# Find Resistance Between A and B

1. Jan 13, 2010

### mmmboh

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

I have to find the input resistance; the resistance between A and B. I think it's Thevinin's theroem, however it is already an open circuit and there are no voltage sources and I can't combine the resistors to make an equivalent. So what do I do

Edit: Am I suppose to remove the 4 ohm resistor connecting the two parallels, and find the equivalent resistance of those two and then add back the 4 as if it were in series?....so the final answer is 16/3 ohm? or is that wrong?

Last edited: Jan 13, 2010
2. Jan 13, 2010

### JaWiB

Have you heard of a Y-Delta transform?

3. Jan 13, 2010

### mmmboh

No I don't believe so.

4. Jan 13, 2010

### vela

Staff Emeritus
Place a voltage source across A-B and calculate the current that it would supply. The equivalent resistance will be the voltage divided by the current.

5. Jan 14, 2010

### mathman44

Ok, I tried it like you suggested vela. Made 3 Kirchhoff loops, put in an imaginary voltage source 10V, calculated the current to be 3.5A, so R should be 10/3.5...

6. Jan 14, 2010

### vela

Staff Emeritus
That's the same answer I got.

7. Jan 14, 2010

### mathman44

Sweet, thanks!

8. Jan 15, 2010

### mmmboh

I tried doing the loop thing, I don't think I did it right. Can you explain it please?

9. Jan 15, 2010

### ideasrule

What loop thing? You won't get the answer with Kirchoff's laws alone. Instead, use Thevinin's theorem, pretending that there's a voltage V between A and B.

10. Jan 15, 2010

### vela

Staff Emeritus
It would be better if you explain what you tried, so we can identify where you're having trouble. It's kind of hard to boil all of circuit analysis down to a forum post.

11. Jan 16, 2010

### mmmboh

Never mind I think I got it. I1=7A/2, R=10V/3.5A.