n oil drop carrying a charge q has a mass m .it is falling freely in air wid terminal speed v de electric feild required to make drop move upward wid de same speed is
When its falling down net force is zero. So drag = mg upwards. If it has to move with velocity v upwards, weight of mg and drag mg will act downwards So E = 2mg/q (This assumes there is no boyancy i.e. density of air is very very less than density of air)
Krishna Gopal Singh
B.Tech Chemical Engg
IIT Delhi 2002
Currently doing PhD from IIT Delhi