Question

If a proton is projected in a direction perpendicular to a uniform magnetic field with velocity $v$ and an electron is projected along the lines of force, what will happen to proton and electron?

• A. The electron will travel along a circle with constant speed and the proton will move along a straight line
• B. Proton will move in a circle with constant speed and there will be no effect on the motion of electron
• C. There will not be any effect on the motion of electron and proton
• D. The electron and proton both will follow the path of a parabola

Answer 1

Answer: (B)

For proton, $v \perp B$ and for electron $v \| B$.
So, force on proton $=q( v \times B )=B q v$ whereas, force on electron $=0$.