Question

Which of the following properties of a proton can change while it moves freely in a magnetic field? (There may be more than one correct answer).
A. Mass
B. Speed
C. Velocity
D. Momentum

Answer 1

Hint: Use the concept of free motion of a proton in a magnetic field. Check whether the force acting on the proton when it moves freely in a magnetic field remains constant or changes. Depending on these results, check which of the physical quantities related with the proton given in the options remains the same or changes.

Complete answer:
The mass of a proton is a constant physical quantity. It remains always the same although it is placed in any magnetic field. Therefore, the mass of the proton does not change when it moves in a magnetic field. Hence, the option A is incorrect.
When the proton moves freely in a magnetic field, its speed always remains the same.Hence, the option B is incorrect.
When a proton is moving in a magnetic field, there is a force acting on the proton in a direction perpendicular to the motion of the proton. This force acting on the proton changes the direction of motion of the proton. Thus, although the speed of the proton remains the same, the direction of motion of the proton changes. Therefore, the velocity of the proton moving freely in a magnetic field changes.Hence, the option C is correct.
The momentum of the proton moving freely in a magnetic field is the product of mass of the proton and velocity of the proton. Since the velocity of the proton changes during its motion in the magnetic field, the momentum of the proton also changes.Hence, the option D is correct.

Hence, the correct options are C and D.

Note: One can also explain the same solution in another way. When the proton enters in a changing or uniform magnetic field, it follows a helical or circular path of motion. When the proton is in helical or circular motion, the speed of the proton remains the same but velocity changes and hence momentum also changes.