Question

If a charged particle enters perpendicularly into the uniform magnetic field then its

• A. Energy remains constant but momentum changes
• B. Energy and momentum both remains constant
• C. Momentum remains constant, but energy changes
• D. Neither energy nor momentum remains constant

Answer 1

The correct option is A Energy remains constant but momentum changes

Force on charge particle $q$ moving with velocity $\overrightarrow{V}$in uniform magnetic field is given by
$$\overrightarrow{F}=q(\overrightarrow{V}\times \overrightarrow{B})$$As this $\overrightarrow{F}$is perpendicular to $\overrightarrow{V}$ as well as $\overrightarrow{B}$,the particle starts moving in the circular path.

As force and displacement are perpendiculars, the net work done will be zero and kinetic energy does not change.

But as it moves in a circular path, the direction of velocity changes periodically, so the momentum will also change.

As the charged particle enters perpendicular to the magnetic field, the force will act perpendicular to the velocity.

Hence, option (a) is the correct answer.