# Electric Field homework problem

Hello all. I am new to this forum and I'm glad I found this place! Anywho, here's the problem I'm stuck on...

Three equal charges 4.4 micro coulombs are located in the xy-plane, one at (0m, 63m), another at (52m, 0m), and th third at (49m, -53m). Find the magnitude of the electric field at the origin due to these three charges.

Here's how I tried to solve it:

d1 = 63m; d2 = 52m; d3 = 72.18m (derived using the distance formula)

E = kq/d^2
Enet = E1+E2+E3 = kq (1/d1^2+1/d2^2+1/d3^2)

But when I plug the numbers in, I end up with ~32 N/C as my answer but the correct answer is 20.25 N/C. What am I doing wrong? Any help would be greatly appreciated.

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Gokul43201
Staff Emeritus
Gold Member
1. Welcome to PF. Notice that we have subforums dedicated to Homework Help. Please use one of those from the next thread on.

2. The electric field is a vector quantity (with a magnitude and a direction in the XY plane), not a scalar. Do you knoow how to add vectors using cartesian (or rectangular) components?

I apologize for posting in the wrong sub-forum..thank you for moving it here. As for the problem, I thought about splitting them into components...field 1 does not have an x component and field 2 does not have a y component. For the third field, it does have x and y compoenets, but how do I find the angle needed for the calculation?

So basically, is it:

Ex = kq2/r2^2 + kq3/r3x^2 cos (?)
Ey = kq1/r1^2 + kq3/r3y^2 cos (?)

sqrt(Ex^2 + Ey^2) = E

where ? = mystery angle

I already tried using this...but didnt know what angle to use and the answer I got was close but not exactly to the answer given in the sample problem. Is this correct? If so, any hints on how I can find the mystery angle?

Hootenanny
Staff Emeritus