Electric Field Lines
Electric charges create an electric field in the space surrounding them. It is useful to have a kind of “map” that gives the direction and indicates the strength of the field at various places. Field lines, a concept introduced by Michael Faraday, provide us with an easy way to visualize the electric field.
“An electric field line is an imaginary line or curve drawn through a region of space so that its tangent at any point is in the direction of the electric field vector at that point. The relative closeness of the lines at some place give an idea about the intensity of electric field at that point.”
Properties of electric field lines:
(1) Electric field lines always begin on a positive charge and end on a negative charge and do not start or stop in mid-space.
(2) The tangent to a line at any point gives the direction of E at that point. This is also the path on which a positive test charge will tend to move if free to do so.
(3) The number of lines leaving a positive charge or entering a negative charge is proportional to the magnitude of the charge.
This means, for example that if 100 lines are drawn leaving a + 4µC charge then 75 lines would have to end on a –3µCcharge.
(4) Two lines can never intersect. If it happens then two tangents can be drawn at their point of intersection, i.e. intensity at that point will have two directions which is absurd.
(5) The electric field lines can never form closed loops as a line can never start and end on the same charge.
(6) In a uniform field, the field lines are straight parallel and uniformly spaced.
(7) Electric field lines also give us an indication of the equipotential surface (surface which has the same potential)
(8) Electric lines of force ends or starts normally from the surface of a conductor.
(9) Electric field lines always flow from higher potential to lower potential.
(10) In a region where there is no electric field, lines are absent. This is why inside a conductor (where electric field is zero) there, cannot be any electric field line.
Electric field lines due to single point charge:
Electric field lines due to pair of positive and negative charge:
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