Question

A charged particle moves through a magnetic field perpendicular to its direction. Then

• A. kinetic energy changes but the momentum is constant
• B. the momentum changes but the kinetic energy is constant
• C. both momentum and kinetic energy of the particle are not constant
• D. both momentum and kinetic energy of the particle are constant

Hint:

Magnetic force can change the direction of velocity but never its magnitude. Hence: K.E. = constant, Momentum (vector) = changing

Answer 1

The Correct Option is B

Step 1: Magnetic force acting on a charged particle is given by: vecF = q(vecv × vecB) 

Step 2: Since the force is always perpendicular to the velocity, it does no work on the particle. W = vecF · vecs = 0 

Step 3: As no work is done, the kinetic energy remains constant. 

Step 4: The force continuously changes the direction of velocity, hence the momentum vector changes (even though its magnitude remains constant).