- #1
j0n
- 2
- 0
Homework Statement
Identical thin rods of length 2a carry equal charges, +Q, uniformly distributed along their lengths. The rods lie along the x-axis with their centers separated by a distance b > 2a. Find the force, F, exerted by the left rod on the right one.
Left rod's center @ (0,0) → rod stretches from (-a,0) to (a,0)
Right rod's center @ (b,0) → rod stretches from (b-a, 0) to (b+a, 0)
Homework Equations
They give that
[tex] F = \frac{k Q^2}{4 a^2} \ln \frac{b^2}{b^2 -4a^2} [/tex]
Also we know that [tex] λ = \frac{Q}{2a} = \frac{dQ}{dx} , \space and \space Q = Q_1 = Q_2 [/tex]
The Attempt at a Solution
Electric field produced by rod 1 at a point x > 2a:
[tex] E_1(x) = \int \frac{k}{χ^2}dQ = \int_{x-a}^{x+a} \frac{k}{χ^2} λ dχ = \int_{x-a}^{x+a} \frac{k}{χ^2} \frac{Q_1}{2a} dχ = \frac{k Q_1}{2a} \int_{x-a}^{x+a} \frac{dχ}{χ^2} [/tex]
[tex] = \frac{k Q_1}{x^2 - a^2} [/tex]
Now I understand that FTOT = Ʃ qiE1(xi),
where qi are the infinitesimal contributions of Q2 (= Q).
So shouldn't I have [tex] F = \int_{x-a}^{x+a} E_1(χ) λ dχ ? [/tex]
If so then I have:
[tex] F = \int_{x-a}^{x+a} \frac{k Q_1}{χ^2 - a^2} \frac{Q_2}{2a} dχ [/tex]
[tex] = \frac{k Q^2}{2a}\int_{x-a}^{x+a} \frac {dχ}{χ^2 - a^2} [/tex]
And I'm stuck.
Last edited: