The properties of electric field lines are:
The electric field lines start from a positive charge and end at negative charges or at infinity. For a positive point charge the electric field lines point radially outward and for a negative point charge, the electric field lines point radially inward. (Fig. (a) and (b)).
Note that for an isolated point charge the electric field line starts from the charge and ends only at infinity. For an isolated negative point charge the electric field lines start at infinity and end at the negative charge.
The electric field vector at a point in space is tangential to the electric field line at that point.
The electric field lines are denser (more close) in a region where the dense in a region where the electric field is of smalller magnitude. In other words, the number of lines passing through a given surface area perpendicular to the lines is proportional to the magnitude of the electric field in that region.
Figure shows electric field lines from a positive point charge. The magnitude of the electric field for a point charge decreases as the distance increases `(|vecE|prop1/r^2)`. So the electric field has greater magnitude at the surface A than at B. Therefore the number of lines crossing the surface A is greater than the number of lines crossing the surfaceB. Note that at surface B the electric field lines are farther apart compared to the electric field lines at the surface A.
No two electric field lines intersect each other. If two lines cross at a point, then there will be two different electeic field vectors at the same point.
As a consequence, if some charge is placed in the intersection point, Then it has to move in two different directions at the same time, which is physically impossible. Hence, electric field lines do not intersect.
The number of electric field lines that emnate from the positive charge or end at a negative charge is directly proportional to the magnitude of the charges.
For example in the figure, the electric field lines are drawn for charges +q and -2q. Note that the number of field lines emanating from the +q is 8 and the number of field lines ending at -2q is 16. Since the magnitude of the second charge is twice that of the first charge, the number of filed lines drawn for -2q is twice in number than that for charge +q.
