Electric field

Electric Field:

Concept of Field :

When an electric charge is placed at a point, the properties of space around the charge get modified. 

The modified space around an electric charge is called electric field. The charge is known as the source of electric field. 

There are other examples in physics where the properties of space get modified due to presence of a body (called source). The modified space in those cases is also stated in terms of a field. 

• Presence of mass of a certain point produces a field, known as magnetic field, around it. 
• Presence of a magnetic pole at a point produces a field, known as magnetic field, around it. 

Origin of force between two sources:

Force between two sources arises due to the interaction of two fields of a similar nature. If we place two electric charges at two points, they will produce their individual electric fields around themselves. These two fields shall interact with each other to produce a force called coulomb's force. If there is only one charge, it will have its own electric field. It will not experience any force since there is no second field to interact with. 

For the production of force, the two fields should be of similar nature. If we have a charge 'q' and a mass 'm'. There is the presence of an electric field and a gravitational field, but there is no force between the two. Similarly, a charge (source of electric field) and a magnetic pole (source of magnetic field) do not produce a force. 

Diagrammatic Representation of Electric :

Following two ways are, generally, used to present an electric field. 

1) Representation by lines of force

      Based upon the characteristics of lines of force, an electric field can be represented by a set of lines of force. 

(a) Magnetic intensity. Magnitude of electric intensity, at any point, in the field is given by the number of lines of force passing through a unit area normally to the lines of force at that point. 

(b) Direction of intensity. Direction of intensity, at any point, is given by the direction of tangent to the lines of force passing through that point. 

Fig. 1. Lines of force representation of electric field. 

1. [Fig. 1(i)] shows a uniform electric field, acting perpendicular to the plane of paper, directed outwards. 
2. [Fig. 1(ii)] shows a uniform electric field, acting perpendicular to the plane paper, directed inwards. 
3. [Fig. 1(iii)] shows a strong uniform electric field, in the plane of paper, directed from left to right. The source of this field is at infinity. 
4. [Fig. 1(iv)] shows a weak uniform electric field, in the plane of paper, directed from left to right. The source is at infinity. 
5. [Fig. 1(v)] shows a non-uniform field having its source (positive) towards left. 
6. [Fig. 1(vi)] shows a non-uniform field having its source (negative) towards right. 

2) Representation by force of vectors :

Electrostatic field can also be represented by electric field vectors or force vectors. 

A vector E representing in the direction of tangent at P and having a length proportional to the strength of field at P is known as electric field vector or force vector 

Consider a charge +q situated at O [Fig. 2]. It is surrounded by an electric field whose strength decreases with an increase in distance. Let A, B and C be the three points situated along a line at increasing distances from q. Let E₁, E₂, E₃ be the electric intensities at A, B and C respectively. These vectors are known as electric field vectors or force vectors. They have the same direction but decrease in magnitude with increase of distance. Therefore, they are represented by arrows of gradually decreasing lengths. A similar set of arrows, along another line, at A', B', C' are also shown. The direction of arrow represents the direction of field at that point. An electric field having these force vectors at its different points is said to be represented by force vectors or field vectors. 

Fig. 2. Electric field in terms of field vectors. 

If the negative charge is placed at O, then the field vector will point towards the charge. 

The main advantage of this representation us that we can have an idea of the relative magnitude and the direction of electric intensity at a point in an electric field directly from the diagram. 

Representation of electric field by force vectors in shown in [Fig. 2]. 

[Fig. 2(i)] represents a non-uniform field while [Fig. (ii)] represents a uniform field.