# Electrostatics question, find distance

#### Scremin34

6.2) F = k*q1*q2/r^2
6.17 = (9*10^9)(5.5*10^-6)^2/r^2
r = 0.21m and not 0.32m

I think that 0.32m is wrong, or am I doing something wrong?

Forgot to add one more question:

6.3) Point S is 0,15 m to the right of the centre of sphere A.

Calculate the magnitude and direction of the resultant field at point S.

EAs = 2.2*10^6 N.c^-1 right
EBs = 1.38*10^7 N.c^-1 left

Enet = 1.6*10^7 N.c^-1 right.............Correct?

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#### ChipB

PHF Helper
I believe you are correct in that your calculation of the distance between the spheres. As for the electric field calculation - don't forget that the electric field from each of the spheres is a vector and thus has direction. You show both electric fields due to the charges on the left and right as being positive, but actually the one on the left resuts in a vector with negative sign. Given r_i is the unit vector from the point of interest to the charge Q_i:

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#### Scremin34

I believe you are correct in that your calculation of the distance between the spheres. As for the electric field calculation - don't forget that the electric field from each of the spheres is a vector and thus has direction. You show both electric fields due to the charges on the left and right as being positive, but actually the one on the left resuts in a vector with negative sign. Given r_i is the unit vector from the point of interest to the charge Q_i:

Yeah, mistake that should be

Enet = Eas+Ebs
= 2.2*10^6 + (-1.38*10^7)
= 1.16*10^7 N.c^-1 left

And I have no idea what that equation is . I have never seen it in my entire life, maybe it is abit too advanced for my level of work.

#### Vaibhav

And I have no idea what that equation is :confused:. I have never seen it in my entire life said:
Equation given is based on Principal of Superposition of electric fields.

#### topsquark

Forum Staff
Equation given is based on Principal of Superposition of electric fields.
The equation you are apparently referring to is the electric field for multiple electric charges. It reduces to E = k*Q1/r^2 for two charges. The equation ChipB used is the vector form of this law.

-Dan

Edit: Whoops! I quoted the wrong post. This is an addendum to Vaibhav's post.