# Electric Field Intensity (Field Strength)

### Electric Field Intensity or Field Strength (E)

The intensity of electric field at any point is determined by the force acting on a unit positive charge placed at that point.

#### Definition

Electric intensity or field strength at a point in an electric field is the force acting on a unit positive charge placed at that point. Its direction is the direction along which the force acts.

Electric intensity at a point,

Where  Q = Charge in coulombs placed at that point

F = Force in newtons acting on Q coulombs

• Since electric intensity is a force, it is a vector quantity possessing both magnitude and direction.
• Electric intensity can also be described in terms of lines of force. Where the lines of force are close together, the intensity is high and where the lines of force are widely separated, intensity will be low.
• Electric intensity can also be expressed in V/m ; 1 V/m = 1 N/C

#### Electric Field Intensity due to a Point Charge

Consider a point charge +Q placed at point O.

Then magnitude of electric field intensity at point P at a distance d from O is given by :

The above expression gives the magnitude of electric field intensity arising from charge +Q at any point at a distance d from +Q.

#### Electric Field Intensity due to a Group of Point Charges

The resultant electric field intensity at a point due to a group of point charges can be found by applying superposition principle.

Thus electric field intensity at a point P due to n point charges (Q1, Q2, Q3,…….. Qn) is equal to the vector sum of electric fields due to Q1, Q2, Q3,…….. Qn at point P i.e.,

#### Electric Flux (ψ)

To compare electric fields, we usually use the term electric flux (ψ) instead of electric lines of force.

The electric flux is measured in coulombs.

Thus, a body charged to Q coulombs emits a total electric flux of Q coulombs i.e.,

Electric flux, ψ = Q coulombs

#### Flux Density (D)

The electric flux density is defined as the electric flux passing normally through a unit area. Thus, if  an electric flux of Q coulombs is passing normally through an area A m, then,

Electric flux density,

The ratio D/E is equal to the absolute permittivity (ε) i.e.,

D/E = ε

Or, D = εE = ϵ0ϵrE