Homework Help: Equivalent Resistance in series-parallel circuit

1. Feb 13, 2015

JustMerc

1. The problem statement, all variables and given/known data
This is a sample problem. I'm trying to figure it out before I do the actual homework. The issue is, the way to figure it out is not included in our notes. I'm absolutely lost (math isn't my strong point) I've also included my attempt at how I think it should be done below, which got my really close. We're rounding to the nearest whole number, but I'm still off by 2. If it's hard to see: R1 = 2900 ohms, R3 = 800 ohms, Req = 3490 ohms. Find R2 in ohms.

2. Relevant equations
The only equations given to us for equivalent resistance in series-parallel is GT = G1 + G2 and Rx = 1/GT which is really not helping to figure this out.

3. The attempt at a solution
Here's my attempt. I subtracted R1 from RT (590 or .001694915) and divided by 1. Then subtracted R3 (.00125) from that (.000444915). Divided by 1 and got within 2 of the answer after rounding (2248). I'm so lost on this.

2. Feb 13, 2015

axmls

I got the same thing as you. If you're given that the answer is 2250, it's likely that it simply has to do with the significant figures. Either way, I'd say you're correct.

3. Feb 13, 2015

JustMerc

So then I'm actually doing it correctly? The problem with that is, the homework is online. There's only one answer which works. If you're close...it's still wrong and there's no partial credit, because we don't show our work. Regardless to that fact, I appreciate you looking at it and leading me to believe I'm on the right path.

4. Feb 13, 2015

Staff: Mentor

5. Feb 13, 2015

Staff: Mentor

BTW, a better, more visual way to do this problem is like this:

$$\frac{1}{\frac{1}{x}+\frac{1}{800}} = 590$$

Then put the two terms in the denominator over a common denominator, and solve for x. :-)

6. Feb 13, 2015

JustMerc

Nice, thank you berkeman! Even though I couldn't get the answer (2250) on the sample question, I attempted the homework and thus far it is working. I appreciate the help from you all. It's not fun going into this stuff blindly.

7. Feb 13, 2015

vela

Staff Emeritus
When you turn in written work, do yourself a favor and don't write stuff like R3 = 800 = 0.00125 because because 800 doesn't equal 0.00125 and R3 isn't 0.00125 ohms. Chances are whoever is grading your work pays attention to mathematical detail and may find what you wrote irritating, and it's best not to irritate the grader.