Final answer:
A particle released from rest in a uniform electric field will move in a straight line with increasing velocity due to constant acceleration by the electric force. In contrast, in a uniform magnetic field, a charged particle moving perpendicular to the field lines will exhibit circular motion.
Step-by-step explanation:
When a particle is released from rest in a uniform electric field, it will move in a straight line with increasing velocity. This is due to the electric force acting on the charged particle in the direction of the field, which constantly accelerates the particle as long as the field is uniform and the particle is within it. If, however, we consider a uniform magnetic field, the motion of a charged particle that is moving perpendicular to the field lines would be a circular path, as the magnetic force acts perpendicular to the velocity of the particle, causing it to follow a curved trajectory within the magnetic field.