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 magnetic field, 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

The correct option is B Proton will move in a circle with constant speed and there will be no effect on the motion of electron.

The magnetic force on the moving charge is,

$\overrightarrow{F}=q(\overrightarrow{v}\times \overrightarrow{B})=qvB\sin \theta$

For Proton; $\theta ={90}^{\circ }$

For Electron; $\theta =0^{\circ }$

Therefore, maximum force will act on proton in a direction perpendicular to both $\overrightarrow{v}$ and $\overrightarrow{B}$ , so it will move along a circular path.

Force on electron will be zero because it is moving parallel to the field.

<!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}--> Hence, $\left(B\right)$ is the correct answer.