Determine the current in this circuit

[tensor0910]

Homework Statement

Find the current Ig[/B]

Homework Equations

[/B]
V1 = R1/(R1 + R2) * Vs

1/Rtotal = 1/R1 + 1/R2 + 1/R3...example

The Attempt at a Solution

I know what I did wrong, but I don't know how to avoid it. Here's what I mean.

How do you know which way its suppose to be?

 

[tensor0910]

I added onto the old picture. Hopefully this clears things up.

 

[phyzguy]

In your "How do I know it's this...and not this" pictures, look at the resistor connections and they are obviously not the same. In the original circuit, the 5 Ohm and the 20 Ohm resistors are in parallel. One end of them is connected to the 6 Ohm resistor and the other end is connected to both the 13 Ohm resistor and the negative of the power supply. This is exactly the same connections as in your "How do i know it's this" picture. But in your "and not this" picture, the ends that are connected to the 6 Ohm resistor are now connected to the negative of the power supply. It's just not the same connections.

 

[scottdave]

Look up about how to determine what a Node is. Wikipedia has some pretty good info on this. I'd put more now, but I'm on my phone.

 

[CWatters]

In original and in the left hand diagram the 13R has one end connected to the -ve side of the voltage source. In the right hand diagram is doesn't. So the right hand ("and not this") circuit is different to the original.