※ 引述《unknown1 (unknown1)》之銘言：
: 請問一下~
: a long cylinder carries a charge density that is proportional to the distance
: from the axis: p=ks, for some constant k.
: Find the electric field inside this cylinder
Assume that the cylinder has radius a.
│
a├───────────
│ r ' ←point q
───────'───────>
│ z z'
a├───────────
│
1 p
E(q)=── ∫ ── dv
4πε v' R^2
1 s a kz 2π
=── ∫∫────rdrdz∫ dφ
4πε 0 0 z^2+r^2 0
k
=─ (s^2+a^2)*Ln(1+(a/s)^2) ...ans
8ε
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