Q: In a Millikan’s experiment an oil drop of radius 1.5 × 10^(-6) m and density 890 kg/m^3 is held stationary between two condenser plates 1.2 cm apart and kept at a p.d of 2.3 kV. If upthrust due to air ignored, then the number of excess electrons carries by the drop will be

Sol: $\large q E = m g $

$\large q (\frac{V}{d}) = (\frac{4}{3}\pi r^3 \rho) g $

$\large n e (\frac{V}{d}) = (\frac{4}{3}\pi r^3 \rho) g $

On putting the known values we get ,

n ≈ 4