An electric field is a vector quantity defined throughout a region of three-dimensional space. It describes the forces that will be experienced by an electric charge if it is placed at various locations in the field. If a positive charge is introduced, the force on it equals the field strength times its own charge magnitude, in the direction of the field. When a negative charge is introduced, the force on it is opposite to the direction of the field. The field strength is measured in newtons per coulomb (N/C). A test charge is a small positive charge used to measure a field.

Point charges generate electric fields. At any distance from the charge, the strength of the field is proportional to the magnitude of the charge and inversely proportional to the square of the distance from the charge.

An electric field diagram is a convenient means of representing the direction and strength of an electric field in a region. The direction of a field line represents the local direction of the electric field. The strength of the field at a particular location is indicated by the proximity of the field lines to each other around that location.

When more than one charge is present, electric fields obey the principle of superposition. This means electric fields at any point can be added as vectors.

In an isolated, charged conductor the excess charges distribute themselves on the conductor’s surface in a state of electrostatic equilibrium. There is no net motion of these charges. The electric field extends perpendicularly outward from all points on the conductor’s surface, and there is no field inside the conductor.

An electric dipole consists of a positive and a negative charge of equal magnitude, separated by a fixed distance. A vector called the dipole moment p points from the negative to the positive charge. Its magnitude is equal to the magnitude of either dipole charge times the distance between the charges. When the dipole is placed in an external electric field, the dipole moment experiences a torque tending to align it in a direction parallel to the field.

Dipoles generate electric fields. At large distances from the dipole, the strength of the field is proportional to the magnitude of the dipole moment and inversely proportional to the cube of the distance from the dipole. The equations on the right apply to a dipole centered at the origin, and oriented toward the right, along the x axis. This is the dipole axis; the y axis is the bisector axis.