# Another question on electric fields.

Another question on electric fields. :(

I have another problem I don't understand on electric fields. I thought I understood these after my last post, but I guess not. I'm guessing it has something to do with the signs of the forces, but I am not sure.

1. Homework Statement
Particles of charge Q1 = +68 µC, Q2 = +49 µC, and Q3 = -80 µC are placed in a line (Fig. 16-37). The center one is 0.35 m from each of the others. Calculate the net force on each charge due to the other two. Figure 16-37

2. Homework Equations
F = KQ_1Q_2/r^2

3. The Attempt at a Solution

F = K * Q1Q2/r^2
F_12 = (K)(6.8 X 10-5)(4.9 X 10-5)/.35^2
F_12 = 244.8 N (goes to the left, repels)

F_13 = K(Q1Q2)/r^2
F_13 = (K)(6.8 X 10-5)(4.9 X 10-5)/.7^2
F_13 ~= 100 N (goes to the right, attracts)

F_q1 = -244.8 + 100
F_q1 = -144.8 N (wrong)

However, the second part of the question asked which direction it went in, and based on my answer, I put "left" and that was correct. I'm confused.

Related Introductory Physics Homework Help News on Phys.org
What does F_13 mean? 1 on 3 or 1 due to 3?

nrqed
Homework Helper
Gold Member
I have another problem I don't understand on electric fields. I thought I understood these after my last post, but I guess not. I'm guessing it has something to do with the signs of the forces, but I am not sure.

1. Homework Statement
Particles of charge Q1 = +68 µC, Q2 = +49 µC, and Q3 = -80 µC are placed in a line (Fig. 16-37). The center one is 0.35 m from each of the others. Calculate the net force on each charge due to the other two. Figure 16-37

2. Homework Equations
F = KQ_1Q_2/r^2

3. The Attempt at a Solution

F = K * Q1Q2/r^2
F_12 = (K)(6.8 X 10-5)(4.9 X 10-5)/.35^2
F_12 = 244.8 N (goes to the left, repels)

F_13 = K(Q1Q2)/r^2
F_13 = (K)(6.8 X 10-5)(4.9 X 10-5)/.7^2
F_13 ~= 100 N (goes to the right, attracts)

F_q1 = -244.8 + 100
F_q1 = -144.8 N (wrong)

However, the second part of the question asked which direction it went in, and based on my answer, I put "left" and that was correct. I'm confused.
Watch out! In F_13 you used the charge of q2 instead of q3!

Doc Al
Mentor
Your answers look good to me. Since they ask for the direction separately, I suspect that for the first part they just wanted the magnitude of the force.

Doc Al
Mentor
Watch out! In F_13 you used the charge of q2 instead of q3!
That was a typo; the answer matches the correct numbers. nrqed
That was a typo; the answer matches the correct numbers. Ah, ok! I should have checked!