Physics 014 Lecture 1 Chapter 21 Electric Charge.
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Transcript of Physics 014 Lecture 1 Chapter 21 Electric Charge.
![Page 1: Physics 014 Lecture 1 Chapter 21 Electric Charge.](https://reader036.fdocuments.us/reader036/viewer/2022062300/56649ca35503460f94963a38/html5/thumbnails/1.jpg)
Physics 014 Physics 014 Lecture 1Lecture 1
Chapter 21 Electric ChargeChapter 21 Electric Charge
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Office hours: Thirkield Hall, Room 215, MWF 12:00-1:00pm (or by appointment)
Phone: (202) 806-6257
Email: [email protected]
My Research: Atmospheric PhysicsNumerical Modeling
Dr. Mengs H. Weldegaber
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Course DetailsCourse Details• See: Syllabus, schedule, grade policy, …
• Text: Fundamentals of Physics, Halliday, Resnick, and Walker, 9th edition. We will cover chapters 21-36 in this class.
• Exams: Midterm: 11 Oct 2013Final Exam (cumulative): 13 Dec 2013
• Quizzes: Weekly.
• Grades: Homework 30%, Quizzes 15% , Midterm Exam 20% Final Exam 35%.
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What are we going to learn?What are we going to learn?A road mapA road map
• Electric charge Electric force on other electric charges Electric field, and electric potential
• Moving electric charges : current • Electronic circuit components: batteries, resistors, capacitors• Electric currents Magnetic field
Magnetic force on moving charges• Time-varying magnetic field Electric Field• More circuit components: inductors, AC circuits. • Maxwell’s equations Electromagnetic waves light waves• Geometrical Optics (light rays). • Physical optics (light waves): interference, diffraction.
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Let’s get started!Let’s get started!Electric chargesElectric charges
• Two types of charges: positive/negative• Like charges repel• Opposite charges attract
Atomic structureAtomic structure : • negative electron cloud• nucleus of positive protons, uncharged neutrons
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Electric Charge
(a) Two charged rods of the same sign repel each other.
(b) Two charged rods of opposite signs attract each other. Plus signs indicate a positive net charge, and minus signs indicate a negative net charge.
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12F1q 21F 2q
12F 1q 21F2qor
12F1q 21F2qor
Force between pairs of point Force between pairs of point charges: Coulomb’s lawcharges: Coulomb’s law
Coulomb’s law -- the force between point charges:
• Lies along the line connecting the charges.
• Is proportional to the magnitude of each charge.
• Is inversely proportional to the distance squared.
• Note that Newton’s third law says |F12| = |F21|!!
Charles-Augustin de Coulomb (1736-1806)
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2q12F1q 21F
12r
Coulomb’s lawCoulomb’s law
212
2112
||||||
rqqk
F
2
212
00
1085.8with 41
mN
Ck
2
291099.8
CmNk =
For charges in aVACUUM
Often, we write k as:
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Materials classified based on their ability to move charge
• Conductors are materials in which a significant number of electrons are free to move. Examples include metals.
• The charged particles in nonconductors (insulators) are not free to move. Examples include rubber, plastic, glass.
• Semiconductors are materials that are intermediate between conductors and insulators; examples include silicon and germanium in computer chips.
• Superconductors are materials that are perfect conductors, allowing charge to move without any hindrance.
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Electric charges in solidsElectric charges in solids• In macroscopic solids, nuclei
often arrange themselves into a stiff regular pattern called a “lattice”.
• Electrons move around this lattice. Depending on how they move the solid can be classified by its “electrical properties” as an insulator or a conductor.
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• In a conductor, electrons move around freely, forming a “sea” of electrons. This is why metals conduct electricity.
• Charges can be “induced” (moved around) in conductors.
Charges in solidsCharges in solids
Blue background = mobile electrons
Red circles = static positive charge (nuclei)
-
-
+
+
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Insulating solidsInsulating solids
• In an insulator, each electron cloud is tightly bound to the protons in a nucleus. Wood, glass, rubber.
• Note that the electrons are not free to move throughout the lattice, but the electron cloud can “distort” locally.
