Time management is very much important in IIT JAM. The eduncle test series for IIT JAM Mathematical Statistics helped me a lot in this portion. I am very thankful to the test series I bought from eduncle.
Nilanjan Bhowmick AIR 3, CSIR NET (Earth Science)
Meena
see the attachments. note (why we integrate from 0 to π/2) : One hemisphere experiences a repulsive force due to other hemisphere. so we find the (repulsive) force experienced by a HEMISPHER i.e. half sphere and not the whole sphere. That is why we integrate theta from 0 to π/2 and not 0 to π