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Scaling problem with potential difference change and calculation of original length 
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#1
Jan813, 05:05 PM

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The electric potential at a distance r from a positive point charge is 45V. The potential
increases to 50 V when the distance from the charge decreases by 1.5 m. What is the value of r? A 1.3m B 1.5m C 7.9m D 15m 1. The problem statement, all variables and given/known data the answer is D 2. Relevant equations V=(1/4∏ε) q/r v proportional to 1/r 3. The attempt at a solution 5=(q/(1/4∏ε))*((1/r1.5)(1/r)) How embarrassing I just can’t get my head around this one. Am I setting the equation up correct to solve. If I have I need desperate help with the algebra. Thank you 


#2
Jan813, 05:16 PM

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Taking the ratio [itex]\displaystyle \ \ \frac{V(r)}{V(r1.5)}\ \ [/itex] might be more helpful. Otherwise, use your equation along with [itex]\displaystyle \ \ V(r)=45=\frac{q}{4\pi\epsilon_0\,r}\ \ [/itex] then eliminate q & solve for r. 


#3
Jan813, 05:21 PM

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You're going to want to get rid of the constants you don't have values for (like the charge value q) so you'll probably want to set it up as a ratio so that they'll cancel out.
EDIT: SammyS got their first! 


#4
Jan813, 05:36 PM

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Scaling problem with potential difference change and calculation of original length
Thank you guys.
I did it like this with your help. Is this mathematicaly sound. Thanks Mr C 


#5
Jan813, 10:40 PM

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