Field charging

The field charging describes the electrical charging of gas-borne particles ( aerosol ) in an electric field in the presence of unipolar charge carriers ( ions or electrons ).

Ions or electrons have a high electrical mobility . In an electric field, the drift speed is proportional to the electric field strength . If a particle is in this field, the field lines are distorted depending on the electrical permittivity and the charge state of the particle. Charge carriers that move along the field lines collide with the particle when the field lines end on the particle surface. With each collision, the number of charges on the particle increases and with it the opposing electric field that the particle builds up. This opposing field displaces the field lines on the side facing the emission source, so that the collision probability decreases with an increasing number of charges until no field lines end on the particle. Once this so-called saturation charge has been reached, no more charge carriers hit the particle. Neglecting the diffusion charge , the particle charge can be estimated at

${\displaystyle n={\frac {3\varepsilon _{\mathrm {r} }}{\varepsilon _{\text{r}}+2}}\cdot {\frac {Ed_{\text{P}}^{2}}{4e}}\cdot {\frac {\pi eZ_{\text{i}}N_{\text{i}}t}{1+{\frac {eZ_{\text{i}}N_{\text{i}}t}{4\varepsilon _{0}}}}}}$

where n is the particle charge as a multiple of the elementary charge. Here ε _{0 is} the permittivity of the vacuum, E is the applied field strength, d _{P is} the particle diameter, e is the elementary charge , Z _{i is} the ion mobility, N _{i is} the ion concentration and t is the length of time the particle remains in this field. The resulting saturation charge is calculated after a correspondingly long dwell time ( t → ∞) in this field

${\displaystyle n=4\pi \varepsilon _{0}\cdot {\frac {3\varepsilon _{\text{r}}}{\varepsilon _{\text{r}}+2}}\cdot {\frac {Ed_{\text{P}}^{2}}{4e}}}$

The influence of field charging increases with increasing particle size. In contrast, the influence of diffusion charging increases with decreasing particle size. In the range between 200 nm <  d _P  <500 nm, the two charging mechanisms are approximately equally strong, depending on the particle properties, the applied electric field strength and the charge carrier concentration. Diffusion charging dominates below about 200 nm.

See also

• Diffusion charging
• Particle charge

literature

• William C. Hinds: Aerosol Technology . John Wiley & Sons, New York 1982, ISBN 0-471-08726-2 .

Individual evidence

1. ^ William C. Hinds: Aerosol Technology . John Wiley & Sons, New York 1982, ISBN 0-471-08726-2 .
2. ↑ B. Hu: An Investigation of Walking Induced Electrostatic Field Effects on Indoor Particle Resuspension . ProQuest, 2008, ISBN 0-549-77125-5 , pp. 45 ( limited preview in Google Book search).