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## Scaling problem with potential difference change and calculation of original length

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

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/r-1.5)-(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
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Mentor
 Quote by mrcotton 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/r-1.5)-(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
You have one equation and two unknowns.

Taking the ratio $\displaystyle \ \ \frac{V(r)}{V(r-1.5)}\ \$ might be more helpful.

Otherwise, use your equation along with $\displaystyle \ \ V(r)=45=\frac{q}{4\pi\epsilon_0\,r}\ \$ then eliminate q & solve for r.
 Recognitions: Homework Help 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!

Recognitions:
Gold Member

## 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

Mentor
 Quote by mrcotton Thank you guys. I did it like this with your help. Is this mathematically sound. Thanks Mr C