Chapter 6 Modern Technology
Science explains many phenomena and the ways that
the things work. By the scientific theories and the nature of matters, they
bring a lot of changes in our lives and raise our standard of
living. For example, scientists invented many useful tools in medical
field that are valuable of diagnosing and treating illness and injury. The
invention of magnetically levitated trains and optical fiber reduces the
distance between peoples. Photocopier, inkjet printer, fax machine and
scanner enhance the efficiency of business operations.
In this chapter, we will discuss the physics behind these tools or apparatus.
The belt (drum) inside the photocopier is made of
photoconductor which control the placement of
static electricity.
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The internal components in a photocopier [ii].
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In order
to make a copy of the original paper, place it over the glass surface. Then
the photoconducting belt (platen drum) is charged positively (but for some models it is negative)
by rolling it with corona wire.
The following discuss concerns a positively charged belt(drum).
Light shines on the original. The black region of the paper block the
light and the static electricity
remains in this area while the light is reflected in the white region and the
photoconductor (i.e. the belt) has conducting electrons to neutralize the positive
charge when light shines on it. No charge remains in
this area and forming the charge image on the belt (drum). The positively
charged region of the belt (drum) attracts the negatively charged toner particles.
Place a white paper to make the reproduction and charged by corona wire.
The negatively charge toner is attracted by the paper and form the image.
Then it is heated and pressed to fuse the image onto the paper creating the
final copy. The charge image left on the drum is erased by exposing
under charge erase lamp and the toner is removed by the cleaner. Now it
is ready to make another copy by repeating the procedures.
Various photocopier drums consisting metal roller covered
with a photoconducting material layer made of semiconductor such as selenium,
germanium or silicon.
Corona wires subjected to a high voltage
transferring to the drum and paper in the form of static electricity.
A strong lamp illuminates the original to be copied. A
mirror is used to reflect light passing through a lens onto the drum. By
adjusting the distance between the lens and the drum, the size of the original
image can be reduced or magnified.
Static Electricity
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Static Electricity is used in the copier to arrange the
toner particles on a belt (drum) and transferring to the paper. It can be
generated by Van der Graaff generator with a roller made by a piece of nylon
covered with silicon tape. Since all matters are made up of atoms consisting of
nucleus (neutrons and protons) and electrons in the surrounding shell. If
the number of protons and electrons are not balanced, the atoms is charged.
Different materials have different strength to hold the electrons. When
two non-conducting materials rub to each other, one
material may capture electron from the other material and becomes charged.
When they are separated, the charge imbalance between these two materials
produces the static electricity. The static electricity can make someone's
hair stand on end.

Demonstration of Van der Graaff generator [iii].

The attractive force due to a negative charged balloon on the water column [vi].
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How Things Work
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Why does the paper always hot when they come out of the photocopier?
The
paper passes through the fuser which is a pair of heated rollers before coming
out from the photocopier. The heat will melt the toner powder and the
toner then fusing with the fiber of the paper. Finally, the finished
copy is rolled to the output tray. The temperature of the finished copy
will not be too hot because the speed of the paper rolling over the fuser is
very fast, otherwise, it will burn up due to the high temperature of the
fuser.
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Inkjet Printer
An inkjet
printer is the printer that create image by placing extremely small droplets of
ink onto paper. The dots are small, about 50-60 microns in diameter which
is smaller than the diameter of human hair (70 microns). The resolution is
about 1440 x 720 dots per inch (dpi). It may have different colors
combining together to form a dot, creating photo-quality images. Inkjet
printers have print head with 300 to 600 firing chambers which are tiny nozzles
used to spray thousands of droplets of ink per second in a precise pattern to
make up the text and images on a page. There are two types of inkjet
technologies to squirt the ink droplets, thermal bubble printing technology and
piezoelectric printing technology.
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View of the nozzles on a thermal
bubble inkjet print head [i].
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Thermal bubble printing
It uses heat to create a tiny bubble in the firing chamber forcing out an ink
droplet.
Each print head has many tiny nozzles that can fire ink droplets simultaneously.
How Things Work |
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Bubble Jet Printing |
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1. To push the ink through the nozzle, bubble jet printers heat minuscule quantities of ink
by passing an electrical charge through a resistor through a resistor, which quickly reaches 900 degrees Fahrenheit.
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Operation of a bubble jet printer, step 1 [v]

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2. The heating element vaporizes a tiny layer of ink at the bottom of the chamber for a few
millionths of a second, forming a bubble, pushing the ink down the nozzle.
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Operation of a bubble jet printer, step 2 [v]

