Transcript Ch33 - Siena College

Welcome to Phys 140!
sites.google.com/site/sienaphys140spring2011/
General Physics
1
Textbook
• Physics for Scientists and Engineers, 2nd Edition
• By Knight
• Required texts
• Volume 4 and Volume 3
• Workbooks for these volumes
• We will start with Volume 4
• YOUR TEXTBOOK IS YOUR #1 RESOURCE!!!
• You are expected to come to class having read the
assigned chapter.
Course Design and Grading
Tests
Exam 1, Feb. 15
Exam 2, Mar. 24
Exam 3, Apr. 14
30%
10%
10%
Other
Homework
Reading Quizzes
25%
Participation
10%
10%
Labs
15%
You must pass lab to pass
class!
You must complete every
lab to pass lab!
Final Exam
20%
Uniform exams for all
sections
Cumulative
Lab Grading
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Lab Write-ups
Formal Lab Write-up
Participation
Lab Practicum
25%
25%
25%
25%
• Lab Write-ups
– Write-up due at the end of lab
– C - lacking , B - adequate; A extra – something we didn’t ask for
• Formal Lab Write-up
– Due the following week; graded and returned for editing
– Average of two grades
• Participation
– Grade in each lab
– C - watching, B - helping; A- doing, explaining, writing
• Lab Practicum
– Individual test of lab skills
Class Time
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Reading Quiz
2 minute problems
Group problems
Lab-style activities
SHORT lectures
Classes will be ACTIVE! They will require
you to participate and engage in the problems
and activities.
General Physics
Lecture 1
5
Preparing for Class
• Assigned Reading
– One chapter each class/week
• Go through exercises in each chapter
– Answers are provided at the end of the
chapter
• Reading quiz
– multiple choice or short answer
General Physics
Lecture 1
6
Class Website
• Website will include:
– Schedule
• http://sites.google.com/site/sienaphys140spring2011/schedule
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Powerpoint slides for each class
Details on Reading Quiz
Mastering Physics HW Questions and due date
Written HW questions and due date
Lab Schedule and link to lab materials
Info on Bike Generator
Extra Credit Opportunities
• Website will be updated each week – check it often
General Physics
Lecture 1
7
Attendance Policy
• Students are expected to attend all classes and
are responsible for all material covered in
class, even when absent.
• Students should understand that some material
discussed in class is not covered in the
textbook.
• In-class problems and activities can not be
made up.
General Physics
Lecture 1
8
Attendance Policy
• Attendance is required.
• We realize that some absences are unavoidable, and
you should inform your instructor prior to missing
any classes.
• Missing more than 3 classes will decrease your
overall grade by a letter grade.
• You will be advised to withdraw from the course if
you miss more than 5 classes.
General Physics
Lecture 1
9
Homework
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Weekly Homework will be listed on the schedule
Mastering Physics Homework is due Saturday by 11:30 pm
Homework to be handed in is due Sunday by 5 pm in the
drop box outside RB 121 (Dr. McColgan’s office) or RB 223
(Dr. Vernizzi’s office)
Grading
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Mastering Physic assignments will be worth 15 points each
Written HW
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4 points per problem
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2 point for working out the problem
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2 point for presentation
Exams will be very similar to homework, group problems,
and labs
Grading Homework – 4 point scale
• Working out problem
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– 0:
good effort with correct results and reasoning
Incorrect results or reasoning
a very poor effort or no effort
• Presentation
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Adequate diagrams
Explanations of model
Units and vector notation
Algebra first (with symbols only), plug in numbers at the end
• Grading
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General Physics
great presentation
presentation problems
extremely poor presentation
Lecture 1
11
Guidelines for working homework
problems
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Given:
Find:
•Draw a picture!
•Write in pencil
•Clearly label units
•Cancel units when appropriate
•WRITE NEATLY
•Keep work in one column
•Box answers
General Physics
Lecture 1
12
Extra Credit
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Opportunities announced throughout the
semester
Examples
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Assignments
Brown bag lunches
Comics
MATLAB
• MATLAB is widely used in engineering, science and mathematics
– Knowledge of MATLAB is a marketable skill
• Physics Department has adopted MATLAB as its official
language
– You will see it in higher level courses
• We will use it this semester in class and in lab
– Data analysis and plotting
– Vector analysis
– Improve understanding abstract concepts such as vector fields
Nature of charge
• Amber is a natural substance that can be
naturally charged
• Few naturally occurring objects can be
electrically charged
• Static electricity is easily shown with plastic,
rubber, and synthetic fibers
• Electrical charges behave like positive and
negative numbers
Electrostatics
• Complete the tutorial on electrostatics (both
sides of handout)
• Tape activity
• http://www.sos.siena.edu/%7Erfinn/phys140s09/tape.pdf
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Charge Conservation
• The total charge of an isolated system
is conserved.
– Rubbing a balloon on hair
No charge
• Triboelectric series
– Decreasing tendency to lose
electrons
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Atoms and Electricity
• An atom consists of a very small and dense
nucleus surrounded by much less massive orbiting
electrons.
• The nucleus is a composite structure consisting
of protons, positively charged particles, and neutral
neutrons.
• The atom is held together by the attractive electric
force between the positive nucleus and the negative
electrons.
• Electrons and protons have charges of opposite sign
but exactly equal magnitude.
• This atomic-level unit of charge, called the
fundamental unit of charge, is represented by the symbol
Charge quantization
• A macroscopic object has net charge
• Where Np and Ne are the number of protons and
electrons contained in the object.
• The process of removing an electron from the
electron cloud of an atom is called ionization.
• An atom that is missing an electron is called a
positive ion. Its net charge is q = +e.
Insulators and Conductors
• In metals, the outer atomic electrons are only
weakly bound to the nuclei.
• These outer electrons become detached from
their parent nuclei and are free to
wander about through the entire metal.
• The metal as a whole remains electrically
neutral, but the electrons are now like
a negatively charged liquid.
•The electrons in the insulator are all tightly
bound to the positive nuclei and not free
to move around.
• Charging an insulator by friction leaves
patches of molecular ions on the surface, but
these patches are immobile.
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Illustrations of Coulomb’s Law
• Electroscope
induction
conduction
Polarization Force
The Electric Dipole
Coulomb’s Law - magnitude
K q1q2
Fe 
2
r
• K = 9.0 x 109 N m2/C2
– Coulomb’s contant
• q1, q2 = charges

