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February 27, 2015

February 27, 2015

Posted by **Physics Lover** on Thursday, June 13, 2013 at 4:45am.

Details and assumptions:

The second ring also has evenly distributed positive charge on it.

- Physics -
**Physics Lover**, Thursday, June 13, 2013 at 4:53amI solved it like this

let charge of the ball be Q

let charge of unkown ring be Q2

let the place where there is equilbrium be

P. This point P is 0.25m from the ring with known charge and and hence 0.75m from the ring with unknown charge.

Now applying coulomb's law at this point P from both the rings and this force must be equal as the point is in chrge equilibrim.

So at point p

(k*Q2*Q)/(0.75)^2 = (k*1*10^-9*Q)/(0.25)^2

[As known charge is 1nC)

Solving we get Q2 = 9 nC.

But the answer is incorrect so what should be the correct answer ?

PLEASE HELP !

- Physics -
**Elena**, Thursday, June 13, 2013 at 6:58amElectric field on the axis of the ring at the distance x from the center of it is

E = k xq/{sqrt(R²+x²)}³

k=1/4πε₀

The test charge is in equilibrium =>

E₁=E₂

k x₁q₁/{sqrt(R²+x₁²)}³=

=k x₂q₂/{sqrt(R²+x₂²)}³,

q= x₁q₁{sqrt(R²+x₂²)}³/{sqrt(R²+x₁²)}³=

=0.25110⁻⁹{sqrt(0.25²+0.75²)}³/{sqrt(0.25²+0.25²)}³=

=2.810⁻⁹ C = 2.9 nC

- Physics -
**Physics Lover**, Thursday, June 13, 2013 at 10:10amHey elena but why is my method wrong ?

- Physics -
**idk**, Thursday, June 13, 2013 at 2:34pmwait 2.8 and 2.9 are both wrong.

- Physics -
**finnt**, Friday, June 14, 2013 at 2:29amPhysics Lover : The Coulomb formula is only valid for a point, not a ring

- Physics -
**Physics Lover**, Friday, June 14, 2013 at 3:23amYeah the answer is wrong so what should be correct ?

- Physics -
**Anonymous**, Friday, June 14, 2013 at 4:22am3.73 is the perfect answer. Elena has done it right, but has made some mistake in calculation

**Answer this Question**

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