Transcript Document

OVERVIEW
• Green sheet
• Online HW assignments
• Practice Problems
• Course overview
See course website
www.physics.sjsu.edu/Becker/physics51
C 2012 J. Becker
PHYSICS 51 GREEN SHEET
Front side:
Course outline
Back side:
Reading assignment
and problems
3 slides printed on a page
Chapter 21A Electric Field and
Coulomb’s Law
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Electric charge
Conductors, insulators,
and induced charge
Coulomb’s Law
Electric field lines
C 2012 J. Becker
(sec. 21.1)
(sec. 21.2)
(sec. 21.3)
(sec. 21.6)
Learning Goals - we will learn:
• The nature of electric charge.
• How objects become electrically charged.
• How to use Coulomb’s Law to calculate the
electric force between charges.
• How to calculate the electric field caused
by electric charges.
• How to use the idea of electric field lines
to visualize electric fields.
Electric charge
•Protons have positive charge
•Electrons have negative charge
•Opposite signs attract
•Similar signs repel
•Electric field – used to calculate force
between charges
C 2012 J. Becker
Photocopiers are amazing devices. They
use electric charge to hold fine dust
(toner) in patterns until the pattern may
be transferred to paper and made
permanent with heat.
LITHIUM (Li) ELEMENT
Atom: electrically neutral 3 protons and 3 elec.
Positive ion: missing one electron so net
charge is positive
Negative ion: has added electron so net
charge is negative
A positive charge and a negative
charge attract each other.
Two positive charges
(or two negative charges)
repel each other.
Figure 21.5
Electric forces in action
Figure 21.1a
Figure 21.1b
Figure 21.1c
Figure 21.6a
Copper is a good conductor of electricity;
Glass and nylon are good insulators
Figure 21.6b
Figure 21.6c
CHARGING A METAL SPHERE BY INDUCTION
Charges are free to move in a conductor but
are tightly bound in an insulator.
The earth (“ground”) is a large conductor
having many free charges.
POLARIZED
insulator
CHARGED COMB ATTRACTS
A PIECE OF PAPER
In an insulator the charges
can move slightly (called
polarization of the insulator).
A piece of paper is attracted
to a charged comb because
the positive charges are
closer to the negatively
charged comb
(in the upper figure).
e - PAPER DISPLAY (Kindle, i - PAD, etc.)
wikipedia.org/wiki/Electronic_paper
Scheme of an electrophoretic display.
e - PAPER DISPLAY (Kindle, i - PAD, etc.)
wikipedia.org/wiki/Electronic_paper
Each capsule contains an oily solution containing
black dye (the electronic ink), with numerous white
titanium dioxide (white) particles suspended within.
The white particles are slightly negatively charged.
Applying a negative charge to the surface electrode
repels the particles to the bottom of local capsules,
forcing the black dye to the surface and giving the
capsule, or pixel, a black appearance. Reversing the
voltage has the opposite effect - the particles are
forced to the surface, giving the pixel
a white appearance.
An uncharged conductor can attract the
charge imparted to paint droplets. Excess
charges can flow to or from “ground”
Car door
-e
The imaging drum is aluminum coated with
selenium, which changes from an insulator to
a conductor when illuminated with light.
Figure 21.2
LASER PRINTER USES CHARGED TONER
FORCE
between two charges
is given by
Coulomb’s Law:
| F | = k | Q qo | / r 2
We can use our notion of the
gravitational field to form the concept of an
ELECTRIC FIELD (E)
Recall force between two masses: F = m g
g is the gravitational field (9.8 m/sec2)
| F | = G | M m | / r2
The force between two charges Q and qo is
given by: F = qo E
| F | = k | Q qo | / r2
Coulomb’s Law:
| F | = k | Q qo | / r 2
Rearranged:
| F | = | qo [k Q/r2] |
Gives us:
F = qo E
where the electric field
E is:
| E | = | k Q / r2 |
ELECTRIC FIELD LINES START AND END
AT ELECTRIC CHARGES
An electric charge is surrounded by an electric
field just as a mass is surrounded by a
gravitational field.
Electric field and equipotential lines
are perpendicular to each other
In Lab #2 a
voltmeter is
used to measure
the equipotential
lines (in Volts)
in order to
determine the
magnitude and
direction of the
electric field
lines.
Forces on electron beam in a TV tube (CRT)
F = Q E and F = m g (vector equations)
TV tube with electron-deflecting charged
plates (orange)
F=QE
Review
see
www.physics.sjsu.edu/Becker/physics51
INTRODUCTION: see Ch. 1
Vectors Review (See Chapter 1)
Used extensively throughout course
C 2012 J. Becker
Vectors are quantities that
have both magnitude and
direction.
An example of a vector
quantity is velocity. A
velocity has both magnitude
(speed) and direction, say
60 miles per hour in a
DIRECTION due west.
(A scalar quantity is
different; it has only
magnitude – mass, time,
temperature, etc.)
A vector may
be composed of
its x- and ycomponents as
shown.
Ax  A cos 
Ay  A sin 
A  Ax  Ay
2
2
2
The scalar (or dot) product of two
vectors is defined as
A  B  AB cos  Ax Bx  Ay By  Az Bz
Note: The dot product of two
vectors is a scalar quantity.
The vector (or cross) product of
two vectors is a vector where the
direction of the vector product is
given by the right-hand rule.
The MAGNITUDE of the vector
product is given by:
A  B  AB sin
Figure 21.14
PROFESSIONAL FORMAT