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Last updated on September 10th, 2019 at 03:21 am
In this article, we will rigorously derive expressions for the Electric Field due to Point Charge and System of Charges. We will review the Coulomb’s quantitative statement about the force between two point charges. However, we have to understand the definition of point charge before we start our discussion. When the dimension of the charged particles is much smaller than the separation between them we can consider them as point charges. Thus point charge may not be an absolutely zero-dimensional object.
The most fundamental law of electrostatics was first published in 1785 by Charles-Augustin de Coulomb and was essential to the development of the theory of electromagnetism. This is an inverse-square law and is analogous to Isaac Newton’s inverse-square law of universal gravitation. According to Coulomb’s law
“The magnitude of the electrostatic force of attraction or repulsion between two point charges is directly proportional to the product of the magnitudes of charges and inversely proportional to the square of the distance between them.”
“The force is along the straight line joining them. If the two charges have the same sign, the electrostatic force between them is repulsive. If they have different signs, the force between them is attractive.”
Thus if two point charges and are separated by a distance r in the vacuum then the magnitude of the force between them is given by
here k is a proportionality constant and is usually put as . is called the permittivity of free space. The value of in SI unit is . Thus in vector notation, Coulomb’s law can be expressed as
Electric Field due to a Point Charge
An electric field is a vector field that associates the Coulomb force experienced by a test charge at each point in space to the source charge. The strength and the direction of the electric field can be determined from the Coulomb force F on a test charge q. If the field is generated by a positive charge Q, the direction of the electric field will be radially outward. Similarly, for a negative point source charge, the direction is radially inwards. The magnitude of the electric field E can be derived from Coulomb’s law. Let there is a point charge Q placed in the vacuum. We introduce another point charge q (or test charge) at a distance r from the charge Q. The electric field at point P due to the point charge Q is given by
In the figure above we have shown the direction of the electric field due to a positive point charge . The direction of is also shown. The magnitude of the electric field is proportional to the length of the shown. However, if a relatively large test charge q is brought within the vicinity of the source charge Q, it is bound to modify the original electric field due to the source charge. A simple way to avoid this conflict is to use a negligibly small test charge q. Thus our definition of electric field modifies to,
According to the definition given above, the electric field at point P due to the point charge Q is
S.I. unit of electric field is .
In the diagram below, we have shown the electric field due to a positive point charge as a function of distance.
Salient Features of Electric field due to Point Charge
- An electric field due to a point charge is a vector quantity.
- This electric field is the property of the source charge and does not depend on the test charge.
- For a positive source charge, the electric field will point radially outward from the source charge and for a negative one, it will direct radially inwards.
- The magnitude of the electric field due to a point charge also depends on the distance from the source charge.
- Electric field due to point charge has spherical symmetry and does not depend on , .
- Electric field at any point in space is tangent to the line of force at that point.
Electric Field due to a System of Charges
Let us consider a system of n number of charges denoted as with position vectors respectively. We have to calculate the electric field at a point P() due to this discrete charge distribution. We introduce a test point charge q at a position P(). The individual electric force at the test point P can be calculated according to the Columb’s law shon in the section above. The total electric force at the test point P can be calculated using the superposition principle. According to which, the net response caused by two or more stimuli is the sum of the responses that would have been caused by each stimulus individually. Electric force at point P due to charge at position is
We have shown the direction of the force vector in the figure below. Similarly, the electric force at the same point due to charge is given by
Thus using the superposition principle we can calculate the total electric force at point P due to the system of charges.
and by definition of an electric field, the total electric field at point P due to this system of charges becomes
In the diagram above, we have shown the electric field due to a system of two source point charges , and . The resuslant electric field is the vector sum of and . We also have shown the direction of and .