# Homework Help: Couloumb's Law 3 charges on a line

1. Apr 12, 2012

### PatrickGeddes

1. The problem statement, all variables and given/known data
Three point charges lie along a straight line as shown in the figure below, where q1 = 6.36 µC, q2 = 1.56 µC, and q3 = -1.84 µC. The separation distances are d1 = 3.00 cm and d2 = 2.00 cm. Calculate the magnitude and direction of the net electric force on each of the charges.

2. Relevant equations

3. The attempt at a solution
I did F of q1 on q2 is ((8.99x10^9) x .00000636 C x .00000156 C)/.0009 m=99.10576 N and F of q3 on q2 is ((8.99x10^9) x -.00000184 C x .00000156 C)/.0016 m=-286.72 N. Then for the Force on the middle charge I just added them together to get -187.8 N.

2. Apr 12, 2012

### Staff: Mentor

Can you explain how you arrived at 0.0016 m in the denominator for the force calculation of q3 on q2? The inferred distance between q3 and q2 is 2 cm so where does the 16 come from? (you didn't include the mentioned figure, so no clues from there). Shouldn't the units of the "distance squared" be meters squared, not meters?

Did you take into account the physical position of the charges when determining the direction that the forces act? Charge signs may determine whether a force is attractive or repulsive, but physical layout determines the actual directions that the forces resolve to on the coordinate axes.

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