Please help electric field and electric potential

[plano1239]

Homework Statement

One positive (+2q) and two negative (-q) charges are arranged as displayed in fgure 1.
Calculate electric feld E and electric potential (a) at points along the positive y¡axis as
functions of their coordinate y. What is the direction of E at those points? In your results,
does y-component Ey of the field satisfy Ey = -dx/dy? Is it supposed to satisfy?

x
-q.......2q.......q)

Homework Equations

E = ke q/r^2

The Attempt at a Solution

i don't know how to start this problem.

 

[malawi_glenn]

I do not understand your picture..

 

[plano1239]

i added the picture

 

[malawi_glenn]

Then the electric field is force/ unit charge.

So you should add all forces that comes from the 3three charges; note their signs! ; as a function of y.

What is the distance from point y to x ? (i.e the r in the formula), hint: triangle..

 

[plano1239]

i will try that thank you.

 

[plano1239]

so for the first part, i got

E = -4ke/a^2
is that all or can i solve this further?

 

[malawi_glenn]

where did the 4 come from? And it should be a function of y, and i forgot: Use also vectors. i.e for a certain point on the y-axis, find the x and y composant of the force for each charge, then add x and y-wise; so you get the direction too.