Electric Field and its physical significance
The space surrounding an electric charge Q in which another charge q experiences an electrostatic force of attraction or repulsion,is called the electric field of charge Q.
![]()
** Here charge Q is called the source charge and charge q is the called test charge.
As we know that electrostatic force is a vector quantity;similarly electric field is a vector quantity.
Electric field Intensity
The electric field intensity at a point due to a source charge is defined as the electrostatic force per unit positive charge acting on a vanishingly small test charge placed at that point.
Note:
-
The point where electric field is measured is called the field point.
-
You can see from the formula that there is a limit q → 0.This is because on placing the test charge at the field point,the test charge may not affect the electric field.
The S.I. unit of electric field is Newton per coulomb (NC-1) or Volt per metre (Vm-1)
-
If the intensity of electric field at a point in an electric field
is known,then we can determine the force
acting on a charge q placed at that point by the equation
Properties of electric field
-
Electric field at a point due to a negative source charge is directed towards this charge as shown in figure below.
-
Electric field at a point due to a positive source charge is directed away this charge as shown in figure below.
-
If the charge is positive,then the direction of force on that charge is same as that of electric field.
-
If the charge is negative,then the direction of force on that charge is opposite to the electric field.
Q: What is the physical significance of electric field?
Ans. Calculations of electric field intensity are of tremendous importance due to the technological applications of electric forces.
-
It helps in finding the electric force experienced by a unit positive charge when placed at a point in the system of multiple charges.Electric field is a thus independent of the amount of the test charge and is property of system of charges i.e the source charges.
-
It gives the magnitude and direction of the electric force.
-
In time varying electromagnetic properties,the concept of electric field intensity becomes very important.
Q: Why is the test charge vanishingly small?
Ans. The test charge must be vanishingly small so that it may not affect the electric field of source charge.