# Homework Help: Calculate the ratio q1/q2 of the two charges

1. Apr 10, 2007

### simplicity12

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

2. Relevant equations
V= kq/r

3. 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?

2. Apr 10, 2007

### 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.

$$\frac{KQ_1}{r_1}=-\frac{KQ_2}{r_2}$$

Does this help?

3. Apr 10, 2007

### 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?

4. Apr 10, 2007

### G01

Yes,
$$V_A = V_1+V_2$$

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

$$V_A = V_1+V_2=0$$

$$V_1=-V_2$$

5. Apr 10, 2007

### simplicity12

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

6. Apr 10, 2007

No Problem!