If the particle is given an electrical chargeq and is surrounded by an electrical field with 
a gradient VV, there will be force exerted on the particle of Ey = q VV. 
The electrical charge resides on the surface of the particle and is a form of surface 
energy. If the droplet is a conductor, the amount of charge will be limited by the charging 
system and the subsequent discharge through air ionization. If a nonconductor, the charge 
will be limited by surface instability, because the surface charge tends to reduce the ef- 
fective surface tension. In the lattercase,themaximum equilibrium surface charge in air 
of a nonconducting drop is q = 2.65 (10 =F he coulombs/cm2, Thus, the maximum charge 
on a particle would be given by q = (2.65 x 10-9) a D%coulombs so max Fo =*(2.65 x 
10-9) mD2VV if D is in cm. If the particle is surrounded by air in which there exists a 
thermal gradientVT, the difference in mean molecular velocity on either side of the 
particle will exert a force on the particle. The force on the particle is given by the 
expression 
— 9mu* DR xa 
t es ee + x] 
Vi 
where R= gas constant, M = equivalent molecular weight of the air, p= air pressure, 
Xq = heat conductivity of theair,andx, is the heat conductivity of the particle. A summary 
sketch of the possible forces that may be acting on a charged particle approaching a hot 
leaf is shown below. 
Laat 
Droplet 
Kg 
Now you can see that it may be possible that a number of forces will be acting ona 
particle. These forces depend, among other things upon the size of the particle, the force 
per unit mass of the particle generally varying with the diameter of the particle. One of 
the things that is enlightening to study is a comparison of these forces for varying 
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