# Question about Electric Fields and Charges

• gamma-guy

## Homework Statement

So I was given some questions as practice, and I can't seem to do them. Here is a link to the questions:

http://i.imgur.com/ZmkqCAV.jpg (the answers are in square brackets)

Ive only been able to calculate Q3 and Q2 so far. I would really especially like help with Q1. I've been stuck on it for ages.

## Homework Equations

Ive been told this would be helpful: http://i.imgur.com/nmrsk2E.jpg

## The Attempt at a Solution

Ive been stuck on Q1 for ages. I've been playing around with F = k q1q2/r^2 but that's not getting me anywhere.

For Q2: I used E = kq/r^2 for q1 and q2. Subtracting the answers gave me -225x10^3, which is somewhat equal to 2.3x10^5. I've got no idea on how to get the j vector component though.
Edit: I just got the j vector. Pythag and then the formula above got it.

I have no idea where to begin for Q4

ANY help appreciated. Thank you

Last edited:
Lets take them one at a time:
Q1: you have the correct equation... you need to use the fact that force is a vector and Q seen a net zero force. However, it may give simpler maths if you use the electric field equation instead.
Work out the force on Q due to each charge ... just using symbols to start with.
What does the force add up to?
How do you write that in maths?

Q2: the electric field is a vector - which way does the electric field of q3 point?

Q4: you need to look at your notes ... you will have an equation about the torque on a dipole.

Lets take them one at a time:
Q1: you have the correct equation... you need to use the fact that force is a vector and Q seen a net zero force. However, it may give simpler maths if you use the electric field equation instead.
Work out the force on Q due to each charge ... just using symbols to start with.
What does the force add up to?
How do you write that in maths?

Q2: the electric field is a vector - which way does the electric field of q3 point?

Q4: you need to look at your notes ... you will have an equation about the torque on a dipole.
Thank you, i really appreciate it. Am I right in assuming the Force from Q to q1 = Q to q3? Or Q to q1 - Q-q3 = 0?

The question says it is the force on Q due to the other three that is zero.
You can use your understanding of forces (or electric fields) to work out the directions.
i.e. a positive charge to the left of Q contribute an electric field pointing to the right and F=QE
The actual value of Q does not matter (you'll see) so you can assume it is positive.

The question says it is the force on Q due to the other three that is zero.
You can use your understanding of forces (or electric fields) to work out the directions.
i.e. a positive charge to the left of Q contribute an electric field pointing to the right and F=QE
The actual value of Q does not matter (you'll see) so you can assume it is positive.

Ok, so the forces (Q to q1) + (Q to q2) + (Q to q3) = 0?

That is what the problem statement says all right - well done.

• gamma-guy