Another question on electric fields.

In summary, the conversation is about a problem involving particles with different charges placed in a line and calculating the net force on each charge due to the other two. The equations and attempts at solving the problem are provided, with a discussion about the direction of the forces and a typo in the calculation. The summary also mentions the importance of paying attention to details such as sig figs.
  • #1
Bensky
82
0
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.

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.

16-37alt.gif

Figure 16-37

Homework Equations


F = KQ_1Q_2/r^2

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.
 
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  • #2
What does F_13 mean? 1 on 3 or 1 due to 3?
 
  • #3
Bensky said:
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.

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.

16-37alt.gif

Figure 16-37


Homework Equations


F = KQ_1Q_2/r^2


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!
 
  • #4
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.
 
  • #5
nrqed said:
Watch out! In F_13 you used the charge of q2 instead of q3!
That was a typo; the answer matches the correct numbers. :wink:
 
  • #6
Doc Al said:
That was a typo; the answer matches the correct numbers. :wink:

Ah, ok! :smile: I should have checked!

To the OP: yes, they surely want the magnitude only. And be careful with sig figs.
 
  • #7
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...
 

1. What is an electric field?

An electric field is a region in which an electric charge experiences a force. It is a fundamental concept in physics and is created by charged particles, such as electrons and protons.

2. How is an electric field measured?

Electric fields can be measured using instruments such as voltmeters or electric field meters. The units of measurement for electric fields are volts per meter (V/m).

3. What is the relationship between electric fields and electric charges?

Electric fields are created by electric charges and are affected by the magnitude and direction of those charges. The direction of the electric field is away from positive charges and towards negative charges.

4. How do electric fields interact with matter?

Electric fields can interact with matter by exerting a force on charged particles in the material. This can result in movement or polarization of the particles, leading to various effects such as conductivity and capacitance.

5. What are some real-world applications of electric fields?

Electric fields have numerous applications, including powering electronic devices, generating electricity, and controlling the motion of charged particles in technologies such as particle accelerators and cathode ray tubes. They also play a crucial role in the functioning of biological systems, such as the nervous system and muscle movements.

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