Find the Zero Potential Point on the X-Axis for a System with Two Point Charges

[ziplock2k]

Homework Statement

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.

Homework Equations

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?

 

[alphysicist]

Hi ziplock2k,

Homework Statement

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.

Homework Equations

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

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?

 

[ziplock2k]

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

 

[alphysicist]

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

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?