How Things Work II
(Lecture #9)
Instructor: Gordon D. Cates
Office: Physics 106a, Phone: (434) 924-4792
email: cates@virginia.edu
Course web site available through COD and Toolkit
or at http://people.virginia.edu/~gdc4k/phys106/spring08
February 6, 2008
Announcements
• Please read through section 10.2 by Friday.
• I believe that the problem with the receipts
from eclass is fixed. Is it?
• There was an error on the due date, it is now
fixed, and I don’t believe that anyone will be
inappropriately penalized.
Electric Charge 1
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Two types: positive and negative.
Like charges repel, opposites attract.
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Forces consist of a matched pair:
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Each charge pushes or pulls on the other.
Forces have equal magnitudes, opposite directions.
Forces increase with decreasing separation.
Charge is measured in coulombs.
Electric Charge 2
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Charge is conserved.
Charge is quantized.
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One fundamental charge is 1.6·10-19 coulombs
Charge is an intrinsic property of matter.
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Electrons are negatively charged.
Protons are positively charged.
Each has one fundamental charge (of opposite sign).
Net Charge
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Net charge is the sum of an object’s charges.
Most objects have zero net charge (they are neutral).
Neutral objects contain equal amounts of
+ and - charges.
Charging objects by rubbing them together
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Rubbing a teflon rod with silk leaves a negative
charge on the teflon.
Rubbing a plexiglas rod with teflon leaves a positive
charge on the plexiglas.
Question:
A woman rubs her feet on the carpet and gives
a shock to her identical twin. If the twin also
rubs her feet on the carpet before being
touched, the shock will be:
A. larger.
B. smaller.
C. the same size.
Coulomb’s Law
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The force between two charged particles is
given by Coulomb’s law:
k q1 q2
F=
r2
where
k = 8.988 x 109 N·m2/C2
Voltage
+
+
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+
+
+
++
+
+
+
Note:
Don’t take this
figure too literally!
+
It takes work to move a charged object toward a bunch of like charges.
Once the charged object is close, it has electrostatic potential energy.
The potential energy PER UNIT CHARGE is called VOLTAGE.
Voltage is measured in volts = joules / coulomb.
Charging an object using voltage
• So an object at positive voltage has net
positive charge.
• and an object at negative voltage has net
negative charge.
Corona Discharge
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Charge tends to accumulate on sharp points.
Once you get enough charge onto a point, it
will start jumping off into the air.
This is called “corona discharge”.
van de Graaff generator
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Charge is sprayed onto
the belt.
Charge is transported
into the sphere.
A conducting “brush”
touches the belt and
allows the charge to
jump off of the belt and
flow onto the sphere.
The voltage builds and
builds.