# What am i doing wrong or right

1. Jul 20, 2008

### ziplock2k

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

A point charge of +2.7 mC is located at the origin of a coordinate system and a second point charge of -7.8 mC is at x = 1.5 m. At what point on the x axis is the electrical potential zero? Calculate to 2 decimal places.

2. Relevant equations

3. The attempt at a solution

net pot. diff will be zero at
x=1/3.
and at x = -1.

pot. diff is a scalar qty. so u can directly add it.

if the net potential diff at a pt (say at x,0) is zero,
we have
(k*3/x) - (k*6/(1-x)) = 0
2x=1-x
x=1/3

Similarly for negative x, u can make n eqn:
-(k*3/x) - (-k*6/(1+x)) = 0
which shal give u: x=-1

WHAT AM I DOING WRONG?

2. Jul 20, 2008

### alphysicist

Hi ziplock2k,

I don't understand what you are doing here. In particular, what are the 3 and the 6 in the numerators? And why is the denominator of the second term 1-x?

3. Jul 20, 2008

### ziplock2k

I'm sorta lost, as you can tell, and I need help.

4. Jul 20, 2008

### alphysicist

You may have done a problem similar to this before when you studied electric fields. This one is done much the same way.

The first step is, what is the formula for the electric potential of a point charge? (What confused me about your post was that you seemed to have the right form, but I did not understand where the numbers came from.)

So what is that formula, and what do they variable represent?