I don't understand these explane me please .... (Two electric lines of force naver intersect each other ?????)with diagram
Lines of force are a way of visualizing the vector sum of the repulsion/attraction forces due to the nearby charges.
Let me begin by pointing out how a test point would behave at a point where two lines of force do intersect. That would mean that the force on a test point introduced at that point would have 2 separate directions that it could move under influence of the lines of force. I think I can explain it to you with this visualization of "lines of force":
Imagine a positive test charge introduced into an area with 2 positive charges fixed to locations relatively close together. The test charge will feel repulsive forces from both of the fixed charges. The net force on the test charge is the resultant of the 2 repulsive forces due to those 2 charges. The line of force at the point of that test charge is in the direction of the net force the test charge experiences.
If you move the test charge a short distance in the direction of that net force, and note the direction of the net force now on the test charge, you will likely notice that the line of force has curved. Continue moving the test charge for a ways, in the direction of the net force, noting change in direction of the net force as the position is changed.
Repeat from the first position as you move the test charge in the direction exactly opposed to the net force. This would be in general toward the 2 fixed charges.
You are tracing the path of one line of force in the space around those 2 fixed charges.
To summarize the definition of lines of force, it shows the vector sum of the repulsion/attraction (electric field) due to the nearby charges.
I hope this helps,