the exponential: This approximation yields the linearized Poisson–Boltzmann equation which also is known as the Debye–Hückel equation: The second term on the right-hand side vanishes for systems that are electrically neutral. The term in parentheses divided by , has the units of an inverse length squared and by dimensional analysis leads to the definition of the characteristic length scale that commonly is referred to as the Debye–Hückel length. As the only characteristic length scale in the Debye–Hückel equation, sets the scale for variations in the potential and in the concentrations of charged species. All charged species contribute to the Debye–Hückel length in the same way, regardless of the sign of their charges. For an electrically neutral system, the Poisson equation becomes To illustrate Debye screening, the potential produced by an external point charge is The bare Coulomb potential is exponentially screened by the medium, over a distance of the Debye length: this is called Debye screening or shielding (Screening effect). The Debye–Hückel length may be expressed in terms of the Bjerrum length as where is the integer charge number that relates the charge on the -th ionic species to the elementary charge . In a plasma For a weakly collisional plasma, Debye shielding can be introduced in a very intuitive way by taking into account the granular character of such a plasma. Let us imagine a sphere about one of its electrons, and compare the number of electrons crossing this sphere with and without Coulomb repulsion. With repulsion, this number is smaller. Therefore, according to Gauss theorem, the apparent charge of the first electron is smaller than in the absence of repulsion. The larger the sphere radius, the larger is the number of deflected electrons, and the smaller the apparent charge: this is Debye shielding. Since the global deflection of particles includes the contributions of many other ones, the density of the electrons does not change, at variance with the shielding at work next to a Langmuir probe (Debye sheath). Ions bring a similar contribution to shielding, because of the attractive Coulombian deflection of charges with opposite signs. This intuitive picture leads to an effective calculation of Debye shielding (see section II.A.2 of ). The assumption of a Boltzmann distribution is not necessary in this calculation: it works for whatever particle distribution function. The calculation also avoids approximating weakly collisional plasmas as continuous media. An N-body calculation reveals that the