24
2 The Kinds of Ordinary Materials
• Electrostatic precipitators can be used to remove colloidal particles such as dust,
smoke, or oil mist from air. They work by creating a net charge on the colloid
particles and then an electric field to sweep them away. This process begins by
negatively charging wires until they emit corona discharge electrons into the
passing air mixture. The electric field near the wires accelerate the electrons,
which collide with air molecules (and colloidal particles), knocking off electrons
from those molecules, making positive ions near the wires. But further away
from the wires, the electrons are not moving as fast, and tend to be captured by
air molecules. So, a negative set of ions are created, which feel a force moving
them away from the wire and toward the collection plates. Some of these negative
ions make contact with and adhere to the colloidal particles, imparting to them a
negative charge. As the colloidal particles are much larger than the air molecules,
they can accumulate a number of such charges. As the air moves through the
device, the negatively-charged colloid particles are attracted to and adhere to
collection plates made positively charged relative to the coronal wires. The plates
are periodically cleaned with a fluid, or by mechanical ‘rapping’.
• Electrophoresis uses differential diffusion in a fluid to separate molecules with
varying mobilities. The process is the following: The molecules are first given a
charge (if they had no net charge initially) by attaching anions or cations to their
surface. They are then pulled through the background fluid or gel by an electric
field. The fluid will exert a viscous force which depends on the size and shape of
the base molecule together with any adsorbed molecules on its surface and on the
tangential forces of the fluid on this molecular complex. After being dragged a
certain distance by the external electric field, the location of differing molecules
along the field direction will vary.
• Advantage can be taken of the adhesion of molecules to both natural and specially
coated surfaces. Oil can be separated from water using treated sponges and other
porous materials. The effectiveness of such an open matrix within the material is
proportional to the surface area of the network boundaries within the matrix as
well as the adhesive properties of the molecules to the fiber surfaces.
• If one or more components of a mixture is ferromagnetic, then magnets can
be used to effect separation of those substances. Magnetic fields with strong
gradients can be used to separate materials of varying paramagnetism and
diamagnetism.
• A ‘mass spectrometer’ is capable of separating atoms and molecules according
to their mass. However, the device requires that the material be first vaporized in
a vacuum, ionized, and then accelerated by an electric field into a magnetic field.
The magnetic field will deflect the particles into a curve whose radius depends
on the particle’s mass to charge ratio. 12 Collection and counting of the particles
according to their separated beams is then possible.
12 Newton’s 2nd law gives a radius of curvature of R = p/(qB), where q is the charge of the
deflected particle, p its momentum, and B the magnetic field strength. This formula for R works
even for relativistic particle speeds v if one takes p = mv/
1 − (v/c) 2 .
2 The Kinds of Ordinary Materials
• Electrostatic precipitators can be used to remove colloidal particles such as dust,
smoke, or oil mist from air. They work by creating a net charge on the colloid
particles and then an electric field to sweep them away. This process begins by
negatively charging wires until they emit corona discharge electrons into the
passing air mixture. The electric field near the wires accelerate the electrons,
which collide with air molecules (and colloidal particles), knocking off electrons
from those molecules, making positive ions near the wires. But further away
from the wires, the electrons are not moving as fast, and tend to be captured by
air molecules. So, a negative set of ions are created, which feel a force moving
them away from the wire and toward the collection plates. Some of these negative
ions make contact with and adhere to the colloidal particles, imparting to them a
negative charge. As the colloidal particles are much larger than the air molecules,
they can accumulate a number of such charges. As the air moves through the
device, the negatively-charged colloid particles are attracted to and adhere to
collection plates made positively charged relative to the coronal wires. The plates
are periodically cleaned with a fluid, or by mechanical ‘rapping’.
• Electrophoresis uses differential diffusion in a fluid to separate molecules with
varying mobilities. The process is the following: The molecules are first given a
charge (if they had no net charge initially) by attaching anions or cations to their
surface. They are then pulled through the background fluid or gel by an electric
field. The fluid will exert a viscous force which depends on the size and shape of
the base molecule together with any adsorbed molecules on its surface and on the
tangential forces of the fluid on this molecular complex. After being dragged a
certain distance by the external electric field, the location of differing molecules
along the field direction will vary.
• Advantage can be taken of the adhesion of molecules to both natural and specially
coated surfaces. Oil can be separated from water using treated sponges and other
porous materials. The effectiveness of such an open matrix within the material is
proportional to the surface area of the network boundaries within the matrix as
well as the adhesive properties of the molecules to the fiber surfaces.
• If one or more components of a mixture is ferromagnetic, then magnets can
be used to effect separation of those substances. Magnetic fields with strong
gradients can be used to separate materials of varying paramagnetism and
diamagnetism.
• A ‘mass spectrometer’ is capable of separating atoms and molecules according
to their mass. However, the device requires that the material be first vaporized in
a vacuum, ionized, and then accelerated by an electric field into a magnetic field.
The magnetic field will deflect the particles into a curve whose radius depends
on the particle’s mass to charge ratio. 12 Collection and counting of the particles
according to their separated beams is then possible.
12 Newton’s 2nd law gives a radius of curvature of R = p/(qB), where q is the charge of the
deflected particle, p its momentum, and B the magnetic field strength. This formula for R works
even for relativistic particle speeds v if one takes p = mv/
1 − (v/c) 2 .