Homework Help: Determin the magnitude of the net electric field of three point charges

1. Dec 6, 2011

Ruth_1392

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

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.

2. Relevant equations
E=Q/4∏ε0r2

3. 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.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Dec 6, 2011

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.

3. Dec 7, 2011

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

4. Dec 7, 2011

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?

5. Dec 7, 2011

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.

6. Dec 7, 2011

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.