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3. The bubble expands and forces a droplet of ink out of the nozzle. Colored droplets are generally so
small that a quart could contain 100 billion such drops or more; black droplets are about four times as big. The
whole process takes about 10 millionths of a second. The heating element cools and the bubble collapses, creating suction
that draws more ink into the chamber from the ink cartridge.
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Operation of a bubble jet printer, step 3 [v]

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Piezoelectric printing
It uses a piezoelectric crystal, which
bends when an electrical charge is applied to fire the ink drop onto the paper.
How Things Work |
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Piezoelectric Printing |
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1. A piezoelectric printer works like a squirt gun, but instead of a trigger and plunger, it uses
a piezoelectric crystal that changes shape when an electrical charge is applied. A small negative charge deflects the crystal
away from the chamber, creating suction.
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Operation of a piezoelectric printer, step 1 [v]

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2. A positive charge bends the crystal in the other direction, which pushes a plate into the
chamber to create the pressure to expel a droplet of ink.
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Operation of a piezoelectric printer, step 2 [v]

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3. An advantage to this approach is that the quantity of ink in the droplet can be precisely controlled.
A small charge causes a slight deflection, enough to discharge as little as three-trillionths of a quart of ink
through the nozzle. Larger charges can produce larger droplets. The ink is forced out through the nozzle.
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Operation of a piezoelectric printer, step 3 [v]

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Laser printer
Laser printer
uses a laser beam to write letters or draw pictures on paper by sending data
from the computer. What is the scientific principles using in the laser
printing process? We will discuss this mystery in this section.
The principle
used in a laser printer is static electricity. Initially, the photoreceptor drum is
charged positively by corona wire by applying an electrical current on it.
Then the
printer shines a tiny laser beam across the surface according to the data sent
by the computer, one horizontal line at a time. The laser beam shines
light on the drum for dot and light off for empty space on the page. The
laser beam does not move itself but shines light through a movable mirror
instead. The light discharge certain points on the photoreceptor drum and
form an electrostatic image.
After
the pattern is set, the toner stored in a toner hopper is gathered by the developer
unit. The positively charged toner clings to the discharged areas of the
drum but not to the positively charged background (area with no light shine on).
A sheet of
paper (with strong negative charges) is moving along the belt and rolls over the drum with affixed toner powder
pattern. The paper pulls the toner powder away from the drum and picks up
the image pattern fixed by the fuser. Then the finished copy is rolled to
the exit tray.
Comparison of laser printer with photocopier
Laser printers work the same basic way
as photocopiers, with a few significant differences.
- A photocopier scans an image by
reflecting a bright light off of it, while a laser printer receives the image in
digital form.
- The electrostatic image is created by different ways:
- For the photocopier, light bounces off a
piece of paper and reflects back onto the photoreceptor from the white areas but
is absorbed by the dark areas. The background is discharged. This method is
called "write-white".
- For the laser printer, the process is
reversed. The laser discharges the lines of the electrostatic image and
leaves the background uncharged. This method is called "write-black".
X-rays
How Things Work
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- How X-rays test works?
- How X-rays is used to see inside the body? What is the difference about bones and
tissues that bones appear white in an X-ray image while tissues appear dark?
X-rays are electromagnetic waves with high
energy. They are produced in a vacuum-filled tube, X-ray tube. An X-ray tube
is an evacuated glass envelope within which a coiled tungsten filament [1]
(the cathode) acts as a source of electrons. A low voltage [2] heats
the filament, "boiling" electrons off its surface. These are focused and
accelerated to high speed to around half the speed of light by a large voltage [3]
between the target [4] (the anode) and the cathode.
- The electrons
collide with the tungsten anode, producing X-rays via two different
mechanisms:
bremsstrahlung and X-ray fluorescence
and give out heat as a by-product. To prevent the heat from
building up, a motor [5] spins the target to 3000 rpm: in addition, a
layer of oil around the tube helps dissipate the heat. X-rays emerge through
an opening in the housing. Before reaching the patient, they pass through a
number of adjustable apertures [6], which limit the size of the X-ray
field according to the size of the film. A lamp [7] and mirror [8]
provide a beam of light that coincides exactly with the path of the invisible
X-rays: this is used to aim the radiation field.

