67) A stream of electrons enters an electric field with?10⁷ms^?1?at right angle to it. If the strength of field is?3?10⁴V/m³, the magnetic field acting perpendicular to electric field for producing no deflection of the beam is

68) A drop of radius one micron carries a charge of 4 electrons. If the density of oil is 2g/cc, the electric field required to balance it is

69) When a photon of energy 7eV is made incident on a metal then the emitted electron is stopped by a stopping potential of -5.5V. The work function of metal will be

70) In an experiment of photoelectric emission for incident light of 4000?A⁰, the stopping potential is 2V. If the wavelength of incident light is made 3000?A⁰, then stopping potential will be

71) Work function of tungsten and sodium are 4.4eV and 2.3 eV respectively. If threshold wavelength of sodium is 5460A⁰, then threshold wavelength of tungsten is