Equivalent Resistance in series-parallel circuit

[JustMerc]

Homework Statement

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, R_eq = 3490 ohms. Find R₂ in ohms.

Homework Equations

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

The Attempt at a Solution

Here's my attempt. I subtracted R₁ from R_T (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.

 

[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.

 

[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.

 

[berkeman]

Agreed. I got 2247.6, which rounds to your answer.

 

[berkeman]

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. :-)

 

[JustMerc]

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. :)

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.

 

[vela]

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.