Electric Field Intensity


An electric field is defined as the space around a charge or charged body in which other charges (less in magnitude than source charge) experience an electric force. It can be further defined as the force experienced per unit test charge.

It is a vector quantity and is directed away from the positive charge as well as directed towards the negative charge.

The Electric fields intensities are important in many areas of physics and are exploited practically in electrical technology as well. Taking an atomic scale, we can define that the electric field is responsible for the attractive force between the atomic nucleus and electrons that hold atoms together and the forces between atoms that cause chemical bonding.

Strength of Electric Field

The electric field is a function of vector position. Electric field intensity at any point in the electric field is defined as the force experienced by unit test charge at that point.

[Image to be added Soon]

E=F/q =Force (Electrostatic force experienced per unit test charge)

Here F =KqQ/r2

Thus, E=KQ/r2

We can see that the electric charge is independent of the test charge that is placed to determine the field thus is totally characteristic of the system of charges.

Here, q is the test charge and Q is the charge due to which field is to be found out.

Thus the SI unit of the electric field is NC-1

Electric Field Intensity Direction

Electric Field Intensity due to a Point Charge

[Image to be added Soon]

Electric field intensity due to a positive point charge is directed towards it while due to negative point charge, it is directed away from it.

Electric Field Intensity due to the System of Discrete Charges

[Image to be added Soon]

The electric field intensity due to the system of discrete point charges is obtained by vector addition of intensity due to the number of charges q1, q2, ……….. ... qn. The resultant electric field intensity at that point due to these charges is given by the superposition theorem.

Electric Field Intensity due to the Uniform Charge Distribution

For finding electric fields due to continuous charge distribution, we consider the term charge density as a measure of electric charge per unit length for linear distribution, the charge per unit area for areal distribution and charge per unit volume for volumetric distribution. A surface charge density σ at an area element is given by

The linear charge density λ of a wire is defined by λ=ΔQ/Δl, where λ is the linear charge density, Δl is a small line element of wire and ΔQ is the charge in that elemental portion

The areal charge density for a sheet of charge is given by σ=ΔQ/ΔA

where ΔQ is the charge and ΔA is the area element.

The volume charge density is defined by 


where ΔV is volume element and ρ is volume charge density.

Force Experienced by a Charge in an Electric Field

The electrostatic force experienced by a charge of magnitude q placed in an electric field of strength E is given by:

If q is positive, the force acts in the direction of E and equals to F=qE

If q is negative, the force acts in the direction opposite to E and is given by F=qE 

This force is experienced on a charge placed in an electric field whether it is at rest or motion and this force is independent of mass as well as the velocity of the charged particle. The force experienced by a proton and an electron in the same electric field is equal in magnitude and opposite in direction.

Also, if the charge has some initial velocity, the force on the proton is accelerating in nature while that on the electron is retarding force.

The ratio of acceleration of proton/Retardation of electron = mass of an electron/mass of a proton.

Electric Lines of Force

Electric lines of force are imaginary lines which are continuous straight or curve, tangent to a point on the line, gives the direction of electric field vector and number of lines per unit area that is the density of lines is proportional to the magnitude of E.

[Image to be added Soon]

The exact strength of the electric field is measured by the total number of lines passing per unit area of the surface which is passing perpendicular to the surface.

Properties of Electric Lines of Force

  1. Electric lines of force start from the positive charge and end at the negative charge.

  2. Number of electric lines of force start or end on the charge is proportional to their magnitude

  3. Electric lines of force do not form a closed loop.

  4. Electric lines of force do not cross each other.

  5. Electric lines of force are always perpendicular to the conducting surface.

FAQ (Frequently Asked Questions)

Q1. Explain how Intensity is Related to the Electric Field?

Ans: The electric field is related to the intensity as the energy carried by any of the waves is proportional to its squared amplitude. For the wave of electromagnet this means that intensity can be expressed as lave= c0E 202 I ave= c0E022, Where average intensity is written as W/m2 and E0 is the maximum electric field strength.

Q2. Electric Field Intensity is Which Quality?

Ans: the electric field is defined as the strength of vector quantity; it has both direction and magnitude. The electric field strength magnitude is defined in terms of how it is measured. Electric field charge can be denoted by symbol Q. 

Q3. Explain the Importance of Electric Field Intensity.

Ans: The electric field intensity is an elegant way for characterizing the electrical environment of charge of a system. At any point in space, the electric field intensity of charges represents the force unit as a positive test charge would experience if placed at a point.

Q4. Explain the Term Electric Field Intensity.

Ans: The electric field intensity is the magnitude at a point of an electric field is equal to the force that would be exerted on a small unit charge placed as a point. This is how we define the electric field intensity of a field at any point.