label each resistor with a current... then get around 12 equations with 12 unknowns... but that seems like the "wrong" way to go about the problem.

It seems to me like mesh analysis is the way to solve this. After getting the current through R4, R5 and R6, you can get 5 equations in 5 unknowns using mesh. It is still a large problem for a high school quiz... Still have the feeling I'm not seeing something.

Have a look at that site I gave you a link to.

Well, here's how I'd do it using mesh analysis:
Draw a loop clockwise inside each of the 6 squares. Each loop represents a current. You already know the current in the square in the top row to the right(with R4,R5,R6). Label each current: I1 (going through R1),I2 (through R2),I3(going through R3),I4(going through R12),I5(going through R13).

Then using KVL... for loop 1:

-(I1)(R11) -V1 - (I1)(R1) + V2 - (I1-I4)(R10) + V7 =0

Loop 2:

-V2-(I2)(R2)-(I2-I3)R8 - (I2-I4)R9=0

Loop 3:

-(I3-I2)R8 - I3(R3) - V3 - (I3-I5)R7=0

Loop 4:

-V7 - (I4-I1)(R10) - (I4-I2)R9 + V5 - (I4)(R12) + V4 = 0

Loop 5:
-V5 + (I5-I3)(R7) - V6 - (I5)(R13) = 0

So you've got 5 equations with 5 unknows. :yuck:

Once you solve for I1,I2,I3,I4,I5... you can get the current anywhere in the circuit. The current through R8 for example is I2-I3 downward. If I2-I3 is negative then the current is I3-I2 (positive value) upward.

Not easy.

Sorry I couldn't help more. Hope someone else sees something I missed.