The concept of electric field was introduced by Faraday during the middle of the 19th century. It is a vector quantity equal to the force experienced by a positive unit charge at any point P of the space.

To get an idea, consider a stationary positive point charge q_{1} like the one represented in green in the following figure.

The force experienced by a 1 coulomb charge situated at any position P of the space in the vicinity of a charge q_{1} is given by Coulomb’s law:

Where **u**_{r} is a unit vector in the radial direction.

k is the Coulomb constant and its value in vacuum and in SI units is:

It can be expressed as the following product of the **vacuum permittivity** (permittivity of free space or electric constant):

The force experienced by the positive unit charge is repulsive, because q_{1} is positive too and two charges with the same sign repel each other. At any point of space, the positive unit charge would experience a repulsive force given by Coulomb’s law. It is represented with the green arrows in the previous figure.

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This situation can be interpreted as a disruption due to the charge q_{1} on its surroundings, so that a **test charge** situated at a point P of space experiences a force. The electric field **E** is the vector magnitude that describes this disruption.

The charge q_{1} creating the electric field **E** is called a **source charge**.

Going back to the definition given at the beginning of this page, the electric field due to a point charge is:

The SI units for the electric field strength are N/C or V/m.

The green arrows in the previous figure are called **field lines** and they are a way to graphically represent the field due to a point charge. As you can see in the figure, the field lines of the electric field start at positive charges, For this reason, **a positive charge is called a source of field lines**.

**The density (number per area) of field lines is proportional to the magnitude of the electric field** and they can never intersect because it they did, the electric field would have two different values for the same point in space.

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If the source charge were negative, the positive unit charge (the test charge) would experience an attractive force. Therefore, the field lines would end at the **negative charge**, which is called a **sink of field lines** (see next figure).

**The electric field satisfies the superposition principle**. At any point of space, the net electric field from multiple charges is the sum of the individual electric fields due to each individual charge.

Therefore, the electric field due to a set of N charges is:

The force exerted by an electric field on a test charge q_{2} at any location of space is given by:

If the electric field is due to a source point charge q_{1}, this force is given by:

This expression is Coulomb’s law. The following figure represents the force experienced by a negative (a) or positive (b) test charge q_{2} in vicinity of the electric field due to a positive source charge q_{1}: