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Homework Statement
A big metal sphere has radius ##r## and charge ##Q##. The distance between a certain point charge and the surface of the sphere is equal to ##s##. The magnitude of the force exerted on the point charge is inversely proportional to:
A) ##s^2##
B) ##r^2##
C) ##(s+r)^2## (correct answer)
D) ##s^2+r^2##
(Source: Fysica Vandaag 5.2, Uitgeverij Pelckmans)
Homework Equations
Coulomb's law
The Attempt at a Solution
Let ##d## be the distance between the two charges.
##F=k\frac{|Q||Q'|}{d^2}=\frac{\mathrm{constant}}{d^2}##
This means the constant of proportionality is equal to ##d^2##.
Why is ##d=s+r##? I would expect it to be equal to ##s## since the charge in a sphere is the same everywhere.
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