# Calculating the magnitude of the electric field

1. Sep 14, 2009

### jc269

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

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

3. 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.

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2. Sep 14, 2009

### 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.

3. Sep 16, 2009

### jc269

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