Electrostatic Field


Electric Field

An Electric field can be considered an electric property associated with each point in the space where a charge is present in any form. An electric field is also described as the electric force per unit charge.

The formula of the electric field is given as,



E is the electric field.
F is the force.
Q is the charge.

Electric fields are usually caused by varying magnetic fields or electric charges. Electric field strength is measured in the SI unit volt per metre (V/m).

The direction of the field is taken as the direction of the force which is exerted on the positive charge. The electric field is radially outwards from the positive charge and radially towards the negative point charge.

 What Is an Electric Field?

An electric field is defined mathematically as a vector field that can be associated with each point in space, the force per unit charge exerted on a positive test charge at rest at that point.

The electric field is generated by the electric charge or by time-varying magnetic fields. In the case of an atomic scale, the electric field is responsible for the attractive forces between the atomic nucleus and electrons which hold them together.

According to Coulomb’s law, a particle with electric charge q1 at position x1 exerts a force on a particle with charge q0 at position x0 of,


r1,0 is the unit vector in the direction from point x1 to point x0

ε0 is the electric constant, also known as absolute permittivity of free space C2m-2N-1

When the charges q0 and q1 have the same sign, the force is positive, and the direction is away from other charges, which means they repel each other. When the charges have unlike signs, the force is negative, and the particles attract each other.

The electric field is force per unit charge,

Electric field intensity

The electric field intensity at a point due to a source charge may be defined as the force experienced per unit positive test charge placed at that point without disturbing the source charge.

The electric field at a point is a vector quantity.

SI unit is Newton coulomb-1 (N C-1).

If E  is electric field at a point, then by definition, a charge q placed at that point will experience a force  given by


Electric Field Line

Electric field lines were first introduced by Michael Faraday himself. Electric field lines are an excellent way of visualizing electric fields.

A field line is drawn tangential to the net at a point. Thus at any point, the tangent to the electric field line show the direction of the electric field at that point. Secondly, the relative density of field lines around a point corresponds to the relative magnitude of the electric field at that point. In other words, if you see more electric field lines in the vicinity of point ‘X’ as compared to point ‘Y’, then the electric field is stronger at point ‘X’.

Properties of Electric Field Lines

  • The field lines never intersect each other.
  • The field lines are perpendicular to the surface of the charge.
  • The magnitude of charge and the number of field lines, both are proportional to each other.
  • The start point of the field lines is at the positive charge and end at the negative charge.
  • For the field lines to either start or end at infinity, a single charge must be used.

Electric Field Lines Attraction and Repulsion

Electric field lines always point away from a positive charge and towards a negative point. In fact, electric fields originate at a positive charge and terminate at a negative charge.

Also, field lines never cross each other. If they do, it implies that there are two directions for the electric field at that point. But this is impossible since electric fields add up vectorially at any point and remember that “A field line is drawn tangentially to the net electric field at a point”. Thus, electric field lines can never intersect one another.

As said before field lines are a great way to visualize electric fields. You can almost feel the attraction between unlike charges and the repulsion between like charges as though they are trying to push each other away.

Coming to our initial example of static charge on hair, the direction in which charged hair stands up traces the local electric field lines. The charges on the hair exert forces on the hair strand as they attempt to leak into the surrounding uncharged space. The hair aligns accordingly so that there is no net force acting on it and inadvertently traces the electric field lines.

Some Basics Rules for Drawing Electric Field Lines

Following are the rules for drawing electric field lines:

1.     The field line start at the charge and ends either at the charge or at infinity.

2.     When the field is stronger, the field lines are closer to each other.

3.     The number of field lines depends on the charge.

4.     The field lines should never crossover.

5.     Electric field and electric field line are tangent at the point where they pass through.