Electric Field Intensity | Definition and Physical Significance.

What are Electric Field and Electric Field intensity?

Electric Field Intensity | Definition and Physical Significance.

Electric field intensity is a very common term in Electromagnetism representing the strength of the Electric field. This article elaborates its definition in simple words.

What is Electric Field?

A field is a general term analogous to function in mathematics. The value of the field at a particular point is defined by the specific function. The field may be a scalar field or a vector field.

We know the Coulomb’s Law stating that there exists a force between any two charges. So, let us consider any charge Q is present as shown in the figure. Now if you try to put any other charge, say q,  in the space around Q then q will experience a force.

Field around Q

So we may consider that there is an invisible field around the stationary charge called as an Electric Field.

Electric Field Intensity (E)

It is defined as the force per unit charge when placed in the electric field. 

Let us say an electric field E is present in the space as shown in the figure.

Test charge in Electric field

If we put other charge q in this field then according to Coulomb’s Law, it will experience a force say F. So electric field intensity E is defined as,

E=\frac Fq

Generally, the charge q is considered as the unit test charge. So,

E=\lim_{q\rightarrow0}\;\frac Fq

As seen from the formula, the unit for E is \frac NC. But it is commonly represented with unit \frac Vm.

Now if the electric field is arising because of the point charge Q then the figure is as shown below. Let us find Electric field intensity E at a distance r from it. So consider the unit test charge ‘q’ is placed at the distance of r from Q.

Electric field because of point charge

Now, according to Coulomb’s Law, the force acted on the q is given as,

F=\frac1{4\pi\varepsilon}\frac{Qq}{r^2}

Now, by definition, the electric field intensity is the force per unit test charge.

E=\frac Fq=\frac1q\times F=\frac1q\times\frac1{4\pi\varepsilon}\frac{Qq}{r^2}

\therefore E=\frac1{4\pi\varepsilon}\frac Q{r^2}

This is the formula for the electric field intensity at a distance of r from the point charge Q.

Physical Significance of E

So, we can think of the electric field around any charge and the strength of that electric field at a particular point is defined by the electric field intensity.

The Direction of E

It is quite obvious from the definition that the intensity of the field is a vector quantity. Recall that it is a force upon charge. Being force a vector and charge a scalar, the E must be a vector quantity.

The direction of E at the given point can be thought of as the direction of the force on the unit positive charge which is placed at that point. So the directions for the electric field arising from positive and negative point charge can be shown as below.

Direction of the Electric field

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