Finding the Force Between Two Point Charges

[PeachBanana]

Homework Statement

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

Homework Equations

F = q1q2 * k / r^2

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?

 

[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?]

 

[PeachBanana]

Aha! 0 N. (right?)

 

[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!

 

[Nickie]

I must point out that the forces between two point charges are equal in magnitude but opposite in direction, according to Coulomb's Law. This means that the force of one charge on the other is 200 N in one direction, while the force of the other charge on the first is 200 N in the opposite direction. Therefore, the net force between the two charges is not 400 N, but rather 200 N, as calculated above. It is important to consider both the magnitude and direction of the forces when calculating the net force between two charges.