"An electric field can be described by four field quantities ... The electric field intensity \overrightarrow{E} is the vector field used in electrostatics when charge is at rest in free space and is defined as the electric force on a test charge. The electric displacement vector \overrightarrow{D} (also called the electric flux density or displacement flux) is a vector field used in studying the electric fields inside material objects. Similarly, magnetic field intensity \overrightarrow{H} is a vector needed in discussing magnetic phenomena, that is the field generated at a point in free space by steady or time-varying electric currents in a source; it is related to the magnetic force acting on a moving charge. The magnetic flux density \overrightarrow{B} is useful in the investigation of the magnetic fields within material objects where the material modifies the field intensity."