Electric Displacement

What is Electric Displacement? 

Electric displacement primarily refers to the displacement of electric charge across a conductor positioned in an electric field. It fundamentally measures the charge per unit area, and you can also call it electric flux density. 

Therefore, it applies to free electric charges, thus eliminating the involvement of charges present in molecules or neutral atoms. It functions mainly because an electric field exists between two uncharged conductors when charge flows through them. 

As a result, one of the conductors assumes a positive charge, while the other possesses a negative charge. This, in turn, leads to the creation of an electric field between these conductors. This concept, therefore, aims to find the free charge surface density. Moreover, it relates to the volume of free charge on one conductor, concerning the surface area of this conductor. 

How Does Electric Displacement Work? 

Electric displacement thus calculates the density of electric flux within a charged field. However, it takes into account the instance when you introduce a dielectric into the apparatus. You must note, a dielectric refers to an insulating material that lacks any free or loosely-bound electrons. 

Consequently, they fail to necessitate the movement of free electric charges along their surface area. However, if you introduce an insulated object between an electric field, it witnesses a degree of polarization. It is because the strongly-bound negative charges move slightly towards the positive conductor. 

On the other hand, bound positive charges move by a certain extent towards the negative plate. This phenomenon results in the overall decrease in the electric field. 

Therefore, electric displacement ventures to find reactions of dielectric materials as they come in contact with electric fields. 

What is the Equation for Electric Displacement? 

First of all, you should note that ‘D’ stands for electric displacement. On the other hand, ‘E’ refers to the electric field that comes into the question. 

However, you should take a look at the following table to understand other components of this equation – 


Vacuum Permittivity 


Polarization Density

Therefore, the equation to find the electric displacement in a dielectric material is -  

D = ε0E + P

Its SI unit is C m-2 or Coulomb per meter square. In this unit, Coulomb stands for the unit of electric charge, whereas m-2 is the area of the material. 

Besides, you should also know what vacuum permittivity and polarization density imply. It will help you understand the factors working in the phenomenon even better. 

Vacuum permittivity: It is a physical constant that refers to the capability of an electric field to permeate a vacuum. It is, however, a relation between the units of electric charge and force. Length also becomes an essential component of this concept. Its value is ε0 = 1/μ0c2

In this case, μ0 stands for magnetic constant, and c refers to a constant value of 299,792,458 m s-1

Polarization density: It indicates a vector field that deals with the density of dipole moments in a dielectric object. As discussed above, the introduction of such an object within an electric field results in a movement of charges. It refers to the electric dipole moment which polarizes a given object. 

Therefore, polarization density finds the electric dipole moment per unit volume of the dielectric material. 

How is Electric Displacement a Vector Quantity? 

When you introduce a dielectric material within an electric field, it undergoes a shift in the electron cloud of the free electrons. Consequently, it is known as a dipole. The negative and positive charges in the dielectric show an affinity towards positive and negative plates, respectively. 

As a result, they travel in specific directions along a particular line. Electric displacement vector thus measures the dielectric polarization vector. 

Therefore, electric displacement density duly measures the vector flux of electric density in a given dielectric material. On the other hand, its unit in the meter-kilogram-second system is Coulombs per meter square or C m-2

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FAQ (Frequently Asked Questions)

1. What is Electric Displacement? 

Ans. Electric displacement refers to the charge per unit area of a dielectric material when in contact with an electric field. 

2. What is the Equation for Electric Displacement? 

Ans. Electric displacement D = ε0E + P. In this case, ε0 is vacuum permittivity, E is electric field, and P is polarization density. 

3. What is the SI Unit of Electric Displacement? 

Ans. The SI unit of electric displacement is Coulomb per meter square or C m-2