# 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
Homework Helper
Gold Member
That was a typo; the answer matches the correct numbers.
Ah, ok! I should have checked!

To the OP: yes, they surely want the magnitude only. And be careful with sig figs.

Wow. That was dumb of me for not seeing magnitude right next to the problem...thank you everyone in this thread. At least I know what I'm doing now...