# Homework Help: Net electric field

1. Feb 2, 2010

### fendercaster

1. The problem statement, all variables and given/known data
A 3.3 μC point charge is on the x-axis at
x = 4.9 m, and a 3.9 μC point charge is on
the x-axis at x = 2.6 m.
The Coulomb constant is 8.98755 ×
109 Nm2/C2.
Determine the magnitude of the net electric
field at the point on the y-axis where y =
2.5 m.

2. Relevant equations
The equation I'm using is kQ/r^2
k being Coulomb's constant
Q being my charge
r being the distance

3. The attempt at a solution
I'm not really sure how to approach this problem.

My first attempt was finding the electric field of each. One of the problems I had was that I did not know what distance to use so I just found the mid point of my two charges (3.75m) and used the pythagorean theorem ( sqrt((2.5^2)+(3.75^2)) ) to find my resultant, 4.5m. I'm positive this is where I went wrong.

9e9(3.9e-6)/4.5^2 = 1467.7 N/C
9e9(3.3e-6/4.5^2 = 1733.3 N/C

Found my x's and y's using 33.7 as my angle.

Subtracted the smaller number from the bigger to get my nets and used pythagorean theorem to get the resultant which ended up being 266.9 N/C which was not right.

2. Feb 2, 2010

### rl.bhat

Hi fendercaster, welcome to PF.

Find the distances of the individual charges from the point on the y-axis. At the same time find the angle subtended by these lines with x-axis using tanθ = y/x.
Then find the electric field due to each charge at the required point. resolve the fields into x and y components. Find ΣEx and ΣEy. Then find the resultant field E.