Calculate the ratio q1/q2 of the two charges

[simplicity12]

Homework Statement

Charge q1 is 0.16 m from point A. Charge q2 is 0.40 m from point A. The electric potential at A is zero. Calculate the ratio q1/q2 of the two charges.

Homework Equations

V= kq/r

The Attempt at a Solution

I did
kq1/r1 = kq2/r2
q1/q2 = r1/r2
q1/q2 = 0.16/0.40
q1/q2 = 0.40
My magnitude is right, but the answer is a negative, -0.40. What am i doing wrong?

 

[G01]

You need the potentials to cancel out at A. Since potential is a scalar, you want the potential from one to be positive at A, ad the potential from the other at A to be the negative of the first at A, so they will add to zero. So, assume one charge is negative and put the negative sign in there. That is why your sign is off.

Your initial equation should read:

$$\frac{KQ_1}{r_1}=-\frac{KQ_2}{r_2}$$Does this help?

 

[simplicity12]

So does that mean that my initial equation is:
Vt=V1 + V2? So that when i make the Vt=0, the equation will read V1=-V2?

 

[G01]

Yes,
$$V_A = V_1+V_2$$

But we want the potential at A equal to zero so:

$$V_A = V_1+V_2=0$$

$$V_1=-V_2$$

 

[simplicity12]

that helps a lot! thank you very much! :D

 

[G01]

No Problem!