Final answer:
A proton moving in the direction of an electric field will be either repelled or attracted depending on the source of the field. Protons in a magnetic field moving perpendicularly will circle due to the perpendicular force that keeps the speed constant, demonstrating centripetal force.
Step-by-step explanation:
When a proton moves in the same direction as the electric field is pointing, we can conclude that the proton will experience a force in the direction of the electric field. This is because electric fields exert forces on charged particles, and the direction of the force on a positive charge, like a proton, is the same as the direction of the field lines. According to Coulomb's law, like charges repel and opposite charges attract, so if the electric field is created by a positive charge, the proton will be repelled from the source of the field, whereas if it is created by a negative charge, the proton will be attracted towards it.
In the context of a magnetic field, a moving charged particle also experiences a force. However, this force is perpendicular to both the velocity of the particle and the magnetic field lines, as described by the right-hand rule. If the proton's velocity is perpendicular to a constant magnetic field, it will execute circular motion due to the magnetic force acting as a centripetal force, keeping the magnitude of the proton's velocity constant.