Electric Flux

Electric Flux

• In 1837, Michael Faraday performed the experiment on electric field. He showed that the electric field around a charge can be imagined in terms of presence of the lines of force around it. He suggested that the electric field should be assumed to be composed of very small bunches containing a fixed number of electric lines of force. Such a bunch or closed area is called a tube of flux. The total number of tubes of flux in any particular electric field is called as the electric flux.

Key Point : Thus the total number of lines of force in any particular electric field is called the electric flux. It is represented by the symbol ψ Similar to the charge, unit of electric flux is also coulomb C.

1. Properties of Flux Lines

• The electric flux is nothing but the lines of force, around a charge. Such electric flux lines have following properties,

1. The flux lines start from positive charge and terminate on the negative charge as shown in the Fig. 3.2.1.

2. If the negative charge is absent, then the flux lines terminate at infinity as shown in the Fig. 3.2.2 (a). While in absence of positive charge, the electric flux terminates on the negative charge from infinity. This is shown in the Fig. 3.2.2 (b).

3. There are more number of lines i.e. crowding of lines if electric field is stronger.

4. These lines are parallel and never cross each other.

5. The lines are independent of the medium in which charges are placed.

6. The lines always enter or leave the charged surface, normally.

7. If the charge on a body is ± Q coulombs, then the total number of lines originating or terminating on it is also Q. But the total number of lines is nothing but a flux.

Electric flux ψ = Q coulombs (numerically)

• This is according to SI units. Hence if Q is large then flux ψ is more surrounding the charge and viceversa.

• The electric flux is also called displacement flux.

• The flux is a scalar field. Let us define now a vector field associated with the flux called electric flux density. 

Review Question

1. Define electric flux and state its unit.