# Homework Help: Electric problem

1. Jan 19, 2009

### redhot209

Note: ^i means carrot on top of variable

1. The problem statement, all variables and given/known data
A particle with charge 4 µC is located on the x-axis at the point 10 cm, and a second particle with charge -3 µC is placed on the x-axis at 4 cm. What is the magnitude of the total electrostatic force on a third particle with a charge 4 µC placed on the x-axis at -2 cm? Answer in units of N.

3. The attempt at a solution
1) I converted all my variables to µC = C , and cm = m
q1 = 4 µC = 4e-6
q2 = -3 µC = -3e-6
q3 = 4 µC = 4e-6
x1 = 10 cm = 0.1 m
x2 = 4 cm = 0.04 m
x3 = -2 cm = -0.02 m

2 By using Coulom's law in vector form:
F13= ke(q1q3/r2)^r13
where ^r13 is a unit vector directed from q1 to q3; i.e., ->r13 = ->r3 - ->r1

x13 = x3 - x1
(-0.02m)-(0.1m) = -0.12m
x23 = x3 - x2
(-0.02m)-(0.04) = -0.06 m

^x13 = x3-x1/ √(x3-x1)^2 = -1 or -^i
^x23 = x3-x2/ √(x3-x2)^2 = 1 or +^i

Since the forces are collinear, the force on the third particle is the algrebracis sum of the forces between the first and third and the second and third particles.

->F = ->F13 + ->F23
= ke [q1/ r213 X ^r13 + q2 / r223 x^r13 ] q3

After putting in the variables, I got 33.9444444445 N as my answer, but was wrong. What did i do wrong?

Last edited: Jan 19, 2009
2. Jan 20, 2009

### rl.bhat

First of all find the force between 4micro C at -2 cm and 4 micro C at 10 cm. It is repulsive force. Next find the force between 4micro C at -2 cm and -3 micro C at 4 cm. It is attractive force. Now find the net electrostatic force on the third charge.