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Transcript
What Are Uses Of Electrostatics?
In this topic, I want to response to my some questions about an interesting phenomenon by the
name of electrostatics and its marvelous uses in environment and industry. First of all, I try to
response this question:
What is electrostatic?
The study of electric charges at rest is called electrostatics. Its other name is ‘static
electricity’. A Greek philosopher ‘Thales’ in 600 BC conducted an experiment. He observed
that when amber was rubbed with some woolen cloth it gets charged and immediately
started attracting small pieces of paper, corks, dust particles, etc. Amber is a yellowish
brown translucent substance found along some of the famous seacoasts and is also used in
jewelry items. Greek name of ‘Amber’ is ‘Electrum’. It was originated from the words
electric charge, electric field, electric force, electricity and electron.
Then William Gilbert published a book named ‘De magneto’ in 1600 A.D. In this book he
assigned the name ‘Electrica’ to the substances like amber, which after rubbing with certain
materials starts attracting the small bodies. After conducting all his experiments on
Frictional electricity, he said that the type of charge generated on a piece of Amber is called
‘Resinous’. Amber is a resin that’s why its charge was named so.
As the time passes Benjamin Franklin introduced his new theory. According to which,
charge present on the piece of amber was ‘negative’ and the charge present on wool was
‘positive’. His theory was absolutely correct. Scientists are still following it.
Electrostatics is also called as ‘frictional electricity’. When two non-insulating bodies are
rubbed with each other than the charges at rest is generated due to the presence of friction
in them.
So, when a glass rod is rubbed with a piece of silk cloth then it gets charged. In other words
we can say that rod is ‘Charged’ or ‘Electrified’.
Some examples: Similar is the case of Comb. It also gets charged or electrified when it is
passed through dry hair. A paper sheet also gets charged when it is passed through the
machine in the printing press.
Electrostatic field
When two objects in each other's vicinity have different electrical charges, an electrostatic
field exists between them. An electrostatic field also forms around any single object that is
electrically charged with respect to its environment. An object is negatively charged (-) if it
has an excess of electrons relative to its surroundings. An object is positively charged (+) if
it is deficient in electrons with respect to its surroundings.
Electrostatic fields bear some similarity to magnetic fields. Objects attract if their charges
are of opposite polarity (+/-); objects repel if their charges are of the same polarity (+/+ or
-/-). The lines of electrostatic flux in the vicinity of a pair of oppositely charged objects are
similar to lines of magnetic flux between and around a pair of opposite magnetic poles. In
other ways, electrostatic and magnetic fields differ. Electrostatic fields are blocked by
metallic objects, while magnetic fields can pass through most (but not all) metals.
Electrostatic fields arise from a potential difference or voltage gradient, and can exist when
charge carriers, such as electrons, are stationary (hence the "static “in "electrostatic").
Magnetic fields arise from the movement of charge carriers, that is, from the flow of
current.
When charge carriers are accelerated (as opposed to moving at constant velocity), a
fluctuating magnetic field is produced. This gives rise to a fluctuating electric field, which in
turn produces another varying magnetic field. The result is a "leapfrog" effect, in which
both fields can propagate over vast distances through space. Such a synergistic field is
known as an electromagnetic field, and is the phenomenon that makes
wireless communications, broadcasting, and control systems possible.
What is EDS?
Electrostatic discharge (ESD) is the release of static electricity when two objects come into
contact. Familiar examples of ESD include the shock we receive when we walk across a
carpet and touch a metal doorknob and the static electricity we feel after drying clothes in
clothes dryer. A more extreme example of ESD is a lightning bolt. While most ESD events
are harmless, it can be an expensive problem in many industrial environments.
ESD first requires a build-up of an electrostatic charge. This occurs when two different
materials rub together. One of the materials becomes positively charged; the other
becomes negatively charged. The positively-charged material now has an electrostatic
charge. When that charge comes into contact with the right material, it is transferred and
we have an ESD event. The heat from the ESD event is extremely hot, although we do not
feel it when we are shocked. However, when the charge is released onto an electronic
device such as an expansion card, the intense heat from the charge can melt or vaporize the
tiny parts in the card causing the device to fail. Sometimes an ESD event can damage a
device, but it continues to function. This is a called a latent defect, which is hard to detect
and significantly shortens the life of the device
Many electronic devices are susceptible to low voltage ESD events. For example, hard drive
components are sensitive to only 10 volts. For this reason, manufacturers of electronic
devices incorporate measures to prevent ESD events throughout the manufacturing,
testing, shipping, and handling processes. For example, an employee may wear a wrist
strap when working with devices or may wear ESD control footwear and work on an ESD
floor mat that causes the electrostatic charge to go into the ground instead of into the
device. Sensitive devices can be packaged with materials that shield the product from a
charge.
