Download Week 7 (Ph-1002) lectures by Ambreen Aslam

Physics-II
(Ph-1002)
WEEK # 7
Electric charge & Electric field:
1. Electric charge, conductors, insulators,
2. electric fields & electric forces, electric field lines, electric
diploes
3. Induced charges, Coulombs law
Book:
“University Physics” by Young & Freedman 13th Edition
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21.1: Electric Charge
►The ancient Greeks discovered as early as 600 B.C.
that after they rubbed amber with wool, the
amber could attract other objects.
► Word “electric” derived from the Greek word
elektron, meaning amber.
► Benjamin Franklin (1706–1790) suggested calling
these two kinds of charge negative and positive,
these names are still used.
► The plastic rod and the silk have negative charge.
The glass rod and the fur have positive charge.
Figure on next slide
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Applications of static charge (Copiers and Computer Printers )
Force of attraction plays a role in their mechanism
Photocopier machine:
Copying process is called xerography, Greek words: xeros
means view and graphos means dry writing.
The heart of a copier is xerographic drum, an aluminum (good
conductor) cylinder coated with a layer of selenium. Selenium
(photoconductor: insulator in the dark but conductor in light)
A positive charge deposited on the selenium surface will
remain in dark.
When the drum is exposed to light then electrons from the
aluminum pass through the conducting selenium and
neutralize the positive charge.
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Operation is as under:
Steps of printing
1.
2.
3.
4.
5.
an electrode called a corotron gives entire selenium surface a positive charge in
dark.
series of lenses & mirrors focuses an image of document onto the revolving drum.
The dark & light areas of the document produce corresponding areas on the drum.
The dark areas retain their positive charge, but the light areas become conducting
and lose their positive charge, ending up neutralized. Thus, a positive charge image
of the document remains on the selenium surface.
dry black powder (toner), is given a negative charge & then spread onto the drum,
where it adheres selectively to the positively charged areas.
transferring the toner onto a blank piece of paper. However, the attraction of the
positive-charge image holds the toner to the drum. To transfer the toner, the paper
is given a greater positive charge than that of the image, with the aid of another
corotron.
Lastly: paper & adhering toner pass through heated pressure rollers, which melt the
toner into the fibers of the paper & produce finished copy.
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Xerographic process steps
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A Laser Printer
• It is used with computers to provide high-quality copies of text & graphics. It is
similar in operation to the xerographic machine, except that the information to be
reproduced is not on paper.
• Instead, the information is transferred from computer’s memory to printer,
and laser light is used to copy it onto selenium-aluminum drum.
• Laser beam, focused to a fine point, is scanned rapidly from side to side across the
rotating drum. While the light remains on, positive charge on the drum is
neutralized. As laser beam moves, computer turns beam off at right moments
during each scan to produce the desired positive-charge image, which is letter
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“A”.
An inkjet printer
1.
Inkjet print-head ejects a thin stream of ink while shuttling back & forth across
paper. Figure illustrates the elements of one type of print head. Ink is forced out of a
small nozzle & breaks up into extremely small droplets, with diameters that can be
as small as 9 × 10−6 m.
2.
About 150 000 droplets leave the nozzle each second & travel with speed of ~ 18
m/s toward
paper. During their flight, droplets pass through two electrical
components, an electrode & deflection plates (a parallel plate capacitor). When the
print-head moves over regions of paper that are not to be inked, charging control is
turned on & an electric field is established between the print-head and the electrode.
3.
As the drops pass through electric field, they acquire a net charge by process of
induction. The deflection plates divert charged droplets into a gutter & thus prevent
them from reaching the paper. Whenever ink is to be placed on paper, charging
control, responding to instructions from computer, turns off electric field.
Uncharged droplets fly straight through deflection plates &strike paper.
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An inkjet print-head ejects a steady flow of ink droplets.
Charged droplets are deflected into a gutter by deflection
plates, while uncharged droplets fly straight onto paper.
Letters formed by an inkjet printer look normal, except when
greatly enlarged & patterns from drops become apparent. 9
Electric Charge
and the Structure
of Matter; p-689
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• The negative charge of the electron has (within experimental error)
exactly the same magnitude as the positive charge of the proton. In a
neutral atom the number of electrons equals the number of protons in
the nucleus, and the net electric charge (the algebraic sum of all the
charges) is exactly zero (Fig. 21.4a).
•
The number of protons or electrons in a neutral atom of an element is
called the atomic number of the element. If one or more electrons are
removed from an atom, what remains is called a positive ion (Fig.
21.4b).
•
A negative ion is an atom that has gained one or more electrons (Fig.
21.4c). This gain or loss of electrons is called ionization.
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Fig. 21.4
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Two very important principles
First: The algebraic sum of all the electric charges in
any closed system is constant.
Second:
The magnitude of charge of the electron or proton is a
natural unit of charge.
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Fig. 21.5
Most of the forces on this water skier are electric.
Electric interactions between adjacent molecules give
rise to the force of the water on the ski, the tension in
the tow rope, and the resistance of the air on the
skier’s body. Electric interactions also hold the atoms
of the skier’s body together. Only one wholly
nonelectric force acts on the skier: the force of
gravity.
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21.2 Energy bands for a conductor, and insulator, and a
semiconductor
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That’s all for today !
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