+ -
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How to charge an objectHow to charge an object• An object can be given some “excess” charge: giving
electrons to it (we give it negative charge) or taking electrons away (we “give” it positive charge).
• How do we do charge an object? Usually, moving charges from one surface to another by adhesion (helped by friction), or by contact with other charged objects.
• If a conductor, the whole electron sea redistributes itself.
• If an insulator, the electrons stay where they are put.
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Conservation of ChargeConservation of Charge
You connect these together with a metal wire; what is the final charge distribution?
? ?
Total amount of charge in an isolated system is fixed (“conserved”)
+1C 2C
Example: 2 identical metal spheres have charges
+1C and –2C.
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Conservation of Electric Charges
• A glass rod is rubbed with silk
• Electrons are transferred from the glass to the silk
• Each electron adds a negative charge to the silk
• An equal positive charge is left on the rod
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Conservation of Electric Charges
• A very hard rubber rod is rubbed with animal fur
• Electrons are transferred from the fur to the rubber
• Each electron adds a negative charge to the rubber
• An equal positive charge is left on the fur
(very hard rubber)
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Quantization of ChargeQuantization of Charge
• Charge is always found in INTEGER multiples of the charge on an electron/proton ([[why?]])
• Unit of charge: Coulomb (C) in SI units• Electron charge = –e = 1.6 x 10-19 Coulombs• Proton charge = +e = +1.6 x 10-19 Coulombs• One cannot ISOLATE FRACTIONAL CHARGE
(e.g. 0.8 x 10-19 C, +1.9 x 10-19 C, etc.) [[but what about quarks…?]]
• Unit of current: Ampere = Coulomb/second
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SuperpositionSuperposition
• Question: How do we figure out the force on a point charge due to many other point charges?
• Answer: consider one pair at a time, calculate the force (a vector!) in each case using Coulomb’s Law and finally add all the vectors! (“superposition”)
• Useful to look out for SYMMETRY to simplify calculations!
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Multiple Forces: If multiple electrostatic forces act on a particle, the net force is the vector sum (not scalar sum) of the individual forces.
Shell Theories: There are two shell theories for electrostatic force
Answer: (a) left towards the electron (b) left away from the other proton (c) left
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2q12F1q 21F
12r
Coulomb’s law, Coulomb’s law, ReviewReview
212
2112
||||||
rqqk
F
2
291099.8
CmNk =
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F12 kq1 q2
r2
8.99 109 N m2 C2 3.0 10 6C 4.0 10 6C 0.20m 2 2.7N
F13 kq1 q3
r2 8.99 109 N m2 C2 3.0 10 6C 7.0 10 6C
0.15m 2 8.4N
5.7NN4.8N7.21312 FFF
Example 1. Find the net force on q1 ?
Soln
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Example 2. Find the net force on q3 ?
• The force exerted by q1 on q3 is
• The force exerted by q2 on q3 is
• The resultant force exerted on q3 is the vector sum of and
13F
23F
13F
23F
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F kq1 q2
r2
8.99 109 N m2 C2 1.60 10 19C 2
5.29 10 11m 2 8.22 10 8N
rmvmaF c2
v Fr m 8.22 10 8N 5.29 10 11m
9.1110-31kg2.18 106 m s
Example 3. Determine the speed of the electron in orbit about the nuclear proton at a radius of 5.29x10-11 m, assuming the orbit to be circular [The Bohr model of the hydrogen atom]?
Soln
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Example 4: Example 4: Find the force onFind the force on qq11
• Three equal charges form an equilateral triangle of side 1.5 m as shown
• Compute the force on q1
• What is the force on the other charges?
d
q1
d
dq2
q3
q1= q2= q3= 20 C
Solution: Set up a coordinate system, compute vector sum of F12 and F13
d
1
2
3
d
d
12F
13F
o60y
x
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SummarySummary• Electric charges come with two signs: positive and negative.
• Like charges repel, opposite charges attract, with a magnitude calculated from Coulomb’s law: F=kq1q2/r2
• Atoms have a positive nucleus and a negative “cloud”.
• Electron clouds can combine and flow freely in conductors; are stuck to the nucleus in insulators.
•We can charge objects by transferring charge, or by induction.
• Electrical charge is conserved, and quantized.