Transcript Electrostatics - Cloudfront.net

Electrostatics
Physics I
By Scott Adams
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Video #1
1. Like charges attract each other
a.
True
b. False
2. What are/is a good example of conductors?
a.
Copper
b. Wood
c. Gold
d. A & C
3. How do you get a negatively charged object?
a. Gain electrons
b. Neither
c. Lose electrons
d. a & b
4. A negatively charged rod touches a neutral
spherical conductor and then removed. The
neutral object is now
a.
Negative
b. positive
c. still neutral
Charge
Like Charges Repel
−
−
Like Charges Repel
+
+
Unlike Charges Attract
−
+
Forces are
equal and
opposite!!!
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Conservation of Charge:
During any process, the net electric charge
of an isolated system remains constant
Conductors – allow electrons to flow easily
– ex. Copper, Aluminum, Silver, Gold
Insulators – resist the flow of electrons –
ex. Rubber, plastic, wood, air
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Charging an object
When a rubber
rod is rubbed
with fur or wool,
it becomes
negatively
charged.
(Cutnell & Johnson, 2004)
Recall from Chemistry:
a)How does an object become negatively
charged? Gains Electrons
b)Positively charged?
Loses Electrons
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Celluloid
Sulfur
Rubber
Copper, Brass
Amber
Wood
Cotton
Human Skin
Silk
Cat Fur
Wool
Glass
Rabbit Fur
Objects at the top of
the list have a
greater affinity for
electrons than
objects at the bottom
of the list.
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(Phet, 2010)
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The Electroscope
Knob
Leaves
(Stw, 2005)
9
Determine the Charge on the rod based on the
activity of the leaves and electrons.
Neg.
Leaves Spread more
Pos.
Leaves
Collapse
(Stw, 2005)
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Charging by Conduction
(Cutnell & Johnson, 2004)
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The Quantity of Charge
The quantity of charge (q) can be defined in terms of
the number of electrons, but the Coulomb (C) is a
better unit for later work. A temporary definition might
be as given below:
The Coulomb: 1 C = 6.25 x 1018 electrons
Which means that the charge on a single electron is:
1 electron: e- = -1.6 x 10-19 C
Slide Author: (Tippens, 200a7)
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Units of Charge
The coulomb (selected for use with electric currents) is
actually a very large unit for static electricity. Thus, we
often encounter a need to use the metric prefixes.
1 mC = 1 x 10-6 C
1 nC = 1 x 10-9 C
Slide Author: (Tippens, 2007a)
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Charge
Sphere A carries a charge of +3 C and an identical
sphere B is -1C. If the spheres touch one another and
then are separated, the charge on sphere B would be
-1C
+3C
+3C
+1C
-1C
+3+-1 = +2C
+1C
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Another Example of Charge
Sphere A carries a charge of -6C and an identical
sphere B is -10C. If the spheres touch one another and
then are separated, the charge on spheres A and B
would be
-6C
-10C
-6C
?
-10 C
?
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Video #2
1.
During induction, the neutral charge gains what type of charge
a. same
b. opposite
2. The electric fields get stronger as you travel outward
a. True
b. False
3. A positive test charge of 3x10-5C is placed in an electric field. The force on
it is 0.50N. What is the magnitude of the Electric field?
a. 1.5x105
b. 6
c. 6,000
d. 17,000
4. When you have two equal but opposite charges the electric field lines
_______________?
a. leave the – charge and enter the + charge
b. leave the + charge and enter the – charge
c. Travel away from each other
d. nothing
Pith Ball Demo (No Sound)
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Charging by Induction
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Charging by Induction
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The Electrophorus
1.
2.
--------
3.
-------4.
How does the
pith ball
behave after
being touched
with the
plate?
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Electrophorus
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Magnitude of Electric Fields
F
E
qo
E = Electric Field Intensity (N/C)
Electric Fields Sample Problem
A positive test charge of 4 x 10-5 C is placed in an electric field.
The force on it is 0.60 N. What is the magnitude of the electric field at
the location of the test charge?
1.
Electric Field Lines
Electric Field Lines are imaginary lines drawn in such a
way that their direction at any point is the same as the
direction of the field at that point.
+
++Q +
+ +
++
-- -Q - -
Field lines go away from positive charges and toward
negative charges.
Slide Author: (Tippens, 2007b)
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Examples of E-Field Lines
Two equal but
opposite charges.
Two identical
charges (both +).
Notice that lines leave + charges and enter - charges.
Also, E is strongest where field lines are most dense.
Slide Author: (Tippens, 2007b)
Various Electric Field Configurations
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The E-Field at a distance d from a single charge Q
E
kQ
d
2
Inverse –Square Relationship
E = Electric Field Intensity (N/C)
E Field
Strength
Decreases
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The E-Field at a distance d from a single charge Q
E
kQ
d
2
F is the force on the test charge, qo.
