Determin the magnitude of the net electric field of three point charges

[Ruth_1392]

Homework Statement

There are three point charges, q₁ has a magnitude of 3.00mC (the sign is not known) and is 5.0cm away from P and 13.0 cm away from q₂. P is 12.0cm away from q₂ and the three points form a triangle. The net electric field at point P is entirely in the negative y-direction. The charge for q₂ is not known.

Determine the magnitude of the net electric field.

Homework Equations

E=Q/4∏ε₀r²

The Attempt at a Solution

I think you have to use the equation above and then add the 3 values together, but I don't know how to calculate E when I'm only given the value for q₁ and none of the other points.

 

[DukeLuke]

If you know the net electric field at p is only in one direction, the sum of the contributions in all other directions at point p must be zero. Try writing the total E-field in different directions at point p (sum the contributions from each charge) and set them equal to zero for all the cases but the y-dir. My guess is you will have a solvable system of equations from which you can get the charge values and the y-dir field at p.

 

[Ruth_1392]

For part a) I had to: Sketch the correct force configuration and work out the signs for the
charges q1 and q2. Do I use my answer for a) in this part?
Thanks

 

[netgypsy]

is the third charge at P?? Nothing is mentioned about the third charge? What three points form a triangle? If the problem is referring to point P, charge1 and charge 2 all this says is that they are not along a line. Where is charge 3?

 

[Ruth_1392]

The question had a picture and P, q1 and q2 formed a triangle. Does the question only use q1 and q2? I'm not too sure what P is.

 

[netgypsy]

The problem says three charges but only talks about two charges and point P. Electric field is defined as a force per unit charge but the problem isn't clear to me. Is there a unit charge at point P that we're supposed to assume? See if something is missing.