# Homework Help: Net Electric Field Problem

1. Feb 1, 2010

### nw0rbrolyat

1. The problem statement, all variables and given/known data
four particles are fixed in place and have charges q1 = q2 = 3e, q3 = 7e, and q4 = -28e. Distance d = 6.7 µm. What is the magnitude of the net electric field at point P due to the particles?

2. Relevant equations
Im not sure what form of the magnitude equation to use for this.

Im new to this forum and am always needing Physics help. Thanks to you all!
Taylor

2. Feb 1, 2010

### fluidistic

Hi and welcome to PF!
Do you know what is the electric field of a single charge?
What you have to do in the problem is to find the electric field at point P due to q1. Then do the same for q2, q3 and q4. At last, sum them all. It will give you the net electric field (which is a vector). Once you have the vector, finding the magnitude shouldn't be hard.

3. Feb 1, 2010

### nw0rbrolyat

for q1 the equation I would use is E1=(8.99e9)(3e/(.0067m)^2).
Do I have this setup correctly? Im not quite sure what to do with the e's will an e remain in the answer?

4. Feb 1, 2010

### fluidistic

I think by e they mean the electric charge of an electron. Isn't $$\mu=10^{-6}m$$?
But don't forget that the electric field is a vector and you should always know where it points out.
I don't find the exercise clear. For example, where is P? How are the charges situated?
d is the distance from what to what?

5. Feb 1, 2010

### nw0rbrolyat

http://edugen.wiley.com/edugen/courses/crs1650/art/qb/qu/c22/fig22_31.gif
that is the figure they give with the problem. You can see what d is with this. You are probably correct about the meters its been a while since I have worked with SI prefixes.

6. Feb 4, 2010

### fluidistic

Sorry for being a bit late. Have you solved the problem? If not, you can set up the x-axis as being the one where q1 and q2 lie and the y-axis the one where q3 and q4 lie.
In post #3 you came up with the good formula, but remember that the electric field is a vector quantity.
So using the formula, the q1 and q2 charges contribute to the E-field at point P only along the x-axis, while q3 and q4 contribute to the E-field at point P only along the y-axis.
Can you take it from there?