Digital Circuit Boards: Mach 1 GHz

1.10 The D Field

A second description of the electric field is called the D or displacement field. This field starts and stops on charges. Unlike the E field, the D field does not change intensity at the boundary of a dielectric unless there are surface charges present.

A changing D field is equivalent to a current flowing in space. This current is usually called a displacement current. In a capacitor, the D field between the plates of the capacitor changes as the charge on the surface of the plates changes. By using this interpretation, the changing D field is an extension of the current that flows in the connecting conductors. In circuit theory, current flows in a capacitor even though the plates of the capacitors are insulated from each other. In antenna theory, the current that flows in the antenna structure returns as displacement current in the nearby space.

The relationship between the D and E field is given in Equation 1.4 as

1.4

where ε ₀ is the permittivity of free space or 8.85 × 10⁻¹² F/m and ε _R is the relative dielectric constant.