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 up thrust 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