Finding the neutral point of two charges

AI Thread Summary
The discussion focuses on finding the neutral point between two charges, q1=+9c and q2=-1c, separated by 2m. The electric field equations are set up correctly, but confusion arises when solving for the neutral point, particularly when encountering a negative square root. Participants clarify that the electric field direction must be considered based on the charges' positions, leading to the conclusion that the neutral point is nearest to the negative charge. After simplifying the equations, the solution reveals that the neutral point is at x=1m. This method effectively demonstrates how to approach the problem using electric field concepts.
kspabo
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Homework Statement


Two charges q1=+9c and q2+-1c are separated by 2m. Where is their neutral point?

Homework Equations


E = E(+) + E(-)
E= k q/r^2

The Attempt at a Solution


k q1/(2+x)^2 = -(k q2/x^2)

When I solve for this equation I end up square rooting a negative number which would then give i and I don't know how to work with this.
 
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kspabo said:

Homework Statement


Two charges q1=+9c and q2+-1c are separated by 2m. Where is their neutral point?

Homework Equations


E = E(+) + E(-)
E= k q/r^2

The Attempt at a Solution


k q1/(2+x)^2 = -(k q2/x^2)

When I solve for this equation I end up square rooting a negative number which would then give i and I don't know how to work with this.
Doesn't the electric field vector point away from a positive charge and towards a negative charge?

If so, the sign you use for the field from each of your charges depends upon the position relative to the charges.
 
SammyS said:
Doesn't the electric field vector point away from a positive charge and towards a negative charge?

If so, the sign you use for the field from each of your charges depends upon the position relative to the charges.
Correct. So the positive field flows right towards the negative which continues a field to the right. I'm looking for the neutral point nearest the negative charge. How can I use this info to solve this if q1 is at 0 and q2 is at x=2?
 
Last edited:
kspabo said:
Correct. So the positive field flows right towards the negative which continues a field to the right. I'm looking for the neutral point nearest the negative charge. How can I use this info to solve this if q1 is at 0 and q2 is at x=2?
Show what you get when you put values for charge into the following and do a little simplifying.
kspabo said:
k q1/(2+x)^2 = -(k q2/x^2)

Also,
What is the quadratic equation you get?
 
SammyS said:
Show what you get when you put values for charge into the following and do a little simplifying.Also,
What is the quadratic equation you get?

k q1/(2+x)^2 = -(k q2/x^2)
q1/(2+x)^2 = -(q2/x^2)
+9/(2+x)^2 = -(-1/x^2)
9/(2+x)^2 = 1/x^2
x^2/(2+x)^2 = 1/9
x/(2+x) = ± 1/3
(2+x)/x = ± 3
2/x + 1 = ± 3
2/x = -1 ± 3
So we have x=1 when -1 + 3

Ah, that makes sense and I believe is the correct answer.
 
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