An oil drop of 12 excess electrons is held stationary under a constant electric field of `2.55 xx 104 NC^(–1)` (Millikan’s oil drop experiment). The density of the oil is `1.26 g cm^(–3)`. Estimate the radius of the drop. `(g = 9.81 m s^(–2), e = 1.60 xx 10^(–19) C)`.

In a chamber, a uniform magnetic field of 6.5 G (1 G = 10^-4T) is maintained. An elecIron is shot into the field with a speed of 4.8 xx10^6 ms^-1 normalto the field. Explain why the path of the electron is a circle. Determine the radius of the circular . orbit.(e = 1.6xx10^-19 C, m_e=9.1xx10^-31kg) .