Electric field

The electric field E at a point is defined as the electric force F on a small positive test charge q divided by that charge: E = F/q, with the test charge taken small enough not to disturb the source arrangement. The field is a vector: it has both magnitude and direction, and its SI unit is the newton per coulomb (N/C), equivalent to the volt per metre (V/m). The direction at a point is the direction of the force that a positive test charge would feel there. A negative charge placed in the same field feels a force opposite to E.

A charge creates an electric field in the space around it. For an isolated point charge, the field is radial: it points outward for a positive source charge and inward for a negative one, and its magnitude in vacuum falls with the square of the distance. For several source charges, the total field is the vector sum of the fields each would produce separately. This superposition principle lets the field be found from the source configuration first, then used to calculate the force on a charge placed in it: F = qE.

Field-line diagrams are a visual convention for showing the direction and relative strength of a field. Lines begin on positive charges and end on negative charges, or extend to or from infinity when necessary. The tangent to a line gives the field direction, and closely spaced lines indicate a stronger field. Lines are not physical threads and do not trace the paths that charges must follow; actual motion also depends on a particle's initial velocity and other forces.

Electric potential describes electric potential energy per unit charge and is a scalar, while the electric field is a vector. In electrostatics, the field points in the direction in which potential decreases most rapidly; in one dimension this is expressed as E = -dV/dx. Potential differences can therefore provide a convenient way to calculate fields, and fields can be used to determine the force and work associated with moving charges.

Electrostatics studies electric charges at rest and the fields they produce in static equilibrium. More generally, electric fields are part of electromagnetism: electric charges produce fields, and a changing magnetic field can also induce an electric field. Together with magnetic fields, electric fields describe electromagnetic interactions and waves.

The electric-field concept is used to analyze forces in atoms and materials, the behaviour of charges in conductors and insulators, capacitors, and the design of electrical and electronic devices. It is also central to understanding how electromagnetic waves propagate and how charged particles respond to electric forces.