Calculating the magnitude of the electric field homework

[jc269]

Homework Statement

Consider three charges arranged as shown:

Q1---------Q2----------Q3

Q1 has a value of 6.8 uC
and is 3.5 cm away from O2
It is also a positive charge

Q2 has a value of 1.2 uC
and is 5cm away from O3
It is also positive

Q3 has a value of -4.4 uC
and is negative.

What is the magnitude of the electric field strength at a point 1.1 cm to the left of the middle charge? The value of the Coulomb constant is K = 8.98755 x 10^9 N*m^2/C^2. Answer in units of N/C.

Homework Equations

I think that I am going to have to use E = k*Q/r^2 (electric field)

The Attempt at a Solution

Well using E = k*Q/r^2 this is what I did. I

I did K*(6.8 x 10^-6)/(2.4 x ^-2) = 106103020.8
K*(1.2 x 10^-6)/(1.1 x 10^-2) = 89132727.27
K*(4.4 x 10^-6)/(6.1 x 10^-2) = 10627578.61
Final summation = 205863327.4 N/C
I used the fact that the question was asking for the point charge to be 1.1cm away from the middle charge, so I used that distance to find the other distances in the upper equations. I think that this has to do with superpositioning. The thing is, I am not too sure that if that concept deals with this question. I basically used the electric field formula and applied it to each charge along the x axis. I tried to submit it into my online home and it was spit back out as wrong. Still trying to work on it.

 

[ideasrule]

The problem is that you can't add the electric fields like that. Q2, being positive, counteracts the effect of Q1 (they push charges in opposite directions); Q3, being negative, helps Q1 and counteracts Q2. You want to add the fields vectorially, not arithmetically.

 

[jc269]

Thanks, solved a little bit ago. Forgot to include the direction.