Electrostatic Machines
Van de Graff Generator
Figure 1. Schematic diagram of a Van der Graff generator
A device that uses many of the principles we have discussed in this section is the Van der
Graff generator. It consists of a large hollow metallic dome supported by an insulating
column. A rubber belt runs on pulleys of which the lower pulley is driven by an electric
motor. At the top and bottom of the column are two electrodes E1 and E2 which terminate
in sharply pointed combs. At the lower electrode a voltage of around 10,000 Volts is
applied with respect to the Earth by a battery. At the points, the electrical field becomes
very high and ionizes the air and positive charges are repelled onto the belt where there
are carried up to the upper electrode. At this comb, the positive charge induces negative
charge on the points and thus positive charge on the sphere to which the upper electrode is
connected. The high electric field at the points of the upper electrode ionize the air and
negative charges are repelled onto the belt discharging it before is passes over the upper
pulley. The charge on the sphere gradually charges as the belt goes around. The large
radius of the dome can reach a potential of a million volts relative to the earth.
Wimshurst Machine
The Wimshurst machine is another device for generating high voltages with the advantage
that it can create both positive and negative charges. It consists of two parallel insulating
discs that are geared to rotate in opposite directions, neutralizing rods and rods that collect
the charge.
Although the discs are made of an insulating material, each disc has metal foil strips that
are spaced radially on the outer surface of the discs. These plates help collect the excess
charge that builds up on the insulating plates. The charge is removed by pairs of collecting
combs. The Wimshurst machine works by induction, when the device is made there will be
a small charge on the surface of the discs, which
Figure 2. Schematic diagram of a Wimshurst machine operation
An insulator is very difficult to discharge completely and there will always be regions on
the discs which have small imbalances in charge. Consider such a small charge at the top of
the disc, the charge induces an opposite charge on the second disc and the charges rotate
away from each other, the foil sectors contain the charge. When the foil sectors pass under
the neutralizing rods they induce the opposite charge on the opposite side of the discs. This
creates more charge on the foil sector. As the induced charges rotate around the disc the
induced charges create a positive feedback of charge, so that on the top half of the front
disc there is a buildup of positive charge, while on the bottom half of the front disc there is
a negative charge. On the back disc, the induction produces a negative charge on the top of
the back disc and a positive charge on the bottom half of the back disc. Positive and
negative charges are removed from the by combs which collect the charge and conduct it to
metal spheres, until it become strong enough to jump across an air gap.
The discs make it difficult to visualize the buildup of charge, so it might become clearer by
considering the discs as opposite rotating cylinders. The inner cylinder represents the front
disc and the outer cylinder the outer disc. On each cylinder the foil sectors are represented
by the rectangular blocks that occupy the perimeter of the cylinders. The neutralizing rod
for the outer disc goes around the outer half of the outer cylinder. The second neutralizing
rod is placed at ninety degrees to the outer neutralizing rod.
If one of the foil sectors on the outer cylinder has a slight charge on its surface, positive say.
It will induce a negative charge on the foil sectors that is closest to it on the inner cylinder.
As the cylinders rotate in opposite directions, the charges are separated. The energy for the
electricity comes from the work done in turning the discs.
At A and B on the two discs, brushes at the ends of a neutralizing rod connect diametrically
opposite sectors momentarily. If we consider one disc, then the sector is opposite, say, a
negatively charged sector on the other disc. A (+) charge is induced on the inner side, while
a (-) charge appears on the outer side, under the brush. At the diametrically opposite
sector, the polarities are reversed, so the (+) charge appears on the outer side under the
other brush. The charges on the outer sides are neutralized as a brief current flow, leaving
the upper sector with a greater (+) charge, and the lower with a greater (-) charge. The
induction of the charges does not depend on how much charge is already there, and equal
signs of charge will not neutralize. The sectors charge to higher and higher potentials each
time they pass beneath a brush and their higher potentials serve to induce even greater
charges.
Kelvin Electrostatic Generator
An electrostatic generator can be made by collecting charge from water droplets. To
explain the operation of this device it is worthwhile considering the how charge can be
collected using the more simple arrangement of a positively charged object brought close
the tip dripping water tap as shown in Figure 3.
Figure 3. Induction Charging Water Drops
Overall water is electrically natural but there are small fractions of the molecules, (the
number comes from the pH of the liquid) which form ions and carry either a positive or
negative charge. In the presence of a positively charged object, the negative ions in the
water are attracted to the positive charge which the positive ions in the water are repelled.
When a drop falls, the negative charge is isolated and the drop becomes negatively charged.
The charge can be collected by catching the water in a metal container. The inside of the
container becomes natural and the negative charge accumulates on the outside of the
container.
All this is fine but it requires a charged object to attract the negative ions to the drop. The
Kelvin electrostatic generator is illustrated in Figure 4. There are two dripping taps, two
metal rings A and B which are connected to the buckets which collect the charge. A is
connected to D and B is connected to C.
