Question

A spring of spring constant 'K' is fixed at one end has a small block of mass m and charge q is attached at the other end. The block rests over a smooth horizontal surface. A uniform and constant magnetic field B exists normal to the plane of paper as shown in figure. An electric field $\overrightarrow{E} = E_{0}\widehat{i}$ (E₀ is a positive constant) is switched on at t = 0 sec. The block moves on horizontal surface without ever lifting off the surface. Then the normal reaction acting on the block is –

• A. Maximum at extreme position and minimum at mean position
• B. Maximum at mean position and minimum at extreme position
• C. Uniform throughout the motion
• D. Both maximum and minimum at mean position

Answer 1

Answer: (D)

Both maximum and minimum at mean position

Answer 2

The velocity is maximum at mean position. Hence the magnetic force on block is maximum, at its mean position.

The magnetic force on the block while it crosses the mean position towards right and left is as shown

Hence normal reaction is maximum in case-1 and minimum in case-2. Hence correct option is D.