Michael Faraday (1791-1867) introduced the concept of lines of force for visualizing electric (and magnetic) fields.
An electric line of force is an imaginary curve drawn in such a way that the tangent at any point gives the direction of the electric field at that point.
Lines of force are imaginary, but the field they represent is real. These lines can be used to represent the field of a system of charges also. This field representation helps us to get an idea of the magnitude and direction of electric intensity at a point in a field.
A line of force AB to find the direction of electric intensity L, at a point C, draw a tangent to the curve at C Similarly the tangent to the curve drawn at D gives the direction of the electric intensity at D2Properties or Characteristics of Lines of Force:
1. Every line of force, in an electrostatic field, originates from a positive charge at one end and terminates on a negative charge at the other end. They are discontinuous curves. A line of force starts from the positive charge at the point A and ends on the negative charge at the point B. On reaching B this line does not return to the point B. [But in the case of a magnet it is not so. The lines of force start from the North Pole reach the South Pole then passes through the body of the magnet and returns to the North Pole. It is a continuous curve]
2. The tangent to the field lines give the direction of the electric intensity at that point.
3. Field lines never intersect. If they intersect at a point then the same electric intensity will have two different directions, even though the magnitude is the same. The two tines of force AB and CD intersect at P. From the figure it can be seen that the electric intensity at the point P has two different directions along PE and PF which is impossible.
4. We can draw lines of force through every point of an electric field. The entire space and surface would be completely filled with lines and no individual line could be distinguished. But by suitably limiting the number of lines of force passing through a given area, the lines can be used to represent the magnitude as well as the direction of the field. If the field is intense the lines are closely spaced and if the field is weak the lines are widely separated.The electric field is intense at A and B. So the lines are crowded. But at C and D the field is weak. So the lines are widely spaced.