Electric field lines :
Electric field lines map the electric field pictorially around a configuration of electric charges. The concept was developed by Faraday and is a nonmathematical way of visualizing electric fields around charged configurations. We may define,
An electric field line as a path, straigth or curved, such that tangent to it at any point gives the direction of electric field intensity at that point.
In fact, it is the path along which a unit positive charge actually moves in the electrostatic field, if free to do so. AB is an electrostatic line of force. The tangent to the line at any point P gives us the direction of electric intensity Ep at P. Similarly, tangent to AB at Q gives us the direction of EQ. The field lines are in three-dimensional space.
Some lines of force are due to a single positive point charge. These are directed radially outward. On the contrary, lines of force due to a single negative point charge are directed radially inwards. The lines of force extend to infinity.
Lines of force due to pair of equal and opposite charges (q, - q), forming a dipole. They show clearly the mutual attraction between the opposite charges.
The lines of force are due to two equal positive point charges (q, q).
As is clear, the field lines due to two charges exert lateral pressure on each other, which is responsible for the repulsion between the like charges. Further, point P represents the neutral point, where net electric field intensity is zero.
Field lines for a section of an infinitely large sheet of positive charge. When the charges are equal, P lies at the center of the line joining the charges. However, when the charges are unequal, the neutral point P is closer to the smaller charge.
Properties of electric field lines :
- Electric field lines are discontinuous curves. They start from a positively charged body and end at a negatively charged body. No electric lines of force exist inside the charged body. Thus, electrostatic field lines do not form any closed loops.
If there is a single charge, the electric field lines may start or end at infinity. For example, field lines would start from the charge and end at infinity due to a single positive charge. Due to a single negative charge, field lines would start from infinity and end at the negative charge.
We shall study in unit 3 that magnetic lines of force are continuous (or endless) as against the electric lines of force which are discontinuous.
- Tangent to the electric field lines at any point gives the direction of electric intensity at that point.
- No two electric lines of force can intersect each other. At the point of intersection P, we can draw two tangents PA and PB, to the two lines of force.
This would mean two directions of electric intensity at the same point, which is not possible. Hence no two lines of force can cross each other.
- The electric field lines are always normal to the surface of a conductor, both while starting or ending on the conductor. Therefore, there is no component of electric field intensity parallel to the surface of the conductor.
- The electric field lines contract longitudinally on account of attraction between unlike charges.
- The electric field lines exert lateral pressure on account of repulsion between like charges.
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