A rectangle with Charges on Each Corner - Find Missing Charge?

[NerdingOut]

Homework Statement

A charge is to be placed at the empty corner of a rectangle to make the net force at corner A point along the vertical direction. What charge (magnitude and algebraic sign) must be placed at the empty corner if the three charges have the same charge of +8.45 μC?

Homework Equations

F = kq1q1/r^2

The Attempt at a Solution

because we know that the net force on A is going to be in the vertical direction we know this is the resultant. Therefore i know the horizontal forces must cancel out..but isn't there only one existing horizontal force coming from the charge at the bottom right corner?

F = (8.99x10^9)(8.45 x10^-6)^2/d^2 [/B]

 

[BvU]

Hi nerd, and welcome to PF :-)

Show us your attempt and only then we are allowed to help. Why ? See the guidelines

 

[NerdingOut]

Okay I added my attempt..but i seriously do not know where to go from this :(

 

[haruspex]

Okay I added my attempt..but i seriously do not know where to go from this :(

You don't have to worry about forces as such, just think about the fields.
Suppose you put a charge q at the fourth corner. What horizontal field will this give at A?

 

[BvU]

I don't see an attempt, and it certainly doesn't look like your own work. Tell me about the forces at A due to the other two yellow bulbs. Magnitude, direction ?