Two small aluminum spheres are separated by 80cm How many electrons are to be transferred from one sphere to the other so that they attract with a force of `10^4N`

Two tiny charged spheres separated in air by a distance exert a force 'F'on each other .If they are immersed in a liquid of dielectric constant 2,then the force is

The number density of free electrons in a copper conductor estimated is 8.5x10^28m^(-3) . How long does an electron take in drifting from one enf of a wire 3.0m long to its other end? The area of cross-section of the wire is 2.0x10^(-8)m^2 and it is carrying a current of 3.0A.