– measured in Coulombs (C)
– 1 C is a lot of charge!
• r = distance between q1 and q2
q1q2
Fe 
2
40 r
1
ε0 =permittivity of free space
(more relevant to future chapters)
Electric Force
Gm1m2
Fg 
r2
K q1q2
Fe 
r2
• Similar form as gravitational force (weird,
huh?), except
 negative charges

– positive and
• Like charges repel; opposite charge attract
Net Force from Multiple Charges
• An electrostatic force exists between each pair
of charges according to Coulomb’s law
Fe  Fe,12  Fe,13  Fe,23
k q1q2 k q1q3 k q2q3



2
2
2
r
r
r
• Add components of forces to get net force
when adding multiple charges
The Electric Field
We begin our investigation of electric fields by postulating
a field model that describes how charges interact:
1.Some charges, which we will call the source
charges, alter the space around them by creating an
electric field.
2.A separate charge in the electric field experiences a
force exerted by the field.
Suppose probe charge q experiences an electric force Fon q
due to other charges.
The units of the electric field are N/C. The magnitude E of
the electric field is called the electric field strength.
The Electric Field
Group Problems
EXAMPLE 26.6 Lifting a glass bead
EXAMPLE 26.8 The electric field of a proton
Group Problems
1. What is the net force on the 48 C charge?
Q1
Q2
+y
+x
Q4
Q3
2. Find net force
on Q1