Electric Field : Definition, Properties

Electric Field

The electric field due to a charge is the space around the charge in which any other charge experiences a force of attraction or repulsion.

Let’s consider an electric charge +Q located at point O in space as shown in Fig.1.

Fig.1

If another charge +q is placed near it at point P , then the charge +q experiences a force of repulsion.

This is because, the charge +Q  sets up an electric field around it. Since the point P is inside the electric field, +Q exert a force on +q.

The charge +Q is called the source charge as it produces the electric field.

The charge +q is called the test charge. The test charge should be as small as possible so that its presence does not affect the electric field produced by the source charge.

Theoretically, the electric field due to a charge extends up to infinity. However the effect of electric field dies quickly as the distance from the source charge increases.

Electric Line of Force

Electric field due to a charge or group of charges is represented by  electric line of force or  field lines.

An electric line of force can be defined as a path along which a small positive test charge would move if it is  free to do so.
An electric line of force always originates from a positive charge and ends on a negative charge.

Fig.2

The relation between field lines and electric field can be described as follows :

• The number of field lines emerging from a positive charge or terminating at a negative charge is proportional to the magnitude of the electric charge.
• The field lines point in the direction of the electric field.
• The field lines may be straight. The direction of electric field at any point is given by the direction of tangent to the field line at that point.
• The separation of neighbouring field lines indicate the field strength in that region. If the field lines are close together, the electric field in that region is relatively strong. If the field lines are far apart, the field is weak. We can see in Fig,2 , the field lines are  close together where the force on the test charge is large.

Thus, representation of electric field by field lines allows us to infer relative field strength as well as direction of the electric field.

Properties of Electric Lines of Force

The properties of electric lines of forces are as follows:

• The electric field lines are directed away from a positive charge and towards a negative charge so that ay any point, the tangent to a field line give the direction of electric field at that point.
• Electric field lines  enter or leave the charged surface normally.
• Electric lines of force can not pass through a conductor. This means electric field inside a conductor is zero.
• Electric lines of force never intersect with each other. If two electric lines of force intersect each other at a point, then two tangents can be drawn  at that point. This would mean at that point there would be two direction of electric field, which is impossible.

• Electric lines of force have tendency to contract in length. This explains attraction between oppositely charged bodies.
• Electric lines of force have tendency to expand laterally. This means they tend to separate from each other in the direction perpendicular to their length. This explains repulsion between like charges.