# Confused about Equiv. Resistance for this Circuit

• Engineering

## Homework Statement

(this wasn't the original problem, but a part of it, but I got this part wrong)
Find the R equivalent of the circuit

http://img508.imageshack.us/img508/2191/circuitam6.jpg [Broken]

## Homework Equations

(parallel) 1/Req = 1/R1 + 1/R2 + 1/R3 + ...
(series) Req = R1 + R2 + R3 + ...

## The Attempt at a Solution

My teacher gave back the paper, and I got it wrong, but I still don't understand why mine is wrong and his is the right way to do it.

1. I took the 10-ohm and 50-ohm in parallel, and got 8.33-ohm as equivalent.
2. (this is where the mistake occurred) I took the 20 ohm and 40ohm in series, so it'll be 60-ohm equivalent.
3. Then I took the 60-ohm and 8.33-ohm and put them in parallel, but that's wrong, but I don't understand why that's wrong. How would I know that I would have to put the 20ohm and 8.33 ohm in series, then parallel with the 40ohm INSTEAD of doing the way I did it?

Please help, thanks

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## Answers and Replies

Janus
Staff Emeritus
Science Advisor
Gold Member
It seems from the diagram you gave that your first mistake was in step 1. Why did you take the 10 and 50 ohm resisters in parallel?

Maybe I should write out the whole problem, I was trying to find the R-thevenin. So I took out the voltage source.

The problem was to find the V-thevenin with terminals at the 50-ohm resistor

in finding v thevanin, break the circuit at the point at which you want the voltage. in this case you would want to remove the 50 ohm resistor and find the voltage across that break.

also in finding r-thevanin, you would remove the 50 ohm resistor, turn off the voltage source, and combine the resistors using the revalent equations you posted. then put them into the circuit that includes the v-thev and r-thev.

The 20 ohms and the 40 ohms resistances share the same current, therefore you should combine them in series. Then go from there, shouldn't be hard.

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R-eqv = 10+ (50||(20+40)) ohm
very simple.
ans: 37.27 ohm (check it)

now if u want to find out the R-thv at 50 ohm resistor,
step-1: short the volt source(88 v)
step-2: uproot the 50 ohm resistor from the ckt.
step-3: then look throgh the ckt from the uprooted resistor, can u feel that (20 n 40 ohm in series) which is parallel to 10 ohm?
so R-thv= 10 || (20+40) =8.57 ohm .. ans

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