Final answer:
Determining the type of point charge (- or +) can be inferred from the direction of electric field lines; lines radiating out indicate a positive charge and converging lines indicate a negative charge. The density of these lines gives insight into the magnitude of the electric field.
Step-by-step explanation:
Understanding whether a point charge is positive or negative can be determined by examining the direction of the electric field lines in space. If the electric field lines are diverging out from a point, it indicates the presence of a positive charge, as the electric field direction is defined by the force that a positive test charge would experience. Conversely, if the electric field lines are converging towards a point, it represents a negative charge, since a positive test charge would be attracted and thus move towards the charge.
In figures such as Figure 18.18 and 18.19(b), it is illustrated that for a positive charge, the electric field lines point away from the charge, and for a negative charge, they point towards the charge. Figures like Figure 18.17 are useful for visualizing this concept in a three-dimensional space, showing how field lines emanate from a positive charge. If one were to sketch the electric field lines near a positive point charge, they would draw the lines radiating outward from the charge, as in Figure 5.29(a).
The density of the electric field lines can also be used to infer the magnitude of the electric field. Closely spaced lines indicate a strong electric field while lines spaced further apart suggest a weaker electric field. In the case of multiple charges, such as two opposite charges forming a dipole in Figure 5.29(b), the electric field lines would originate from the positive charge and terminate at the negative charge, indicating the direction of the field at every point is tangent to these lines.