An electric charge influences the space around it. Any other charge placed in this region experiences an electric force.
The region around a charged body where another charge experiences an electric force is called an electric field.
The concept of electric field was introduced to explain how charges interact even when they are not in physical contact.
What is an Electric Field?
Consider a charge Q placed in space.
If a small positive test charge q is brought near Q, it experiences an electric force.
This force exists because charge Q creates an electric field around itself.
The electric field is present whether or not the test charge is introduced.
Electric Field Intensity
Electric field intensity at a point is defined as the electric force experienced by a unit positive test charge placed at that point.
If a test charge q experiences a force F, then the electric field intensity is:
Where:
- E = electric field intensity
- F = electric force
- q = test charge
SI Unit of Electric Field
From the definition:
The SI unit is:
Thus, electric field intensity is measured in newton per coulomb.
Another commonly used unit is:
Both units are equivalent.
Nature of Electric Field Intensity
Electric field intensity is a vector quantity.
It has both magnitude and direction.
The direction of the electric field at any point is the direction of the force acting on a positive test charge placed at that point.
Electric Field Due to a Point Charge
Consider a point charge Q.
A test charge q is placed at a distance r from Q.
According to Coulomb’s law, the force on the test charge is:
Using the definition of electric field:
Substituting the value of force:
Therefore,
This is the electric field intensity due to a point charge.
Direction of Electric Field
The direction of the electric field depends on the nature of the source charge.
For a positive charge:
The electric field is directed radially outward from the charge.
For a negative charge:
The electric field is directed radially inward toward the charge.
Variation of Electric Field with Distance
The electric field due to a point charge is inversely proportional to the square of the distance.
This is known as the inverse square law.
If the distance is doubled:
If the distance is tripled:
Thus, the electric field decreases rapidly as distance increases.
Electric Field Due to Multiple Charges
When several charges are present, the resultant electric field is obtained using the principle of superposition.
The net electric field is the vector sum of electric fields due to individual charges.
Each field must be added vectorially.
Uniform Electric Field
An electric field is said to be uniform if its magnitude and direction remain constant throughout the region.
Between two large oppositely charged parallel plates, the electric field is approximately uniform.
In a uniform field, electric field lines are parallel and equally spaced.
Non-Uniform Electric Field
An electric field is non-uniform if its magnitude or direction changes from point to point.
The electric field around a point charge is non-uniform.
The field becomes weaker as the distance from the charge increases.
Electric Field at the Center of a Charged Ring
If identical charges are distributed symmetrically on a ring, the electric fields due to individual charges cancel at the center.
Therefore, the resultant electric field at the center becomes:
This result follows from symmetry.
Solved Example
Find the electric field intensity at a point located 0.5 m from a charge of 2 μC.
Given:
Using:
Substituting values:
Hence,
Important Points to Remember
- Electric field is the region around a charge where another charge experiences force.
- Electric field intensity is force per unit positive charge.
- Electric field is a vector quantity.
- SI unit of electric field is N/C.
- Electric field due to a point charge is inversely proportional to the square of the distance.
- Electric field is outward from a positive charge and inward toward a negative charge.
- The net electric field is obtained by vector addition of individual fields.
Frequently Asked Questions
What is electric field?
The region around a charge where another charge experiences an electric force is called an electric field.
What is electric field intensity?
Electric field intensity is the force experienced by a unit positive test charge placed at a point in the field.
Is electric field a scalar or vector quantity?
Electric field is a vector quantity because it has both magnitude and direction.
What is the SI unit of electric field?
The SI unit of electric field is newton per coulomb (N/C).
What is the electric field due to a point charge?
where Q is the charge and r is the distance from the charge.
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