A positively charged particle enters a magnetic field oriented perpendicular to its direction of motion. Does the particle: A) Change its velocity, B) Change its speed, C) Accelerate in the direction of the magnetic field.

To answer this question you must use the magnetic field "Right Hand Rule". We know that if a charged particle enters a magnetic field it will experience a force ​perpendicular to both​ the magnetic field and its direction of motion.

From this result we know that the particle experiences no force along its direction of motion thus no work is done on the particle by the magnetic field. This means that B is ​False​ since the kinetic energy of the particle can not change. This also shows that C is False​ since the field exerts a "Central Force" on the moving particle causing it to enter a circular trajectory.

By elimination then, A is ​True​. Though the speed of the particle does not change, the direction does as the particle begins circular motion. Since the velocity depends on both the speed and the direction of motion of an object we see that the magnetic field must be causing a change in velocity of the particle.

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