Electrostatics 7 Question 20

22. Two non-conducting solid spheres of radii R and 2R, having uniform volume charge densities ρ1 and ρ2 respectively, touch each other. The net electric field at a distance 2R from the centre of the smaller sphere, along the line joining the centre of the spheres, is zero. The ratio ρ1/ρ2 can be

(2013 Adv.)

(a) -4

(b) 3225

(c) 3225

(d) 4

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Solution:

  1. At point P

If resultant electric field is zero, then

KQ14R2=KQ28R3Rρ1ρ2=4

At point Q

If resultant electric field is zero then

KQ14R2+KQ225R2=0ρ1ρ2=3225 ( ρ1 must be negative )



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