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PHYSICS 632 SUMMER 2008 9:00 – 10:50 Room 203 Electricity, Magnetism and Light Richard A. Lindgren, Office Room 302. Text: Halliday, Resnick, and Walker, 7’th edition, Extended Version, Starting with Chapter 21 on Charge. Your Goals and Interest. get a degree crossover teaching
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Text: Halliday, Resnick, and Walker, 7’th edition, Extended Version,
Starting with Chapter 21 on Charge.
Bohr Model
3
Consider solid material like a piece of copper wire.The proton core is
fixed in position in a lattice like structure. In a conductor, the valence electrons
are free to move about. How many electrons are free to move about?
1cm long and a radius of 0,005 cm
Copper (Face Centered Cube)  good conductor
Copper atom:
Z=29(protons), N= 34(neutrons),
29 Electrons
Carbon or diamond  poor conductor
Demo: Show electronic electroscope with cage: gives magnitude
and sign of charge. Use teflon and silk to show + and .
Elementary unit of charge
Six billion units of charge
q1
r
Lab Experiment
In 1785 Charles Augustin Coulomb reported in the Royal Academy Memoires using a torsion balance two charged mulberry pithballs repelled each other with a force that is inversely proportional to the distance.
Electrical Permittivity of vacuum
+
+
q2
Repulsion
Point charges
+

Attraction
Spheres same
as points


Repulsion
1 nC
1 nC
1 cm
1 cm
q2
1 nC
q1
1 nC
r
(Equivalent to the weight
of a long strand of hair)
Repulsion
What is the direction of the force in unit vector notation?
1 nC
1 cm
1 nC
1
2
y
x
.
.
q2
q1
r
The positive sign means the force on q2 is in the same direction as the unit vector
.
.
q1
q2
r
The positive sign means the force on q1 is in the same direction as the unit vector.
2
3
1
x
Example
Three charges lie on the x axis: q1=+25 nC at the origin, q2= 12 nC at
x = 2m, q3=+18 nC at x=3 m. What is the net force on q1 and the direction of
the force? We simply add the two forces keeping track of their directions.
(Directed in the positive x direction)
Demo: Show uniformity of charge around sphere using
electrometer.
Demo: Show charging spheres by induction using
electrometer
(Use Elmo)
q3
+q1
+q2
q3
r13
+q1
r12
+q2
In practice you find the x and y comonents of all the vectors and add them up
q3=  2 nC
+y
2 cm
1 cm
+x
q1= + 1 nC
q2= + 1 nC
(Use Elmo)
Example: Three point charges in a plane
Question: What is the net force on q1 and in what direction?
Hint : Find x and y components of force on q1 due to q2 and q3 and add them up.
q
q
m
m
r
r
In an atom can we neglect the gravitational force between the electrons and protons? What is the ratio of Coulomb’s electric force to Newton’s gravity force for 2 electrons separated by a distance r ?
Huge number  pure number  no units
Demo: Put wood on the spinner and place charged teflon
and plastic rods near it.
What is the explanation of all of these phenomena?
Repulsion
Explanation: The neutral objects atoms and molecules orientthemselves in the following way so that the Coulomb forces due to attraction are greater than those due to repulsion because the latter are further away. (Inverse square Law)
Acrylic Rod Wooden block
Acrylic Rod Wooden block
Attraction
+
+
++
++






+
+
++
++
+
+++++
Attractive forces >> Repulsive Forces
Problem: Two equally charged hanging pith balls (Use Elmo)
In crystals of a certain salt, singly charged cations form the eight corners of a cube and a singly charged anion is at the cube's center (Figure 2132). The edge length of the cube is x = 0.32 nm. The cations are each deficient by one electron (and thus each has a charge of +e), and the anion has one excess electron (and thus has a charge of e).
Fig. 2132
(a) What is the magnitude of the net electrostatic force exerted on the anion by the eight cations at the corners of the cube?
(b) If one of the cations is missing, the crystal is said to have a defect. What is the magnitude of the net electrostatic force exerted on the anion by the seven remaining cations?
Figure 2123
In Figure 2123 particles 1 and 2 are fixed in place, but particle 3 is free to move. If the net electrostatic force on particle 3 due to particles 1 and 2 is zero and L23 =2.00 L12, what is the ratio q1/q2?