A point charge q is situated on the axis of a thin conducting rod of length L, at a distance r from one end. The rod has a charge Q uniformly distributed along its length. The magnitude of the electric force between the two is

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(4) (KQq)/(r(r+L)). Idea: split the rod into small elements, each a point charge. Every element pushes on q along the axis, so the forces simply add. Charge per unit length: λ=Q/L. An element dx at distance x from q carries λ dx. dF=(Kqλ dx)/(x²), with x running from r (near end) to r+L (far end). F=Kqλ∫ᵣ^r+L(dx)/(x²)=Kqλ(1/r-1/(r+L))=Kqλ L/(r(r+L)) F=(KQq)/(r(r+L)) Check: when r is much larger than L this becomes (KQq)/(r²), the force between two point charges, as it should.

A point charge q is situated on the axis of a thin conducting rod of length L, at a distance r from one end. The rod has a charge Q uniformly distributed along its length. The magnitude of the electric force between the two is

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Why the other options are wrong