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الكلية كلية الصيدلة
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المرحلة 2
أستاذ المادة ظافر قحطان سعيد الامين الماشطة
5/27/2011 2:05:20 PM
Particle shape
A sphere has minimum surface area per unit volume. The more asymmetric a particle, the greater is the surface area per unit volume. As discussed previously, a spherical particle is characterized completely by its diameter. As the particle becomes more asymmetric, it becomes increasingly difficult to assign a meaningful diameter to the particle, hence, as we have seen, the need for equivalent spherical diameters. It is a simple matter to obtain the surface area or volume of a sphere because for such a particle
where d is the diameter of the particle. The surface area and volume of a spherical particle are therefore proportional to the square and cube, respectively, of the diameter. To obtain an estimate of the surface or volume of a particle (or collection of particles) whose shape is not spherical, however, one must choose a diameter that is characteristic of the particle and relate this to the surface area or volume through a correction factor. Suppose the particles are viewed microscopically, and it is desired to compute the surface area and volume from the projected diameter, dp, of the particles. The square and cube of the chosen dimension (in this case, dp) are proportional to the surface area and volume, respectively. By means of proportionality constants, we can then write
where as is the surface area factor and ds is the equivalent surface diameter. For volume we write
where av is the volume factor and dv is the equivalent volume diameter. The surface area and volume "shape factors" are reality, the ratio of one diameter to another. Thus, for a sphere,
There are as many of these volume and shape factors as there are pairs of equivalent diameters. The ratio as/av is also used to characterize particle shape. When the particle is spherical, as/av = 6. The more asymmetric the particle, the more this ratio exceeds the minimum value of 6.
Specific surface
The specific surface is the surface area per unit volume, Sv or per unit weight, Sw, and can be derived from (19-12) and (19-13). Taking the general case, for asymmetric particles where the characteristic dimension is not yet defined,
where n is the number of particles. The surface area per unit weight is therefore
where r is the true density
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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