Transcript Electrostatics

The study of electrical charges
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Two possible states of charge:
 Positive and negative
▪ Named by Benjamin Franklin
▪ He decided what was considered “positive” and what
was “negative”
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When 2 objects are rubbed together, one
becomes “positive” and one becomes
“negative”…electrons are transferred from
one object to another.
Flow of electrons = electricity
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Insulators: materials that charges do not
easily move through.
 Plastic, rubber, dry wood, glass
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Conductors: materials that allow charges to
move.
 Examples: metals…copper, zinc, nickel, lead
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Electric forces are the interaction of charged
particles.
Things to note:
 2 kinds of electrical charge: positive and negative
 Charges exert force on other charges over a
distance
 Like charges repel…opposite charges attract.
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Charging a neutral body by touching it with a
charged conductor. Electrons transfer to or
from the conductor.
The charge spreads throughout the neutral
body.
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A charged rod is held near a conductive
material. Like charges repel, opposites
attract.
 Material is grounded to allow transfer of electrons
to earth.
 Ground then removed.

Grounding: touching an object to Earth to
eliminate excess charge.
 Lightning rods, grounding gas trucks, etc.
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Read Section 16.1 (pages 558-563)
Do Section Review #1-6
16.2

Charles Coulomb
 Worked with charged spheres to understand
relationships between charge, distance, and force.
 Found that:
▪ F α 1/d2
▪ F α qq’
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Coulomb’s Law
 K = 9.0 x 109 N*m2/C2
 q = charges (In Coulombs (C))
 d = distance (meters)
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1 Coulomb = the charge of 6.25x1018 electrons
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1 Electron = 1.6x10-19 C
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Object A has a positive charge of 6.0x10-6 C.
Object B, carrying a positive charge of 3.0x106 C, is 0.03 m away.
 A. What is the force on A?
 B. What would be the force on A if the charge on
B were negative?
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Object A has a 6.0x10-6 C charge and has 2
other charges nearby. Object B is 0.04 m to
the right of A and has a charge of -3x10-6 C
and object C is 0.03 m directly below A and
has a charge of 1.5x10-6 C. What is the net
force on A?
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Pg. 566 #1,3
Pg. 568 #2ab
Pg. 570 #1-2
Pg. 571 #1,3,4
16.3
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Concept of electric field developed by
Michael Faraday
 A charge creates an electric field about it in all
directions.
 A 2nd charge inserted into the field interacts with
the field at that point.
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Only observed by measuring impact on other
charged particles.
 A small positive test charge is placed a certain
distance away
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Electric fields are always considered using a
small positive test charge
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Electric field lines show the direction of the
electric field radiating from a charge.
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When considering electric fields produced
from multiple charges, the field lines become
curved and more complex.
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Homework:
Pg. 575 #1-2
Pg. 581-585
 #3,5,15,18,21,24,31,32,34,35,41,45,46
Section 17.1
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The work done on moving a charged particle
in an electric field can result in the particle
gaining potential or kinetic energy.
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Peelectric = -qEd (in uniform electric field)
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Electric potential increases as positive test
charge is separated from negative charge
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Electric potential increases as positive test
charge is moved toward positive charge.
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Change in potential energy per unit charge
Measured in J/C = V
 V = Volt
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ΔV = ΔPE/q’
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Reference level for zero potential in a field is
arbitrary
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Only differences in electric potential are
important
Potential difference
 Measured with a voltmeter
 Often called “voltage”
 V = VB - VA
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Uniform field can be produced by oppositely
charged conducting parallel plates.
ΔPE = +Fd
V = +Fd/q = +(F/q)d
E = F/q
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V = Ed
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All systems reach equilibrium when energy is at
a minimum.
Charges on 2 objects try to spread out to reduce
electrical potential.
Grounding: touching an object to Earth to
eliminate excess charge.
 Lightning rods, grounding gas trucks, etc.
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Charges spread out as far apart as possible
 A Hollow conductor will have all charges on
outside surface.
▪ Shields the inside from electric current
▪ People in a car with power lines across it.
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Pg. 599 #1-3
Pg. 601 #5-9
Pg. 626-630 #9, 59, 61, 62, 64
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Capacitance (C)
 = the ratio of charge to potential difference
 C = q/V
 Measured in farads (F)…1 F = 1C/V
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A device designed with a specific capacitance
is a capacitor.
 Made of 2 conductors with equal and opposite
charge separated by an insulator.
 Most capacitors are between 10 picofarads and
500 microfarads.