# Homework Help: Equivalent resistance of the circuit

1. Oct 12, 2014

### Krowek

Hello!

1. The problem statement, all variables and given/known data

2. Relevant equations
$\frac{1}{R} = \frac{1}{R1} + \frac{1}{R2}$

$R = R1 + R2$
3. The attempt at a solution

I have been trying to re-draw the schematics, but with no avail. So far I have figured out that, looking at the right side, resistors labeled 20, 40, 40 Ohms seem to be connected in parallel, so I could replace them with one resistor (10 Ohms). Then that resistor would be also in parallel with 30 Ohms, that was just a bit on the left. Is any of this correct?

Last edited by a moderator: May 7, 2017
2. Oct 12, 2014

### Staff: Mentor

Hello Krowek, Welcome to Physics Forums.

Consider that 30 Ω resistor you mentioned. Can you follow a continuous wire path from its bottom terminal to its top terminal? If so, what does that tell you about that resistor and the places where it connects?

3. Oct 12, 2014

### Staff: Mentor

Hi Krowek! http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

See that connecting wire that goes from the top of the 30 Ω resistor then all the way around to end up at the bottom of that 30 Ω ? How many ohms is that wire equivalent to, I wonder? ;) ;)

Last edited by a moderator: May 7, 2017
4. Oct 12, 2014

### Krowek

Thanks for the answers. Yes, I figured it out, there will be no resistance on that segment as the potential is the same at both ends and there will be also no current flow. Is this right? I have connected both terminals of 30 Ohms resistor and re-drawn the circuit like this:
http://i0.simplest-image-hosting.net/picture/untitled167.jpg [Broken]
Is this correct? (a,b,c,... start at top-left corner and go clockwise in the original picture.)

Last edited by a moderator: May 7, 2017
5. Oct 12, 2014

### Staff: Mentor

Whoa, that's a big redraw! EDIT: Sorry, I got disoriented!

I don't think you have simplified/clarified it, though. The resistors still don't neatly present as all parallel and series. You are going to have to use techniques of circuit analysis.

Choose nodal or mesh analysis ....

Last edited: Oct 12, 2014
6. Oct 12, 2014

### Krowek

The arrow thing is voltage source - I didn't know how to put just the wire alone - so I think it can be ignored. Originally, there were two 30 Ohms resistors. The one we were talking about was removed. The one on the left I think I cannot erase just like that?

7. Oct 12, 2014

### Staff: Mentor

An alternative you could try, just for interest, is a Y-to-Δ transformation if you can spot where you might apply it. Look it up on google to see the details.