Help with Coulomb's law: Net electrostatic force

AI Thread Summary
The discussion revolves around the application of Coulomb's law to calculate net electrostatic forces and the confusion regarding vector components. The original poster attempted to calculate forces using both the overall force and individual vector components but encountered discrepancies, particularly with the modulus of force components exceeding the total force. A key misunderstanding was identified: the poster was incorrectly using squared distances in calculations instead of applying the correct trigonometric functions to derive the components from the overall force magnitude. The solution involves multiplying the overall force by the cosine and sine of the angle to find the respective vector components accurately. Clarification on this method helped resolve the confusion.
mousey
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Homework Statement
Given the arrangement of charged particles in the figure below, find the net electrostatic force on the
q1 = 5.35-µC charged particle. (Assume q2 = 14.33 µC and q3 = −18.12 µC. Express your answer in vector form.)

The three charges (q1, q2, and q3) are located at the following points:
q1: (-2.00cm, 0cm)
q2: (1.00cm, 1.00cm)
q3: (0cm, -1.00cm)

I converted uC into C and cm into m, found the distance between q1 and q2 (r_12) and between q1 and q3 (r_13), and I know I have to compare q2 to q1 and q3 to q1. I can figure out the force just by plugging numbers into Coulomb's law, but I'm not sure how to calculate the vector components.

Thank you!
Relevant Equations
F =(k{q_1}(q_2))\ d^{2)
(Coulomb's Law)
I tried just calculating the force with Coulomb's law, then calculating the forces for each vector individually and adding, but I got it wrong both ways
 
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We need your solution to find what went wrong.
 
Gordianus said:
We need your solution to find what went wrong.
Thank you! I tried uploading a picture of my work but I couldn't figure out how to initially. Also here's the original problem in all it's glory.
 

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The modulus of F12 Is 690 N/C. However its components are larger than that value. There's something wrong there.
 
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Gordianus said:
The modulus of F12 Is 690 N/C. However its components are larger than that value. There's something wrong there.
Right, I tried that answer and it was marked wrong, so I tried finding the force for the individual vector components (table in the lower left), by using just the distance in the x and y directions, but in meters in place of "d" for d^2. Then I added the components (i of q2 + i of q3, j of q2 +j of q3), but that answer was also wrong.
 
Haven’t checked the details of arriving at F12 and F13, but they look about right. After that I am lost. How did you get those component force values in the table?
 
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haruspex said:
Haven’t checked the details of arriving at F12 and F13, but they look about right. After that I am lost. How did you get those component force values in the table?
these are my equations. Basically in terms of a right triangle, I used the "legs" to calculate for each vector component instead of the hypotenuse as the whole force.
 

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I checked the modulus in a hurry and they look OK. I don't understand the table
 
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Gordianus said:
I checked the modulus in a hurry and they look OK. I don't understand the table
I tried to explain it further in my reply to haruspex.
 
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mousey said:
these are my equations. Basically in terms of a right triangle, I used the "legs" to calculate for each vector component instead of the hypotenuse as the whole force.
You have a basic misunderstanding in how to find the components. You should not be dividing by e.g. 0.03^2.
 
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  • #11
Having found the overall force magnitude, multiply it by the cosine of the angle (x/r) to find the i component and sine (y/r) to find the j component.
Watch the signs carefully.
 
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  • #12
haruspex said:
Having found the overall force magnitude, multiply it by the cosine of the angle (x/r) to find the i component and sine (y/r) to find the j component.
Watch the signs carefully.
That makes perfect sense. Thank you.
 
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