dE = ke(dq/r2)
How to change to complicated equation in the following picture?
See http://www.physics.udel.edu/~watson/phys208/exercises/kevan/efield1.html, section on field from disk.
σ=Q/ πR2 and
Then ρ=Q/ (πR2 x dz) in here?
In fact, I don't understand although I can deduce it.
The volume of sphere is (4/3)πR3, (πR2 x dz) = base area x height = volume of cylinder
Whether I got wrong about the picture is talking about the cylinder not sphere?
dz indicates an arbitrarily thin slice. Such a thin slice of a sphere is effectively a very short cylinder. This is the basis of calculus.
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