Strength of electric field or electric field intensity

 Electric field intensity (E):

To measure the strength of the electric field at any point we place a test charge 'q₀' at the point. The test charge 'q₀' shall have it's own electric field. But we will keep in mind that the magnitude of the test charge 'q₀' to be so small that it doesn't disturb the location of the source charge or the charge distribution. 

Definition:

The strength of electric field or electric field intensity is measured by the force expresses per unit positive charge placed at that point and the direction of the field is given by the direction of the motion of the unit positive charge if it were free to do so. 

E=F/Q

Unit of E

 in S.I= Newton/Coulomb

In C.G.S (esu) = dyne/stat coulomb

In C.G.S (emu) = dyne/ab coulomb

The unit of E= joule/coulomb*meter        

                       or

The unit of E= volt/meter

Dimension of Electric field intensity:

Electric field intensity, 

       E = F/q₀

       F = [M¹ L¹ T-2 ] and q₀ = [A¹ T¹]

∴ E = [M¹ L¹ T-2] / [A¹ T¹]

E = [M¹ L¹ T-3 A-1 ]

So, dimension of electric intensity are, 1, 1, -3 and -1 in mass, length, time and electric currents respectively.

Characteristics of Electric Intensity:

1) Electric intensity at any point, inside an electric field is the force experienced per unit charge at that point.

2) Electric intensity is a vector quantity. Its direction is given by the direction of motion of positive charge.

3) Electric intensity varies inversely as the square of distance if the given point from the source charge. A graph showing variation of E with r is shown in Fig.

Variation of 'E' with 'r'

4) Dimensional formula of electric intensity E :

[E] = force/charge = [M¹ L¹ T-2 ]/[A¹ T¹]

=  [M¹ L¹ T-3 A-1 ]

5) Units of electric intensity E :

E = force/charge = N/C = NC-1

E = N/C = N×m/C×m = J/C×m = JC-1m-1

E = N×m/C×m = J/C × 1/m = Vm-1

6) Electric intensity at any point, due to a source charge varies inversely as the relative permittivity (εr) of the medium in between.

7) Electric intensity of an electric field is the negative gradient of potential at a given point.

8) Electric intensity inside a charged conductor is zero.

Important notes:

1. Electric intensity at any point, is due to the interaction of two electrostatic fields, one due to source charge 'q' and the other due to test charge q₀.
2. If q > 0, the electric intensity vector points radially outward from the point. 
3. If q < 0, the electric intensity vector points radially inwards towards the point. 
4. Coulomb's law holds good strictly for charges at rest. A moving charge whose velocity is very small as compared to velocity of light shall produce the same electric field as that produced by a charge at rest. 
5. Force experienced by a charge moving in an electric field is independent of its velocity.