An electric line of force is the path along which a unit positive charge would move , if it is free to do so .
The electric line of force points in the direction of electric field. The electric line of force may be straight or curved . In case , electric line of force is a curve , then direction of electric field at any point is given by the tangent to the electric line of force at that point as shown in the figure below
The strength of the electric field at any point is measured as the number of lines of force crossing a unit area held normal to the lines of force at that point.
Properties of electric lines of force
The electric lines of force have the following properties:
1. The lines of force Start from the positive charge and end at the negative charge.
2. The lines of force originate ( from a positive charge) or terminate ( at a negative charge) always at right angles to the surface of the charge.
3.The lines of force do not pass through a conductor. It indicates that the electric field inside a conductor is always zero.
4.The lines of force never intersect each other.In case,two lines of force intersect at a point,then it will mean that at that point,electric field has two directions.As it is not possible,the lines of force can never intersect each other.
5.The relative closeness of lines of force in different regions of space gives the idea about the relative strengths of the electric field in different regions.
The region,where lines of force are closer,the electric field is strong and the region, where the lines of force are farther apart, the field is weak.
6.The lines of force contract longitudinally I.e., lengthwise.
7.The lines of force exert a lateral pressure on each other.
This property of lines of force leads to explain the repulsion between two like charges.
An system of two equal and opposite charges separated by a certain distance is called an electric dipole. .
Given figure represents an electric dipole consisting of two charges -q and + q are separated by distance AB = 2a . The distance AB is called length of the dipole and is a vector 2a , whose direction is from the charge -q to charge + q .
The molecules of water, ammonia, etc behave as electric dipoles . It is because, the centres of positive and negative charges in these molecules lie at a small distance from each other.
ELECTRIC DIPOLE MOMENT:
It is defined as the product of either charge and the length of the electric dipole.
It is denoted by vector p , which has the same direction as that of vector 2a.
Vector p. =. q (vector 2a)
In SI unit of electric dipole moment is Coulomb metre ( C m).
ELECTRIC FIELD ON AXIAL LINE OF AN ELECTRIC DIPOLE.
Consider an electric dipole consisting of charges – q and + q , separated by a distance 2a and placed in free space . Let P be a point on the line joining the two charges. ( axial line ) at a distance r from the center O of the dipole.
The electric field E (vector) at point P due to the dipole will be the resultant of the electric fields Eá ( due to charge – q at the point A ) and EB ( due to charge + q at the point B ) I.e.
E = EA + EB
When dipole is of very small length.
If the dipole is of small length, such that
ELECTRIC LINES OF FORCE