Electric field line : Electric field line is a straight or curved imaginary line in a region such that the tangent at any point in the field line gives the direction of the electric field at that point in the region.
The theoretical concept of electric field lines was introduced by Faraday.
(i) Electric field lines for a single static positive point charge have direction radially outwards.
(ii) Electric field line due to a static negative point charge have direction radially inwards.
(iii) Electric field line for two equal postive point charges : Pont N represents the neutral point , where net electric field intensity due to both charges is zero and no electric field line passes through this point. Neutral point lies at the exact middle of the distance two similar and equal charges.
Type of electric field :
(i) Uniform electric field : An electric field which has same magnitude and direction at every point in a region is called uniform electric field. It is represented by Straight , equispaced parallel field lines.
(ii) Non- Uniform Electric field: The electric field which has different magnitude and directions at different points in a region is called non-uniform electric field. It is represented by unequally spaced and curved electric field lines.
Properties of Electric Field Lines:
1. The electric field lines begin from a positive charge and terminate or end on a negative charge . For an isolated point charge, electric field lines may start or end at infinity.
For example, for a positive point charge, electric field lines start from the positive charge and end on at infinity. Similarly, for a negative point charge, electric field lines start from infinity and end on the negative charge.
2. Electric field lines are imaginary lines but they are used as a pictorial representation of the electric field, which is real physical quantity. Electric field lines show the electric field at various positions in the space.
3. The tangent at any point on an electric field line gives the direction of the electric field at that point.
4. Two electric field lines do not cross each other. If two electric field lines cross each other, then, there will be two tangents at the point of intersection. It means that there are two directions of the electric field at the point of intersection, which is not possible. Hence, two electric field lines do not cross each other.
5. The number of electric field lines per unit cross-sectional area perpendicular to the field (i.e., density of electric field lines) is proportional to the magnitude of the intensity of electric field in that region. That is, density of electric field lines represents the magnitude of the electric field. Thus, the electric field lines are closer (crowded), where the electric field is stronger and the field lines spread out, where the electric field is weak.
6. Electric field lines do not form closed loops. Electric field lines start from a positive charge and end on negative charge. They never go from a negative charge to a positive charge. Hence they do not form closed loops.
7. In a charge free region, electric field lines are continuous curves without any break. An electric charge experiences a continuous force in an electrostatic field. The electric field line cannot have a sudden break because the charge in the electric field moves continuously and does not jump from one point to another point in the field. That is, why electric field lines are continuous curves with any break.
8. Electric field lines contract lengthwise to represent attraction between two unlike charges.
9. Electric field lines exert lateral (sideways) pressure on each other to represent repulsion between like charges.
10. Electric field lines are perpendicular to the surface of a positively or negatively charged sphere.
11. Electric field lines do not pass through a conductor and the interior of a conductor is free from the influence of the electric field.
12. Electric field lines pass through dielectrics.
13. The number of electric field lines are proportional to the magnitude of the charge. If $N_1$ be the number of electric field lines due to a charge of magnitude $|Q_1|$ and $N_2$ be the number of electric field lines due to a charge of magnitude $|Q_2|$, then
$\frac{N_1}{N_2} = \frac{|Q_1|}{|Q_2|}$