# Electric potential and magnitude

1. Feb 15, 2016

### blee22

1. The problem statement, all variables and given/known data
You start at a distance r from a point charge. The electric potential is 4500V at that point, and the electric field is 2.3x105N/C. a) What is r? b) What is the electric potential and magnitude of the electric field at a distance of r/4?

2. Relevant equations
v= kq/r E=kq/r^2 substitute everything and get q= v^2/kE?

r=kq/v

solve for q which will then allow you to solve for r?

3. The attempt at a solution

V = 4500, k = 8.9x10^9 and E = 2.3x10^5 that will give us a q

This q will then be put into the equation r=kq/v and that will give you the answer for a?

Part b would you take that distance solve for the magnitude and e-field?

2. Feb 15, 2016

### TSny

Welcome to PF! Your outline of a solution should work. However, you don't actually need to find q first. Consider the ratio V/E.

3. Feb 15, 2016

### Isaac0427

He will, however, eventually solve for q, but I would suggest to solve it after you find r. And yes, you would just plug in the values you get into the equations to do part b (but use r/4 instead of r).

4. Feb 15, 2016

### TSny

If you think about how V and E depend on r, you can answer part (b) without knowing the charge. Of course there is nothing wrong with finding q if you want.

5. Feb 15, 2016

### Isaac0427

Ah, I see what you are talking about.

OP: using this strategy will take away most of the calculations you think you need to do.