Mesh Current - Difficult #2

Thread Starter

Godsninja

Joined Apr 30, 2016
16
I have 1 more problem for the mesh current method which I'm a little hesitant to ask for help for as it's my third post for circuit analysis problems, and I don't really have any work to show. I did try it, but I'd rather show a clean picture without equations that don't lead anywhere.

What I first did was the easy equations for loop ib and ic, but I know I won't get anywhere if I don't somehow relate the 10 mA source or loop ia in with them. The constraint. I don't want the answer given to me, if anyone knows, don't say, just hint, I'd really like the satisfaction of seeing the constraint.

10 mA flows in through R1 & R2, down towards the bottom. Whatever passes through R1 and R2 - 10 mA = 0. But how can I express "whatever passes through R1 and R2"? I was thinking about labeling the nodes, and doing (Va is top) Va/R1 + Va/R2 = 10 mA.... and then Va/R2 = ib....so
Va/R1 + ib = 10 mA......ib = 10 mA - Va/R1. But that's completely wrong cuz it's not Va, it's Va - the voltage in the middle.

I know it's a simple KVL equation, which I currently can't see. Anything I try just ends up saying ia = 10 mA, or something flat like that, which doesn't help. I want to say something like, 2ia = ib + ic...

20170714_210845.jpg
 

WBahn

Joined Mar 31, 2012
25,745
Your two loop equations include all three currents, correct?

What does the current ia absolutely have to be?

That leaves you with two equations and two unknowns.
 

Thread Starter

Godsninja

Joined Apr 30, 2016
16
Yes, I was completely overthinking this problem. I was thinking about it last night, I simply forgot to include -ia in both the loops for R1 & R4!

This thread can be deleted. I need to pay more attention...
 

WBahn

Joined Mar 31, 2012
25,745
We don't delete threads unless they violate the ToS in such a way as to require it.

Others have and will make the same mistake you did, so coming across this thread may help them out.

This may not have helped in this case, but it strengthens the argument for writing out the loop and constraint equations simply and by themselves and then checking them carefully to see if you agree with them before proceeding. Check each term of each equation. Check the mesh and make sure that all of the elements (and only those elements) are represented that should be. Make sure that all of the currents are properly represented with the proper sign. It should take you at least twice as long to verify the equations as it took to write them down originally, but the time is time very well spent. Mistakes here are EE mistakes that can be seldom be caught by later math manipulations.
 
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