# Bridge circuits - just need help with this notation

#### galway

Joined Jul 9, 2007
4
I'm studying for my PE (licensing) exam, and I need help with this problem. I don't need help solving it (at least not yet), just with the way it's drawn. I've never seen a circuit with arrows coming out of the nodes like this. I'm guessing that they're supposed to be currents entering or exiting the nodes, but I was hoping someone here would know for sure. I'm supposed to be solving for the current across R5, if that helps any.

Thanks for any help! #### JoeJester

Joined Apr 26, 2005
4,390
I couldn't access your pic. If it's small enough, just upload it on the forums.

#### galway

Joined Jul 9, 2007
4
Sorry about the pic, I've attached a slightly modified version.

So taking the sources and sinks, I'm supposed to use Kirchoff's Law to solve for the current across the load. Note that the bridge isn't balanced.

@A: 100 - I1 + I4 + I5 = 0
@B: -20 + I1 - I2 = 0
@C: -50 + I2 - I3 - I4 = 0
@D: ID + I3 - I5 = 0

If I assume that all of the sinks are going to the same place, then ID would equal 30A, but that still leaves me with 5 unknowns and only 4 equations. What am I missing?

#### galway

Joined Jul 9, 2007
4
Anyone? #### recca02

Joined Apr 2, 2007
1,212
see if this gives u independent equations:
express currents in terms of voltages at nodes.
u might get equations something like (R1I1+R2I2)/R5 = I5;
see if this helps.

1 more thing Id shud be 30A since no accumulation of charge takes place in the bridge.

#### techroomt

Joined May 19, 2004
198
galway - interesting problem. don't think i can help you other then i think
equation c should read: I2 - 50 - I3 - I5 = 0
equation d should read : ID + I4 - I3 = 0

how do you start variable elimination? multiple loop equations?

#### galway

Joined Jul 9, 2007
4
Thanks for the suggestions, I'll try them this weekend.

#### recca02

Joined Apr 2, 2007
1,212
post if u do get/(or not get) answer along wit doubts.
as in my earlier post u wud get your 5th equation from the idea that voltage at two points is same for different branches.

#### hgmjr

Joined Jan 28, 2005
9,027
Anyone? The condition for a balanced bridge is for I5 = 0.

How about setting I5 = 0, then you will have four equations and four unknowns.

I tried this approach and was able to arrive at ID = 30 amps.

I guess this is the same thing as saying that the fifth equation in the mix is I5 = 0. This equality is dictated by the goal of balancing the bridge circuit.

hgmjr