Science > Physics > Electrostatics > Normal Electric Induction
In this article, we shall study the concept of normal electric induction, electric field, and electric flux.
Normal Electrical Induction:
The number of tubes of induction passing normally through a unit area in an electric field is called Normal electric induction.
Total Normal Electrical Induction:
The total number of tubes of induction passing normally through a given surface in an electric field is called the total normal electric induction
Electric Flux:
The number of tubes of force passing normally through a given surface in an electric field is called electric flux.
Electric Field:
When an electric charge is kept in a medium, it creates around it, what is called an electric field. If another charge is kept in this field it experiences a force of attraction or repulsion. Thus electrical charge modifies properties of surrounding space by producing an electric field. Hence we can consider the electric field is a characteristic property of the system of charges.
Electric Lines of force:
The electric field near a charge is represented by drawing a line of force. A line of force in an electric field is an imaginary line drawn in such a way that the direction of the line of force at any point is the same as the direction of the field at that point.
An electric line of force is defined as the path along with a free positive charge, moves when it is placed in an electric. However, since the direction of the field varies from point to point, the lines of force are usually curves.
Characteristics of Electric Lines of Force:
- Lines of force are imaginary. They start from a positive charge and end on a negative charge.
- A tangent drawn to a line of force at any point shows the direction of the electric field at that point.
- Two lines of force never intersect each other. Only one line of force can pass through one point in the electric field.
- The number of lines of force passing normally through a unit area in the direction of the field represents the magnitude of electric intensity.
- The line of force always originates perpendicular to the surface of a charged conductor.
- Lines of force do not pass through the conductor. They can pass through a dielectric.
- Lines of force have a tendency to contract in length. This explains electrostatic attraction between unlike charges.
- Lines of force exert lateral pressure on each other. This explains electrostatic repulsion between like charges.
Concept of Tubes of Force:
Imagine a small area of the surface of a positively charged conductor and suppose that lines of force are drawn from every point on the boundary of this area. These lines enclose a tube of force. The sides of a tube of force are formed by lines of force and the tube of force has the same properties as lines of force.
In order to obtain a relation for the tube of force, it is assumed that, in a medium of permittivity ε = εok , the number of tubes of force originating from a unit positive charge is (1/εok). Therefore the number of tubes of force originating from a charge +q is (q/εok). This is also an expression for electric flux from charge +q.
Suppose that charge +q is situated at the centre of a sphere of radius ‘r’. The total number of tubes force is (q/εok) passes normally from the surface of the sphere of normally through unit area of the surface is 4πr².
Therefore, the number of tubes of force passing normally through the unit area of the surface is
This value is also equal; to the magnitude of electric intensity (E) at a distance `r’ from a charge +q.
Concept of Electric Flux:
The number of tubes of force passing normally through a given surface in an electric field is called electric flux. (∅). As the number of tubes of force passing normally per unit area is the electric intensity (E), it is also the electric flux per unit area. S.I. unit of electric flux is Nm²/C.
Consider a small area dS in an electric field of intensity E. Let θ be the angle made by the normal drawn to area vector dS and the direction of E.
The component of electric intensity i.e. E cosθ is parallel to dS.
Electric flux per unit area = E cosθ
and Electric flux through dS area = ∅ = E cosθ . dS ……….. (1)
Also, electric flux from change q is given by ∅ = q/εok ………(2)
From equation (1) and (2) we get
Concept of Tubes of Induction:
A tube of force in any medium other than a vacuum is called tube of induction. The number of tubes of force originating from a charge depends on the permittivity of the medium and is therefore different for different media.
In order that this number does not depend on the nature of the medium, the concept of tubes of induction was introduced. According to this concept, only one tube originates from a unit positive charge, whatever be the medium surrounding this charge. Such a tube is called a tube of induction. The number of tubes of induction originating from charge +q is q.
Concept of Normal Electric Induction:
The number of tubes of induction originating from charge +q is q. If this charge is situated at the centre of a sphere of radius ‘r’, then the number of tubes of induction passing normally through unit area 4πr² of the sphere is q/4πr².
The number of tubes of induction passing normally through the unit area in the electric field is called Normal electric induction.
Normal Electric Induction is also called an electric displacement vector represented by
Concept of Total Normal Electric Induction:
The total number of tubes of induction passing normally through a given surface in an electric field is called the normal electric induction (T. N. E. I.)
If E cos θ is the component of electric intensity E parallel to the area vector of dS, then
T.N.E.I. through area dS = Normal Electric Induction × area dS.
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