Bremsstrahlung [i]
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X-ray fluorescence [i]
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Photoelectric effect

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- When the X-rays have enough
energy, the photoelectric effect happens.
One of the atom's electrons absorbs
the X-ray photon and is tossed completely out of the atom. Part of the
photon's energy is used to remove the electron from the atom and the rest is
given to the emitted electron as kinetic energy. For the relatively weakly
bound electrons in a small atom, the X-ray photon would give the ejected
electron a large kinetic energy. A small atom usually just ignores the
passing X-ray. In contrast, some of the electrons in a large atom are quite
tightly bound and require most of the X-ray photon's energy to remove. These
electrons would depart with relatively little kinetic energy. A large atom is
likely to absorb a passing X-ray. Thus the small atoms found in tissue
(carbon, hydrogen, oxygen and nitrogen) rarely absorb medical X-rays, while
the large atoms found in bone (calcium) absorb X-rays frequently. That's why
bones cast clear shadows onto X-ray film and tissue shadows are less obvious.
X-ray film [9] is basically the same as photographic film, but has
greater sensitivity. It is coated on both sides with emulsion [10]
that detects X-rays and light alike. The film is sandwiched between two
fluorescent screens [11], which emit light when they are struck by
X-rays. The light emitted exposes the film, thereby forming a more intense
image than X-rays alone. Above the film is a grid of many tiny holes [12].
This allows through those X-rays that have passed straight through the body [13],
but not those that have been scattered-deflected by structures within the
patient [14]. These scattered rays would otherwise blur the image.
A X-ray film

