Second year physics chapter 1.What is electric field lines|write about electric field line:

What is Electric field Line |defined electric field line | different cases of electric field line  |electric field line due to a positive charge| electric field line due to a negative charge| Field Lines Between Two Opposite Point Charges|Electric Field Lines in case of Two independent Positive Point Charges.

 DEF. Of Electrical Field Line

An electric field line is defined as the  path along which a unit positive charge will move in an electric field. It may be straight or curved and tangent to the line of force at any point gives the direction of electric intensity E at that point.

The concept of electrical lines of force was introduced by Michael Faraday An electric field can be represented by lines known as electric lines of force.  The direction of the electric force at a point can be indicated by a small arrow at that point. 

 Electrical field lines can be considered as a "map" that provides  information about the direction and strength of the electric field at various places 

DIFFERENT CASES OF ELECTRIC FIELD 

1. (a) Electric Field Lines Due to a Positive Point Charge: 

In order to introduce electric field lines, positive test charges having the same magnitude of charge q.  are placed at different places but at equal distances from a positive charge + q .Each test charge will be repelled by charge + q as indicated by arrows  and will move radially outward .Thus  the  electric lines of force due to positive  charge  +q  are radially  directed  outward  i.e away  from  it

(b) Electric Field Lines Due to a Negative Point Charge: 

If a negative point charge - q is placed at the center the field lines are directed radially inward, because now an attractive force is acting on the positive test charge. and thus the direction of electric field strength is inward 

As the figures show that field lines are emerging from the charges in a Fig all three dimensions, so an infinite number of lines could be drawn. From these field lines "map" it is also clear that the field is stronger near the charges where the field lines are closer to each other and weaker when the field lines spread out continuously from the charges. It is concluded that 

"The number of lines per unit area passing perpendicularly through an area is proportional to the magnitude of electric field" 

2 . Electric Field Lines in case of Two independent Positive Point Charges : 

Suppose two positive point charges are placed at some distance as shown in fig .When fields due to two identical charges are considered, then it is observed that lines almost repel each other in the same way as two like charges repel each other. Two identical negative charges willwill behave in the same way .The middle  region between  them  shows  the  presence of  a zero field spot or neutral zone .

3 . Field Lines Between Two Opposite Point Charges: 

The fig shows the pattern of two opposite charges of the same magnitude. The field lines from positive charge and end (?) on a negative charge.  The electric field at points 1,2,3 is the resultant of fields created by the two charges at these points.  The directions of the resultant intensities are given by the tangents drawn to the field lines at these points.  

4 . Electric Field Lines Between Two Parallel Plates having opposite :

 The fig shows the electric field lines between two parallel plates having opposite charges.  The field is uniform in the region where the field lines are parallel and equally spaced.  In this case, same number of lines passes per unit area at all points.  Therefore, the field is uniform on all points.

  Fig shows the field lines between the plates of a parallel plate capacitor.  The field lines are directed from the positive plate to the negative plate and are equally spaced, the before the field is uniform in the middle region.

 The fig shows when the plates are of finite (f) size, the lines are uniformly paced, parallel to the  plates inside the plates.  But the lines are curved at the edges of the plates, showing that the field is not uniform at the ends but uniform within the plates.

Properties of Electrical Fields line:

 Electrical field lines start from positive charges and end (negative charges. 

The lines of force are at right angle to the surface of conductor. 

The tangent to a field line at any point gives the direction of the electric field at that point .

The lines of force are closer together at the points where the field is stronger and are further apart at the points where the field is weaker.

 Two lines of force cannot cross each other This is because E has only one direction at any given point If the lines cross, E could have more than one direction.

 The lines of force have no physical existence (ie they are imaginary paths) They expand laterally (side ways) and contract longitudinally.