If a charge q is brought near a charge Q placed, somewhere in space, the charge q will experience a force. According to coulomb’s law, the value of this force would depend upon the distance between the two charges. If the charge q is moved away from q this force would decrease till at a certain distance, the force would practically reduce to zero.
Now the charge q is out of the influence of charge q. this region of space surrounding the charge Q in which it can exert a force on a charge q is known as electric field of the charge Q. thus the electric field of a charge is defined as follow:
The electric field of a charge is a region of space surrounding it, in which a charge can feel its electrostatic effect in the form of a force.
The strength of the field at any point is known as electric intensity. In order to find the value of electric intensity at a point in the field, we place a test charge +q at that point. If F is the force acting upon it, the electric intensity E would be given by
Thus the electric intensity at any point is defined as the force acting on a unit positive charge placed at that point.
Electric intensity being a force is a vector quantity. According to Eq. 15.3, its direction is the same as that of the force acting on the positive test charge. If the charge q is free, it will move in the direction of electric intensity. In case of negative charge the force would be F = -qE, so the negative charge would move in a direction opposite to the electric intensity.
By using Eq. 15.3, we can find out the electric intensity due to a point charge +Q. suppose a point is located at a distance r from the charge +Q at which the electric intensity is to be determined. We place a point charge +q at this point (fig.15.7).
Fig. 15. 7 a charge q is places at a distance r from charge +Q
The force acting upon this charge as given by coulomb’s law is
The electric intensity s directed away from the charge +Q along the line joining the two charges (fig. 15.7).
Example 15.2: Two points P and Q are located at a distance of 5cm and 20 cm respectively from a point charge of + 10 µC. find the magnitude of electric intensity at these points.
Solution:
It can be seen that the magnitude of electric intensity decreases with the increase of distance.