Final answer:
The assertion that a charged particle always follows the electric lines of force within an electric field is not always true, as it can be influenced by other factors such as magnetic fields. The reason given is correct regarding the nature of electric field lines, but it is not the direct explanation of the particle's path. Hence, the answer is that both Assertion and Reason are correct, but Reason is not the explanation for Assertion.
Step-by-step explanation:
The assertion that a charged particle free to move in an electric field always moves along an electric line of force is not universally correct. While electric field lines do illustrate the direction of force experienced by a positive charge in the field, particles can move in different paths due to other forces or constraints. The reason provided suggests that the electric lines of force diverge from positive charges and converge at negative charges, which is correct as it follows Coulomb's law stating that like charges repel and unlike charges attract. However, this is not the reason charged particles always move along the field lines, as there could be cases where other factors influence the particle's motion, such as the presence of a magnetic field.
It's important to note the conditions under which the assertion could be invalid. For instance, in a conductor, free charges move until the field is perpendicular to the surface, meaning there can be no component of the field parallel to the surface in electrostatic equilibrium. Moreover, a charged particle in a magnetic field follows a curved path due to the magnetic force being perpendicular to its velocity. This detail is crucial since it indicates situations where a charged particle would not travel along the electric field lines if other forces are present.
Therefore, the correct response would be B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.