Electrostatic devices

Electrostatic air cleaners

Electrostatic air cleaners remove dust, soot, and ash particles from normal air.  Each dust, soot, or ash particle has mass.  The air exerts two types of forces on the particles, the buoyant force and the viscous drag force.  Since the particles are denser than air, the buoyant force alone cannot support the particles.  The viscous drag force keeps the particles from descending quickly.  For small particles the terminal velocity can be lower than 1 mm/s.  The drag force opposes relative motion between the particles and the air, and moving air tends to carry the particles along with it.  The slightest upward breeze can keep the dust, soot, and ash particle aloft.

Electrostatic air cleaners use electrostatic forces to pull these particles from the air.  A typical air cleaner gives each particle a negative charge and then collects it on a positively charged surface.

How does a dust grain become negatively charged?

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The air cleaner uses a corona discharge to give the dust grain a negative charge.  A power supply does work maintaining a potential difference of approximately 10000 V between the corona wires and the collecting surfaces.  The negatively charged dust flows with the air through the air cleaner.  When it passes a positively charged surface, it experiences an electrostatic force strong enough to overwhelm the viscous drag.  The dust particles quickly leave the air stream and collect on the charged surface.  The air continues on without the dust.  The air cleaner precipitates clumps of dust on its collecting plates, and therefore is called an electrostatic precipitator.  It can accumulate large amounts of dust on its plates without blocking the airflow and it is easy to clean.  When several centimeters of dust have accumulated on the collecting surfaces, it is removed by rapping the plates with a stick.  The sudden blow causes the plates to accelerate rapidly and they leave the dust behind.  It falls in clumps to the bottom of the precipitator, where it's collected for recycling or disposal.

Ion Generators

Household ion generators are also effective at removing dust and smoke from room air.  These machines resemble electrostatic precipitators, but they have no internal collecting plates.  They use a corona discharges to charge passing molecules and dust grains and then let those charged particles drift into the room.  When the charged particles come near a surface, they polarize the surface and are attracted to it.  Although this method is cheap and effective, it slowly dirties the walls and the furniture.

Photocopiers

At the heart of the photocopier is a thin layer of photoconductor.  A photoconductor is a solid material through which electrons can move only when it is exposed to light.  In the dark, it is an insulator, in the light, it's a conductor.  This property allows light to determine the pattern of static electricity on a copying drum and hence the placement of toner on a piece of paper.

The light sensitive component in a photocopier is a metal drum that is covered with a thin layer of photoconductor.  This metal drum is grounded.  The copier coats the photoconductor with electrons, which remain in place as long as the photoconductor is in the dark.  But wherever light strikes the photoconductor, it becomes conductive and allows the electrons to escape through the metal and flow into the ground.  Only the portions of the photoconductor which are not illuminated retain their static electric charge and eventually attract black toner particles.  In that manner, the darkened parts of the photoconductor produce the dark parts of the final copy.

The copier starts by applying a uniform negative charge to the surface of the photoconductor.  This charge is applied by a corotron, a fine wire centered in a half-cylinder of metal.  A power supply pumps electrons onto the fine wire until they are emitted into the air as a corona discharge.  When these electrons approach the photoconductor they polarize it and stick to it.  The photoconductor becomes uniformly charged, with about 10-7 C of negative charge per cm2 of surface.  After the charging, the copier exposes the photoconductor to light from the original document.  It uses a lens to cast an image of the original onto the photoconductor's surface.  Light only hits the photoconductor in certain places, which correspond to the white parts of the original document.  When the exposure is over, the photoconductor carries a charge image of the original document.

To develop this charge image into a visible one, the photocopier exposes the photoconductor to charged toner particles.  The toner is a fine insulating plastic powder.  A spinning brush with extraordinarily soft bristles wipes toner particles out of their storage tray onto the photoconductor.  During this transfer, the toner particles become positively charged so that they stick to the negatively charged portions of the photoconductor.  The photoconductor now carries a black image of the original document.  But to create a copy, this black image must be transferred to paper.  To begin this transfer, the copier illuminates the photoconductor with a charge erase lamp so that the photoconductor's negative charge escapes into the metal.  The toner remains in place but it is only very weakly attached.  The copier then transfers the toner to a nearby sheet of paper by applying negative charge to the paper's back.  The positively charged toner is attracted to the negatively charged paper and the two leave the photoconductor together.  The copier then heats and presses the copy, permanently fusing the toner into the paper.  Sometimes, when a copier jams, you may remove a sheet before it has been fused.  The image looks completely normal but wipes off when you touch it because it's held in place only by electrostatic forces.  Once the image has been transferred to the paper the drum is cleaned.  The photoconductor is then ready to be used again.

A laser printer is also a photocopier device, but it uses a laser beam to write a charge image directly onto its photoconductor drum.  Wherever laser light hits the drum, charge flows through the photoconductor.  A computer in the printer turns the laser on and off as it systematically constructs the charge image, one dot at a time.  The photoconductor and the toner supply are contained in a single disposable cartridge.