Question

when we draw electric field lines between a positive charge and a negative charge of equal magnitude ,we don't consider the field lines coming from infinite whereas when we draw alone the negative charge we bring the electric field lines from infinity .. explain it?

Answer 1

Because, at infinite distance, the field magnitude is nearly 0.

Answer 2

them from a negative charge gets electric field

Answer 3

This happens because when you add the vectors from a negative and positive charge together they end up adding to be on top of each other. For instance:
So consider at every point along that line. As you're closer to the negative charge the electric field is pointing out to infinity very strongly, but once you move away, the electric field gets weaker from the negative and the positive gets stronger, bending the line back towards the charge.

Answer 4

The cancelation of the negative and positive e-fields give rise to an E-field that is "circular" in nature away from the axis of the dipole. The YouTube video referenced below gives more detail on this
http://youtu.be/tWC3tttsfNk

Answer 5

We don't consider the field lines coming from infinity because if we consider the case of looking this charge configuration from infinity, we can only see that a point charge of net magnitude 0 is visible as +Q and - Q at infinity are as good as (Q-Q) at infinity. This is an analogy which can be observed in the real world as we can see that a distant horizon appears like a single point where as the resolution increases as we move closer to the point.

Thus since the net charge adds up to be zero, we can conclusively say that the field at infinity is 0.

Also, we know the electric field falls off as 1/r2 with increasing r. Therefore at infinity the field will be negligible and hence we consider it to be zero which is a very good approximation for large distances.