Charged Particle Acceleration in Electric Field

Question:

A charged particle moves in a straight line through an electric field. Is it necessarily accelerated?

Answer:

Any charged particle which is free to move will be accelerated by an electric field. If the particle is constrained by other forces to move in a straight line at constant velocity regardless of the electric field then the answer to the question could be "no". In general though we consider particles which are free bodies. If a charged particle in the absence of any other influence moves in a straight line through an electric field it must be moving parallel to the field lines. If there were any transverse component the particle would be deflected from its straight path. The particle will experience a force directed along its path and so be accelerated, changing its speed.

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