Electric potential of point charge

[pyninja]

Homework Statement

A charged particle is fixed in place at the origin. A second particle of charge $$+10^{-6}$$ C is released from rest from very far away ($$\approx (\infty, 0)$$). The second particle passes the point (9 m, 0) with a kinetic energy of 1.0 J.

Find the electric potential due to the fixed charged particle at the point (9 m, 0).

Homework Equations

$$F = \frac{q_1q_2}{r^2} = ma$$
$$V = \frac{1}{4\pi\epsilon_0} \frac{q_1}{r}$$

The Attempt at a Solution

We need to the find the charge of the fixed particle to apply the formula for V... I tried using conservation of energy, but it seems to me that the initial potential energy and kinetic energy are both 0. Not sure what I'm doing wrong there...

 

[madah12]

ok why don't you use foruma U= Q*delta V if as you say the total energy is zero then U2 = -K2

 

[pyninja]

ok why don't you use foruma U= Q*delta V if as you say the total energy is zero then U2 = -K2

Oh, for some reason I thought the energy has to be positive, haha.

So then I get $$q_1 = -\frac{k r}{q_2}$$.

And $$V = -\frac{k^2}{q_2}$$?

(where $$k = \frac{1}{4\pi\epsilon_0}$$)

Thanks for your help!

 

[madah12]

Only kinetic energy has to be positive.