Electric field lines originate from positive charges because they indicate the direction in which a positive test charge would move when placed in the field.

Electric field lines serve as a visual representation in physics, illustrating how electric charges influence their surroundings. These lines are drawn starting from positive charges and extending to negative charges, showing the path a positive test charge would take under the influence of the electric field. This behavior is grounded in the fundamental principle that like charges repel each other, while unlike charges attract.

The concept of electric field lines was introduced by the renowned physicist Michael Faraday, who utilized this method to explain the behavior of electric fields. Each line is drawn so that the tangent at any point along the line indicates the direction of the electric field at that specific location. The density of these lines in a given area corresponds to the strength of the electric field: a greater concentration of lines signifies a stronger field.

The reason electric field lines emerge from positive charges lies in the nature of electric forces. Electric forces are vector quantities, possessing both magnitude and direction. For a positive test charge, the direction of the electric force is always away from other positive charges and directed towards negative charges. Consequently, when we illustrate electric field lines, we initiate them at positive charges (to represent the repulsive force) and terminate them at negative charges (to represent the attractive force).

This convention reflects the inherent properties of electric charges. It is essential to understand that the choice of a positive test charge is a standard convention. While we could theoretically select a negative test charge, in that case, the field lines would point in the opposite direction. Nonetheless, the convention of using a positive test charge is widely accepted, as it simplifies the understanding and calculation of electric fields.

In summary, electric field lines emanate from positive charges as they visually represent the direction and magnitude of the electric field, which is fundamentally determined by the properties of electric charges.