Particle field charging

To this point, we have considered only the radiation field. We now turn to the interaction between the matter and the field. According to classical electromagnetic theory, the force on a particle with charge e due to the electric and magnetic fields is... 

E causes a particle of charge q to experience a force and hence a displacement. Both the force and the displacement are proportional to E therefore the energy of the field-charge interaction-the product of the force and the displacement-is proportional to E. ... 

Fig. 12. Comparison of actual and predicted charging rates for 0.3-pm particles in a corona field of 2.65 kV/cm (141). The finite approximation theory (173) which gives the closest approach to experimental data takes into account both field charging and diffusion charging mechanisms. The curve labeled White (141) predicts charging rate based only on field charging and that marked Arendt and Kallmann (174) shows charging rate based only on diffusion.

Particles carrying charges of opposite polarity due to contact electrification will be attracted to opposite electrodes when passing through an electric field and thus can be separated from each other. 

These equations are based on the assumption of zero particle space charge. In practice, however, charged particles can significantly affect the electric field... 

According to the field charging theory, the external electric field drives ions to the aerosol particle until the repelling electric field prevents ions from reaching the surface of the particle. This condition corresponds to the saturation i.e, the particle has reached a stable value which cannot be exceeded. The relationship between the net charge of the particle and charging time is given by... 

FIGURE 13.11 Field charging (ion parhs in the vicintty of a charged particle in an electric field>. 

According to Eq. (13.52), saturation charge is directly proportional to the square of the particle diameter and the external electric field. Particle charging depends also on the composition of the particle, which is taken into account by the relative dielectric constant e,. It is worth noticing that the field charging model should not be applied for small particles (dp < 0.5 pm). 

The most straightforward approach is to assume that the field charging and diffusion charging are independent processes i.e., particle charge can be presented as a sum of charges due to field (s ) and diffusion (sj) charging. Another simple approach to estimating the combined effect is... 

These simple models are based on the assumption of constant drift velocity i.e., particles are assumed to achieve their final charge instantaneously. This is a reasonable assumption in the case of large particles, the charging of which is governed by field-driven ion motion. The characteristic distance, x% corresponding to the time constant in Eq. (13,53) is given by... 

Physics and chemistry are carried out in laboratory frames using coordinate systems to set up experimental devices. Before discussing quantum mechanical processes let us recall the form of the total Hamiltonian for a set of particles having charges qa and masses ma interacting with an electromagnetic field A. This Hamiltonian is given by ... 

On this subject notice that, possibly combined with various heating methods, several physical effects may be considered which allow free flotation of solid and even liquid matter. Materials may be levitated for instance by a jet of gas, by intense sound waves or by beams of laser light. Conductors levitate in strong radiofrequency fields, charged particles in alternating electric fields, magnets above superconductors or vice versa. A review on levitation in physics with the description of several techniques and their principles and applications was made by Brandt (1989). 

Photoemission is unlikely to be a significant contributor to the charging of particles. Field emission, however, could be involved in some of the intensive electrostatic atomization processes where very fine highly charged particles are produced. Because of the dubious current applicability, no effort has been made to review the literature in these cases. 

High-energy beams involving the above particles are created in particle accelerators, in which a charged particle is drawn forward by an electro-sfafic field wifh a charge opposite that of the particle (like charges repel one another, opposites attract) as the particle passes the source of each field, the charge of fhe field is reversed so that the particle is now pushed onto another field source. 

The coating is applied electrostatically. The powder is pumped by air through a gun, which generates an electric field and imparts a static charge to the powder particles. These charged particles are attracted uniformly to a grounded part. The part has to be conductive or coated by a conductive primer coat so that it can be grounded (see Section 7.7.2). 

In formulating quantum electrodynamics (QED), it has been found convenient to introduce the electromagnetic interaction with charged particles via the potentials instead of the fields. Consider a particle of charge q traveling on some path P from i to 2. Then the magnetic change in phase of the wavefunction is just... 

Upon illumination, semiconductor particles become charged, allowing even for electrophoretic mobility under an applied electrical field When appropriately prepared, colloidal TiO2 can apparently accumulate charge to effect directly multiple quanta redox reactions The efficiency of such charge accumulation is surely related to doping level for the doping level can alter band positions and may improve the efficiency of photoinduced electron transfer. For example, the dispersal of FcjOa... 

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