(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

A resistor with resistanceRis connected to a battery that has emf 15.0[V] and internal resistance r = 0.38[Ω].

For what two values of R will the power dissipated in the resistor be 78.0 W?

2. Relevant equations

P=I^{2}R=(ε^{2}/(R+r)^{2})R

3. The attempt at a solution

ε^{2}R=(R^{2}+2Rr+r^{2})P

R^{2}+(2r-(ε^{2}/P))R+r^{2}=0

R=1/2[((ε^{2}/P)-2r)±√(((ε^{2}/P)-2r)^{2})-4r^{2}

At this point I should be able to plug in what I know, and I get roughly

R=1.06±0.77

However that is not right, I have tried again making sure I kept my sig figs in order and all that good stuff, I get a couple similar but different answers, still all wrong though.

I have only one attempt left and it's worth 27% of the grade on this HW so any help would be appreciated greatly.

This is an image of my attempts, the problem, and how the solutions manual did the quadratic.

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# Homework Help: Finding Resistance from internal resistance and Power in a circuit

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