Homework Help: Finding the Force Between Two Point Charges

1. Dec 8, 2012

PeachBanana

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

Two point charges with charges of +5μC and -100μC are separated by 15 cm. The magnitude of the force between these charges is:

a) 1.99 N
b) 4.02 N
c) 20 N
d) 199 N
e) 400 N

2. Relevant equations

F = q1q2 * k / r^2

3. The attempt at a solution

F = (9.00*10^9)(5.0*10^-6 C)(1.0*10^-4 C) / (0.15 m)^2

F = 4.5 / 0.0225 m^2

F = 200 N

Why is it incorrect to think the force of one on two is 200 N and the force of two on one is 200 N so the force between them is 400 N?

2. Dec 8, 2012

TSny

The phrase "force between two charges" is kind of fuzzy. It just means to find the force that one particle exerts on the other. The other particle will then exert the same magnitude of force on the first particle but in the opposite direction. [If for some reason you decided to add these two force vectors, what would you get?]

3. Dec 8, 2012

PeachBanana

Aha! 0 N. (right?)

4. Dec 8, 2012

TSny

Right! The net force for the system is zero. Good. But the answer to the question "what is the magnitude of the force between them" would be "200 N". At least that's my interpretation!