Question:

Space between two concentric conducting spheres of radii a and b (b>a) is filled with a medium of resistivity ρ. The resistance between the two spheres will be :

ρ2π(1/a - 1/b)

ρ2π(1/a + 1/b)

ρ4π(1/a - 1/b)

ρ4π(1/a + 1/b)

Correct option is A. ρ4π(1/a - 1/b)
dR = ρ.dx/4πx²
∫dR = ∫ ρ.dx/4πx² (from a to b)
R = ρ/4π [-1/x] (from a to b)
R = ρ/4π (1/a - 1/b)