Figure 4. Kelvin electrostatic generator
An initial imbalance in charge caused through the ions in the water causes the first initial
drop to become very slightly positively or negatively charged. The small charge difference
is enough to start the charging process. Let us say for the sake of argument, that the first
drop falling into the bucket C is positively charged. The positive charge causes the ring B to
become slightly positively charged which create attracts negative ions from the water. This
cause drops falling from the second tap to become negative. The negative charge stored in
D also causes the metal ring A to become negatively charged which attracts positive ions
from the dripping tap.
Thus the water is separated into two streams of positive and negatively charge drops and
as water is collected in the buckets the charge on each of the buckets increases and
reinforces the charge on the rings. The charge on the buckets builds up until it is great
enough to cause a breakdown in the field across the spark gap at E or if the distance is too
great between the spark gap, coronal discharge from sharp edges causes the charge to leak
into the air.
I have built a Kelvin water dropper from Lego and various bits of junk. It has not been as
easy to make work as you might expect.
Where does the energy come from to make the electricity? In order to raise water to the
height of the nozzles work must be done and this takes the kinetic energy of the falling
drops and slowing them down.
Uses of electrostatics in everyday life
Static electricity has many uses in everyday life. You should know about the following
examples:
Photocopiers and laser printers
The diagram shows how a photocopier works. A laser printer works in a similar way.
Defibrillators
A defibrillator is a machine that can be used by paramedics to restart your heart if it stops.
Two paddles with insulated handles are charged from a high voltage supply. They're put
in good electrical contact with the patient’s chest. It's important that no one else gets a
shock, which is why the paddles have insulating handles and the operator tells any one
nearby to 'stand clear' before charge is passed through the patient to make the heart
contract.
Electrostatic dust precipitators
Smoke is produced when fossil fuels burn. Smoke is made of tiny solid particles, such as
carbon. To remove these particles from the waste gases an electrostatic precipitator is
used.
1. Smoke particles pick up a negative charge.
2. Smoke particles are attracted to the collecting plates.
3. Collecting plates are knocked to remove the smoke particles.
Paint spraying
Car manufacturers can save money by using charged paint spray guns. They work because
like charges repel and unlike charges attract.
The spray gun is charged positively, which causes every paint particle to become
positively charged. Like charges repel and the paint particles spread out. The object to be
painted is given a negative charge and so attracts the paint particles. The advantages of
using this system are that less paint is wasted, the object receives an even coat and the
paint covers awkward ‘shadow’ surfaces that the operator cannot see.
Industrial pollution control devices
These are some of the most common devices used to control industrial pollution.
Cyclone device
Particle removal—cyclone
A cyclone works on the principle of creating a high-speed spiral airflow to remove particles.
This spiral motion applies a centrifugal force on the particles.
The inertia of the particles forces them to the outside walls of the cyclone, where they fall
to the bottom and are collected.
The speed of the airflow and the width of the cyclone influence particle removal—the
smaller the diameter the more efficient the particle removal.
The conical section at the base of the cyclone gradually decreases the diameter of the
spinning gas stream, resulting in better removal of smaller particles.
However, in practice, reducing the diameter increases the pressure drop across the
cyclone. Combining several units together as a ‘multicyclone’ can prevent this.
Electrostatic precipitator
Particle removal—electrostatic precipitator
The electrostatic precipitator works by removing particles and smoke from a gas stream
using an electrostatic charge.
Dust particles travel past wires that have a high direct current (DC) voltage applied, which
ionizes the surrounding gas. This corona discharge causes the particles to pick up a small
electrostatic charge.
An oppositely-charged plate attracts the particles, neutralizes and retains them. A regular
tapping (rapping) of the plates then physically removes the collected particles.
Introducing a water spray can reduce the electrical resistance of the dust particles,
allowing them to accept the charge more easily.
Electrostatic precipitators are an efficient way of removing particles and do not cause a
significant pressure drop across the unit.
Particle removal—bag filters
Bag filters use fabric bags to remove particles from dust-laden gas.
They can achieve high efficiencies for fine particles because of the build-up of particles on
either the inside or outside surface of the bag depending on the design.
As dust builds up, the pressure across the bag increases, so the filters need regular
cleaning. Usually bag filters work in tandem—one filter is working while the other filter is
being cleaned.
Mechanical shaking, using a reverse airflow or a pulse of compressed air are used to
dislodge the dust from the fabric surface, which then falls by gravity into a collection
system.
Packed bed wet scrubber
Particle and gas removal—packed bed wet scrubber
A packed bed wet scrubber removes gaseous and particle contaminants from a gas stream.
It works by bringing the gas stream into contact with a scrubbing liquid (usually water).
The scrubbing liquid absorbs the gaseous pollutants, and the spray droplets physically
capture the particles.
The packed bed of non-reactive material gives a greater gas-to-liquid contact which
improves the removal of particle pollutants.
Although the packed bed wet scrubber can remove gaseous and particle pollutants, the
particular design makes it more efficient at one or the other.
Using certain scrubbing liquids can increase gaseous pollutant removal, e.g. using an
alkaline liquid to remove acidic pollutants.
Trapping and treating droplets of the scrubbing liquid can allow it to be recycled.