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CT scanner
How Things Work
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- How can a CT scan show a cross-sectional view of a living person?
Computed tomography (CT) scan
is used to determine exactly where things are located inside a patient.
It can separate the superimposed internal structures of image obtained from
simple radiographs which are difficult to interpret. An X-ray beam is
passed through a thin slice of the body and is detected by a bank of detectors
as it emerges. The beam is then rotated around the subject, and another
exposure made until the same slice has been surveyed from all angles.
The scanner then shifts the patient's body to work on the next slice. A
computer reconstructs an image of the slice by mathematical method.
Many slices can be stacked on screen to form a three-dimensional view of a
patient's internal organs.
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A scanned liver slice [i]
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Using X-ray to destroy a tumor
How Things Work
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- How can X-ray destroy a tumor?
When a patient is exposed to 1,000,000eV
photons, most of the photons pass right through them but a small fraction
undergo Compton scattering and leave some of
their energy behind. This energy kills tissue and can be used to destroy a
tumor. By approaching a tumor from many different angles through the
patient's body, the treatment can minimize the injury to healthy tissue around
the tumor while giving the tumor itself a fatal dose of radiation.
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Compton scattering
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The first picture of a baby child
How Things Work
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- How can ultrasound be used to take the first picture of a child before birth?
In a medical scanner [B],
ultrasound of between 1 and 15MHz are transmitted into the body, and the
returning echoes are detected. The scan head [C] is made of more than
100 separate piezoelectric transducer elements. Each of these is a block of
synthetic piezoelectric material lead
zirconate titanate (PZT). Under computer control, these are made to emit
sound in a precise sequence. This produces a highly focused "spot" of sound,
which is scanned in a single plane over the fetus [D].
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Ultrasound side view image of a growing fetus showing (left to right) the head, neck, torso and legs. [i]
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By detecting the intensity and return time
of the echoes from the boundaries of different tissue types, the depth and
density of the tissues are revealed. The intensity of the echo at each point
of the scan is converted electronically into a shade of gray, which is
displayed on a screen. Ultrasound is entirely reflected at junctions between
tissue and air, but moves well in liquids. A layer of aqueous gel is applied
between skin and scan head, and a full bladder ensures that sound has an
air-free path to the fetus.
Ultrasound scan can also be used to detect tumors
or fluid-filled cysts. Relatively large echoes are sent back from organ
boundaries, while small structures within the tissue give "grainy" low-level
echoes which can be distinguished from healthy tissue.
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A Maglev in Shanghai
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Maglev train,
short for magnetic levitation train, is a new form of transport mode that
enhances the speed of carrying passengers from one end to another. It uses
the basic principles of magnets floating over a guideway to replace the old steel
wheel and track that is a breakthrough of the limitations of friction between
the train wheels and its rails.
Basic knowledges of Magnetic fields
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A bar magnet has two poles, called the north pole and the south pole.
Magnetic field leaves
the magnet from the north pole and goes to the magnet again at the south poles.
This magnetic field forms closed lines as presented in the diagram of the
bar magnet.
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Other than the permanent magnet, magnetic field can be produced by moving electric charge called electric
current. When electric current flows through a loop of wire, magnetic field is so produced, as shown in the
above figure.
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Consider a current in a wire, the direction of the magnetic field is perpendicular to the electric
field. The lines of the magnetic field form concentric circles around
the wire. Right hand rule
can express this direction relationship. The direction of the magnetic
field is perpendicular to the wire and is in the curling direction of your
right hand fingers. The direction of your thumb is the direction of
electric current.
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An electromagnet can be obtained in the following way. You can wrap the wire around a nail several times,
the nail behaves just like a bar magnet when current runs through the circuit.
Putting iron or other magnetic material inside the coil can make the
electromagnet even stronger. Besides, increasing the number of turns
of wire wrapping can also enhance the magnetic field generated.
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Forces between Magnets
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Suppose magnets are placed at the bottom of a train
and over the top of a track facing each other with like poles,
repulsion forces exert on one another and support the train without any direct contact.
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Illustration of levitation magnets [iii]. Ring magnets sit
around a large wood dowel. The top magnet is pushed down and oscillations are
observed.
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The repulsion becomes stronger as the magnets approach one
another and exceeds the train's weight. The train can remain
suspended over the track on a magnetic cushion and forms the magnetic
levitation. Although the height of the train can keep stable, this
suspension is unstable in horizontal direction. If the train is not
perfectly centered, the train will tend to slip sideways until it falls. The only way to stabilize the train and keep it centered above the
track is using adjustable magnets---electromagnet, to push the train back to the center of
the track if it starts to fall.
In a latter section, we will introduce a sophisticated control system which governs
the lateral guidance. The system monitors
the train's position and adjust the electromagnet to maintain a
constant separation between those poles by the information of the current
situation.
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Electromagnetic induction
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A changing magnetic field creates an
electric field which pushes on the conductor's
mobile electric charges to move and form an electric current. This
process is called
electromagnetic induction.
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Demonstration of electromagnetic induction [iii]
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In the above demonstration, a small light bulb is connected to a large
coil of wire. The coil is moved in and out of a magnet. This changing
magnetic field induces an electric current. The bulb lights as the current
passing through the circuit. In fact, the current runs in a direction such that
