Prof. E. Carson’s lectures are quite clear, supplemental to Prof. Shankar’s Yale courses: https://www.youtube.com/watch?v=v57B_1ZBAho&list=PLZ6kagz8q0bvxaUKCe2RRvU_h7wtNNxxi .

Coulomb’s law

What’s if it’s not a point charge in the center, but a sphere shell? This is when the superposition principle comes into play:

Placing a point charge in a place, and then place an atom to a distance to the point charge, there will be polarization.

From the point charge, we extend to calculate the field at a fixed location from an infinite rod and a ring.

Then filed of a finite disc

When the R goes to infinitive, it’s an infinite plane,

From this simple succinct equation, we know that the distance z doesn’t matter anymore, you always get the same field.

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