# Electric fields two point charges

1. Jan 30, 2008

### mussgo

i just need to get this question over with
i tried and tried but i couldnt get the right answer plss anyone u.u

1. The problem statement, all variables and given/known data

point charges q1 = -6q and q2 = +3q are separated by distance d. Locate the point (measured from the origin at q1) at which the electric field due to the two charges is zero.
<img src=http://www.webassign.net/hrw/23_30.gif> [Broken]

2. Relevant equations
E=kQ/x^2

3. The attempt at a solution

well i did 0= E1+E2

i got this

0 = 3kQ/x^2 - 6kQ/(x+d)^2

6kQ/(x+d)^2 = 3kQ/x^2

cross multiplied

2x^2 = x^2 + 2dx + d^2

x^2 - 2dx - d^2 = 0

and then i did quadratic formula and got d-+d(sqrt2)
like 2.414d and .414 but as i put them in the webassign i got em wrong
can anyone help or tell what i did wrong or any feedback would be apreciated u.u

Last edited by a moderator: May 3, 2017
2. Jan 30, 2008

### Ronnin

There is only one point where the charges cancel each other out. It is one of the two you have calculated for, but only one has the E field vectors pointing opposite directions, which give a net charge of zero.

3. Jan 30, 2008

### mussgo

but something must be wrong because like i said earlier
the webassign says its wrong :S u___U

4. Jan 30, 2008

### Shooting Star

> point charges q1 = -6q and q2 = +3q are separated by distance d. Locate the point (measured from the origin at q1) at which the electric field due to the two charges is zero.

> got this

0 = 3kQ/x^2 - 6kQ/(x+d)^2

You origin is given at q1 = 6q, but you are measuring x from the other charge.

5. Jan 30, 2008

### mussgo

true :O
im gonna try it
and see what comes out :O