Mastering Coulomb's Law: Troubleshooting Sign and Distance Issues

[luap12]

1. Homework Statement [http://img44.imageshack.us/img44/1104/58339946.jpg ][/URL]



2. Homework Equations [F=k*abs(q1)*abs(q2)/r^2]



3. I understand the equation, but I am having trouble with my signs and also how to include the distance in the equation. I know that the force can't be zero between the charges, and I think it has to be farthest away from the larger positive charge to make them equal to zero. The forces point in the same direction because they are opposite.]

 

[rl.bhat]

Consider a point at a distance x from -q toward right, and find the forces on a unit positive charge due to two charges. Equate them to get the value of x.

 

[luap12]

I don't understand what you mean by that... I think I'm over-thinking this.

 

[rl.bhat]

Force on unit positive charge due to -q at a distance x*1o^-2 m from it is...?
Force on unit positive charge due to 2q at a distance (10 + x)*10^-2 m is...?
Since net force is zero, equate them to find x.

 

[luap12]

Thanks very much, I got it now.

((k*q)/x^2)-((k*2q)/(x+10)^2)=0, solving for x gives 24.142cm. Accounting for the distance of 5cm on the x-axis give the point to be 29.142cm. This was the correct answer. Thanks!