-qo
d
F
+Q
+qo
Draw Electric Fields at C in
the square below:
-q
-q
In which region(s) could
the electric field be zero?
I
C
II
+q
+q
r
III
-2q
+q
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Electric Fields and Conductors Summary
• The electric field inside a conductor
is zero when the charges are at rest.
•Electric fields do not cross each other
•Electric fields do not loop together
• Any net charge on a good conductor is
distributed equally
on the surface.
(Cutnell & Johnson, 2004)
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Electric Fields and Conductors
Summary
• The electric field is always
perpendicular
to the surface of a conductor.
• Inside a nonconductor, an electric
field can exist.
• On an irregularly shaped
conductor, charge tends to
accumulate where the radius of
curvature of the surface is smallest,
that is at sharp points.
+
-
-
+
+
+
-
-+
+ +
++
+
++
+
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1.
Video
#3
What is the value for K?
a. 1.6x10-19 Nm2/C2
b. -1.6x10-19 Nm/C
c. 9x109 Nm2/C2
d. -9x109 Nm/C
2. Two charges double the distance between them, the new
force will be what __________ the original force?
a. 4
b. ½
c. 2
d. ¼
3. Two charged spheres both with the charge of 4x10-5 C are
held a distance of 2 meters apart. What is the magnitude of
the force?
a. 3.6
b. 1x10-5
c. 2x10-5
d. 14.4
4. A positive charge moves to the right through a magnetic
field pointing up. What direction is the force on the charge?
a. Down
b. out of page
c. into page
c. left
Like charges experience forces away
from each other while unlike charges
experience forces towards each other
Coulomb’s Law: F  k q1 q2
2
d
k = 9x109 Nm2/C2
q(charge) = Coulombs (C)
F
F
1
d2
Inverse square
relationship
r
(Cutnell & Johnson, 2004)
-e = -1.60 x 10-19 C , mass of electron = 9.11x10-31 kg
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Coulomb’s Law - Sample Problems
1.
Find the magnitude of the force between the two charges.
2m
---------------------------
+5mC
-3mC
Solution:
q1= +5mC = 5x10-6 C
q2 = -3mC = -3x10-6 C
k = 9x109 Nm2/C2
r = 2m
F=?
Formula:
Substitute:
F
k q1 q2
d2
(9 x109 Nm2 / C 2 )(5 x106 C )(3x106 C )
F
 0.03375 N
2
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(2m)
2.
Find the resultant force on each charged particle.
5 cm
---------------------------
+6mC
-2mC
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3.
A force of 1 N exists between the two charges below. How far
apart are the charges?
?
---------------------------
+5mC
-20mC
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Faraday Cage Video – Teacher Tube
University of Tehran High Voltage Lab.
Person in cage: Prof. H.Mohseni
www.eng.ut.ac.ir/hvlab
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Fun with a Giant Tesla Coil - Video
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Faraday Cage & Cars – You Tube
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References:
Adams. S. (1999). Dilbert Cartoon. Received from 2007 AP Conference complimentary resource
CD. (http://dilbert.com/strips/comic/1999-07-02 )
Cutnell & Johnson Physics. (2004). [Text Art CD]. John Wiley & Sons.
Nave, R. (2010). Hyperphysics.[Illustration]. Permission granted to use illustrations. Retrieved from
http://hyperphysics.phyastr.gsu.edu/hbase/hframe.html
Phet (2010). Balloons and static electricity. Interactive Simulations. University of Colorado.
Retrieved from http://phet.colorado.edu/en/simulation/balloons
Ross Shepard Physics (n.d.) Electroscope Applet [video of Animation]. Retrieved from
http://www.shep.net/resources/curricular/physics/p30/unit2/electroscope.html
Sal44sal44. (2006). Faraday Cage. [Video]. Retrieved from
http://www.youtube.com/watch?v=bZwlD-Z0zmE&feature=related.
Studyvilla.com (n.d.) Equipotential surfaces. Electrostatics II. [Illustration]. Retrieved on
December 22, 2010 from http://www.studyvilla.com/electrostaticsII.aspx
Tippens, P. (2007a). Chapter 23 Electric Force. [PowerPoint Slides]. Received from 2007 AP
Conference Complimentary Resource CD
Tippens, P. (2007b). Chapter 26 Electric Field. [PowerPoint Slides]. Received from 2007 AP
Conference Complimentary Resource CD.
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Tippens, P. (2007d). Electric Potential. [PowerPoint Slides]. Received from 2007 AP
Conference Complimentary Resource CD.
Tyrtle, T. (2008). Van de Graaf Generator – Boston Museum of Science. Retrieved on
December 22, 2010 from http://www.flickr.com/photos/ttyrtle/2734669311/.
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