Final answer:
A constant magnetic field only changes the direction of a charged particle's velocity, causing it to move in a circular path without changing its speed. This is because the magnetic force is perpendicular to velocity and does no work on the particle.
Step-by-step explanation:
In the context of a charged particle moving in a magnetic field, the magnetic force only affects the direction of the particle's velocity, not the magnitude. When a charged particle, like an electron or a proton, experiences a constant magnetic field, it undergoes uniform circular motion because the force is always perpendicular to the velocity. Since the force does no work on the particle, its kinetic energy and speed remain constant, hence only its direction changes. This is crucial for applications like particle accelerators and for explaining phenomena in particle physics.
As for two particles with the same velocity, like an electron and a proton, they would experience the same magnetic force magnitude (provided they have the same charge magnitude), but their accelerations would differ due to their different masses. An increased magnitude of the uniform magnetic field would not increase the speed of the charged particle, but it will increase the magnetic force, leading to a tighter turning radius of the circular path.
Answer to the initial question: For a charged particle, a constant magnetic field can be used to change (a) only the direction of the particle's velocity.