Solving for Net Force on Third Particle: What's Missing?

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Homework Help Overview

The problem involves determining the position of a third particle with an unknown charge such that the net electrostatic force acting on it from two other charged particles is zero. The first two particles have known charges and are positioned along the x-axis, while the goal is to find the coordinates of the third particle.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to express the net electrostatic force on the third particle using angles and the Pythagorean theorem but finds the expression too complex due to multiple unknowns. Some participants question why the net force would only be zero along the x-axis, considering the opposing charges. Others suggest that placing the third charge on the y-axis would not yield a zero net force due to the directional nature of the forces involved.

Discussion Status

Participants are exploring the conditions under which the net force can be zero and discussing the implications of charge placement. There is an ongoing examination of the forces acting on the third particle and the potential positions it could occupy to achieve equilibrium.

Contextual Notes

There is a focus on the assumption that the net force can only be zero along the x-axis, with some participants challenging this notion. The discussion also highlights the complexity introduced by the unknown charge of the third particle and the need for further clarification on its placement.

Warlax
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particle 1 of charge q1 = +1.2 µC and particle 2 of charge q2 = -2.5 µC, are held at separation L = 14 cm on an x axis. If particle 3 of unknown charge q3 is to be located such that the net electrostatic force on it from particles 1 and 2 is zero, what must be the (a) x and (b) y coordinates of particle 3?

I tried expressing the net electrostatic force on the third particle both on the x-axis and on the y-axis. The net force on both axis is =0. I introduced angles alpha and beta to express the angle between the x-axis and a line from particle 1 or particle 2 to particle 3, respectively. I can find the net electrostatic force using this and the Pythagorean theorem - but the expression has too many unknowns to be solved for x and y of the 3rd particle.

What am I missing here? How should I go about solving this?

Thanks.
 
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Net force will be zero only on x-axis.
The neutral point will be out side the line joining the two charges nearer to the smaller charge.
 


rl.bhat said:
Net force will be zero only on x-axis.
The neutral point will be out side the line joining the two charges nearer to the smaller charge.

Thanks for your reply,

Why would the force only be zero on the x-axis?
q1 and q2 are of opposite charge so one is pulling on q3 and another is pushing against it... wouldn't that make q3 have a zero net force on the y-axis as well.

In addition, how do I approach this problem? I need to find the x,y coordinates of q3.
 


Warlax said:
Thanks for your reply,

Why would the force only be zero on the x-axis?
q1 and q2 are of opposite charge so one is pulling on q3 and another is pushing against it... wouldn't that make q3 have a zero net force on the y-axis as well.

In addition, how do I approach this problem? I need to find the x,y coordinates of q3.
If +q3 is placed on y-axis, force between q1and q3 will be away from q1 and force between q2 and q3 will be towards q2. The resultant of these forces cannot be zero. because they are not in a line.
 


rl.bhat said:
If +q3 is placed on y-axis, force between q1and q3 will be away from q1 and force between q2 and q3 will be towards q2. The resultant of these forces cannot be zero. because they are not in a line.

Oh, I see... so this reduces the problem to something more like:
What if q3 is on the x axis, either to the right or to the left of both q1 qnd q2?

Am I correct?
 

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