a magnetic field is created to oppose the change. This is the
Lenz's law.
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When an alternating
current is flowing through a coil of wire wrapping around an iron
core, the poles of the electromagnet is reversed by changing the direction of
electric current flowing. If an electric conductor (the metal bar in the
above figure) is placed near the
electromagnet, the magnetic field is so induced on the metal bar to
oppose the changing magnetic field above it. It is, of course, due to the induced alternating current
in the metal bar. Simply, a changing magnetic field in the iron core turns an electric
conductor nearby into a magnet.
For example, if the south pole of the electromagnet is near the
conductor, then the conductor becomes magnetic with its south pole up
pointing towards the electromagnet to repel it. When the current
now reverses the direction changing the pole direction with north pole down,
the conductor becomes magnetic with its north pole up to repel the
electromagnet. The currents generated by electromagnetic induction
always produce magnetic fields that oppose the magnetic field change.
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Image of the guideway for the Yamanashi maglev test line
in Japan [iii]
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Maglev train rides above the track by magnetic suspension instead of rolling on
wheels conventionally. The working principles are as follows. [iv]
- The magnetic levitation
The "8" figured levitation coils are installed on the sidewalls of the guideway.
When the on-board superconducting magnets pass at a high speed about several
centimeters below the center of these coils, an electric current is induced within
the coils, which then act as electromagnets temporarily. As a result, there are
forces which push the superconducting magnet upwards and ones which pull them upwards
simultaneously, thereby levitating the Maglev vehicle.
- The lateral guidance
The levitation coils facing each other are connected
under the guideway, constituting a loop. When a running Maglev vehicle,
that is a superconducting magnet, displaces laterally, an electric current
is induced in the loop, resulting in a repulsive force acting on the levitation
coils of the side near the car and an attractive force acting on the levitation
coils of the side farther apart from the car. Thus, a running car is always
located at the center of the guideway.
- The propulsion
A repulsive force and an attractive force induced between the magnets
are used to propel the vehicle (superconducting magnet). The propulsion
coils located on the sidewalls on both sides of the guideway are energized
by a three-phase alternating current from a substation, creating a shifting
magnetic field on the guideway. The on-board superconducting magnets are
attracted and pushed by the shifting field, propelling the Maglev vehicle.
However, the ordinary permanent magnet is
heavy and expensive, so most electrodynamics levitation schemes use
electromagnets instead. They use special wires made of
superconductors that
the currents flow perfectly
and freely while currents flowing in metal experience frictionlike effects and
gradually slow down. Superconductor can behave like a light and
superstrong permanent magnet at low temperature. A maglev train using
superconducting magnets must be kept cold to make them magnetic and can hover
easily without requiring much electric power.
Reference
- Ruppel, Tom, The Way Science Works, New York: Macmillan, 1995.
- Harris, Tom, "How X-rays Work" at www.howstuffworks.com.
- Gould, Todd A., "How MRI Works" at www.howstuffworks.com.
- Freudenrich, Craig C., "How ultrasound Works" at www.howstuffworks.com.
- Bloomfield, Louis A., How things work: the physics of everyday life.
New York: John Wiley, 1997.
- Meeker-O'Connell, Ann, "How Photocopier Works" at www.howstuffworks.com.
- Tyson, Jeff, "How Inkjet Printers Work" at www.howstuffworks.com.
- Fountain, Henry, The New York times circuits: how electronic things
work. New York: St. Martin's Press, 2001.
- Harris, Tom, "How Laser Printers Work" at www.howstuffworks.com.
- Nave, Rod, Hyper Physics at hyperphysics.phy-astr.gsu.edu/hbase/hframe.html.
- Bonsor, Kevin, "How Maglev Trains Work" at www.howstuffworks.com.
- Bigelow, Ken, Play-Hookey Website at www.play-hookey.com.
- A website of Railway Technical Research institute at "http://www.rtri.or.jp/rd/maglev/html/english/maglev_frame_E.html".
Glossary
- Alternating current
- An electric current
flowing in the direction that reverses periodically. While a current flows
continuously in one direction called a direct current.
- Bremsstrahlung
- It refers to cases in
which a charged particle accelerates extremely rapidly attracted by the nucleus
of an atom and emits a very high
energy photon. In X-ray tube bremsstrahlung, a fast-moving electron arcs
around a nucleus of a heavy atom and accelerates so abruptly that it loses
energy in the form of X-ray photon.
The closer the electron comes to the nucleus, the more it accelerates
and the more energy it gives to the X-ray photon. However the electron is more
likely to miss the nucleus by a large distance than to almost hit it, so bremsstrahlung is more likely to produce a lower energy X-ray photon than a
higher energy one.
- Compton scattering
- It occurs when
an X-ray photon collides with a single electron so that the two particles bounce
off one another. The X-ray photon knocks the electron right out of the atom and
then scattered away.
- Electromagnet
- It is a coil of wire that
is magnet only when an electric current flowing through it.
- Electromagnetic induction
- The process of a changing magnetic field inducing an electric current.
- Lenz's law
- Current induced by a
changing magnetic field always produces a magnetic field that opposes the
change.
- Photoconductor
- It is a material which is an electric
conductor allowing electric charges to move in the light and being an electric
insulator preventing any movement of electric charges in the dark..
- Photoelectric effect
- One of the electrons in
an atom absorbs the photon energy and is ejected out of the atom. The
energy of the photon is equal to the sum of the energy needed to eject out the
electron and the kinetic energy gained by that electron.
- Piezoelectric crystal
- It has remarkable
properties. When a voltage is applied, it changes shape; as soon as the voltage
is switched off, the crystal snaps back to its original conformation. An
oscillating voltage therefore makes the crystal vibrate, producing
high-frequency sound as it does so.
- Right hand rule
- The direction of the
magnetic field is perpendicular to the wire and is in the curling direction of
your right hand fingers. The direction of your thumb is the direction of
electric current.
- Superconductor
- Superconductor is a material that at low
temperature become perfect conductor of electricity.
- X-ray fluorescence
- The fast moving electron
collides with an heavy atom, it knocks out the electrons in one of the inner
orbitals and leaves the atom as a positive ion with a vacant orbital.
A higher orbital electrons fills the empty orbital and release its excess energy
creating X-ray photon.
Acknowledgement
- HowStuffWorks
- The University of
Minnesota
-
http://hyperphysics.phy-astr.gsu.edu (experimenter: Jennifer Craigo)
-
Railway Technical Research Institute
- Fountain H., The New York Times Circuits: How Electronic Things Work, New York: St. Martin's Press, 2001.
- Walker J.S., Physics, 3rd edition, New Jersey: Pearson Education, Inc., 2007.
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