1. Magnetic force acting on a particle moving in magnetic field is _________ [ answer using q, B and Vd]

2. An electron rotates on circular path of radius r in normal direction of magnetic field B. The kinetic energy gain by the electron when it completes half rotation is ___________[half mv^2, one fourth mv^2, [pi]rBev or zero?]

3. Two metal spheres of radii 5cm and 4cm are charged to same potential. the surface charge densities of two spheres are in the ratio ___________

4.stopping potential for one polished surface is 2 volt. the maximum speed of electron emitteed from surface is ____________m/s

5. A magnetic needle placed in uniform magnetic field has magnetic moment 2x10^-2Am^2 and moment of inertia 16x10^-6kgm^2. It performs 10 oscillations in 20 seconds. what is the magnitude of magnetic field. (consider pi^2 = 10)