# Homework Help: Electric field and charges

1. Feb 25, 2007

### jwl322

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

Two point charges are on the y-axis. One, with charge a of +q, is located at point a. The other, with a charge of +2q is located at point -a. Determine the magnitude and direction of the electric field at the origin. Express answers in terms of q,a, and constants.

2. Relevant equations

E=kq/r^2

3. The attempt at a solution

Do i just do k(2q)/a^2 - k(q)/a^2 and thats the answer?

K(2q)/a^2 - k(q)/a^2= kq/a^2

this in preparation of the ap exam and i learned this last semester so i just need a little refreshing

2. Feb 25, 2007

### turdferguson

Yes, and the direction is the direction a positive test charge would go

3. Feb 25, 2007

### ranger

Yup, thats how I would do it.

Since both are a distance "a" from the origin and we only care about what happens at the origin, you equations makes sense to me.

EDIT: oops, turdferguson beat me to it.

4. Feb 25, 2007

### jwl322

the direction is in the -y direction right?

also, if i were to find the electric potential at the origin i would use E=V/d? if i use that, im not sure what i would use for d

thanks

5. Feb 25, 2007

### turdferguson

Think about the direction again, all charges are positive

d is just the distance a. A good thing to remember for the AP is that fields are vectors and potentials are scalars. Voltage is also the scalar sum of each kq/r

6. Feb 25, 2007

### jwl322

ok so its in the positive y direction because the the electric field points outward from positive charges and since the 2q charge is greater the field at the origin will point in the positive y direction