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Welcome to PY212 General Physics II

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Welcome to PY212 General Physics II Prof. Meenakshi Narain Please come up and Get a copy of the syllabus Get the remote transmitter that has been assigned to you. – PowerPoint PPT presentation

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Title: Welcome to PY212 General Physics II


1
Welcome to PY212General Physics II
Prof. Meenakshi Narain
  • Please come up and
  • Get a copy of the syllabus
  • Get the remote transmitter that has been
    assigned to you. There's a list of names and
    transmitter numbers at the front. See me if you
    can't find your name on the list.

2
Welcome to PY212General Physics II
Prof. Meenakshi Narain
3
Course Information
  • Course requisites PY211, MA124
  • Webpage http//webct.bu.edu in Spring 2003
    listing (or click on the link MyWebCT)
  • What is/will be posted on WebCT? PY212 Homepage
  • Syllabus
  • Lecture and Lab schedule
  • Office hours and contact information for Profs
    and TFs
  • Solutions to conceptual excercises, assignments
  • Practice Exams, Exam solutions
  • Grades (as they become available for labs,
    discussions, assignments, etc)
  • Also please use bulletin board for
    communication with Profs, TFs and peers.

4
Lectures
  • Lecture schedule is posted on WebCT.
  • Read the chapter sections BEFORE coming to the
    class.
  • 7 of the grade includes
  • surprise quizzes and
  • class participation (via the transmitters)
  • Office Hours
  • Mon 1-2pm, Tues 9-10am ,Wed 3-4pm, in SCI 121
  • Also feel free to stop by PRB 369 or make an
    appointment

5
Exams and Grades
  • Exams
  • Midterm 1 Monday Feb 10th, from 6-730pm
  • Midterm 2 Monday March 31st, from 6-730pm
  • Final exam date/time to be determined
  • Grades
  • 20 discussion homeworkconceptual excercises
  • Require at least 50 on the homework grades
  • 14 Laboratory section.
  • 7 lecture quizzes
  • 17 exam 1
  • 17 exam 2
  • 25 Final exam

6
Discussion Sections
  • Begins Wed 1-15-2003 (ie tomorrow)
  • During the discussion sections the TFs will
  • Provide help with any concepts you may still want
    clarified.
  • Provide helpful hints on HW problems of your
    choice.
  • There will be a 15 minute Conceptual Exercise
    during the discussions.
  • Conceptual exercises count towards 1/3 of
    discussion grade.

7
Homeworks
  • Require at least 50 on the homework grades to
    pass the course
  • Due Tuesdays at 1159pm.
  • Use WebAssign to enter the answer online.
  • You have 3 chances to submit each assignment.
    Grade for assignment is grade for last
    submission.
  • Numbers are randomized for different students.
  • You will need access code for WebAssign. Comes
    bundled with the book, or buy it directly from
    https//www.webassign.net/secure
  • You are required to hand in your neatly worked
    out assignment in the mailboxes of the TFs by
    Wednesday noon.
  • We will select one or two problems at random for
    detailed grading. If your work is incorrect or
    not understandable, but your online answer is
    correct, then partial grades will be given for
    those problems.
  • You may discuss homework with other students, but
    the work you turn in should be your own.

8
Labs
  • Begins next week Monday 1-20-2003
  • Buy the manual Physics Intro II Laboratory
    Experiments
  • They are also posted on Webct (as PDF files)
  • Do the pre-labs BEFORE the lab and hand it in at
    the start of the lab period. These are posted our
    web site.
  • These count 20 of the lab grade.
  • Late pre-labs or completing them after coming to
    the lab will not receive credit
  • Lab reports - due at the end of each lab session.
  • You can hand these in using neatly stapled
    looseleaf, or in a lab book (you will need two
    books).
  • All 7 labs count.
  • Must complete 6 labs to receive credit for the
    course.
  • Read the one-page handout describing what a lab
    report should look like.

9
My Philosophy
  • My philosophy regarding what goes on in the
    course is that we're all part of the same team.
  • My role is to help you learn. Your role is to
    work hard at learning both outside of class
    (reading, preparing for class, doing all the
    assignments, etc.) and in class.
  • In class please talk to each other about the
    material, ask me questions, and slow me down if
    I'm going too fast. Always feel free to challenge
    anything I say, or to add useful information if
    you think it would add something to the
    discussion.
  • To get anything out of this course you can't
    simply sit back and hope to absorb the material.
    You have to be an active participant in the
    process.

10
Cell Phones
  • Please turn OFF your cell phones now.
  • It is disrupts the lecture!!!!
  • Please do not compel me to impose strict rules
    for the future

11
Lightning
12
Static Electricity
13
Chapter 21Electric charge and Electric Field
  • Outline
  • Historical Timeline
  • Static Electricity, Electric Charge and
    conservation
  • Insulators, Conductors
  • Process of Charging
  • Conduction
  • Induction
  • Coulombs law
  • Describes the nature of force between charges
  • Examples using point charges and collection of
    charges (discrete and continuous)

14
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15
Properties of Charges
  • Type of charges
  • Positive () and Negative (-) (Charles Du
    Fay 1733)
  • Example Charge diff rods by rubbing with silk
    cloth
  • Silk on Glass ? Glass acquires positive charge
  • Silk on Plastic ? Plastic acquires negative
    charge
  • (definition by Benjamin Franklin in 1770)
  • Like charges repel, Unlike charges attract
  • Atom EM force keeps electron(-) orbiting the
    nucleus() and holds the atom together
  • Conservation of Charge
  • No net positive or negative charges can be
    created
  • Examples
  • Salt solution
  • Radioactive decays
  • Particle Physics

16
Conservation of Charge
  • Positron interacts with an electron in the
    bubble chamber, turns into a photon with zero
    charge.

Original positron track
Photon decays back into an electron and a positron
17
Properties of Charges
  • Unit of Charge Coulomb (C)
  • Quantization of Charge
  • Electron charge is the smallest unit of free
    charge 1.6 x 10-19 Coulombs
  • Types of Materials
  • Conductors electrons move freely in the object
  • Insulators electrons tightly bound to the
    nucleus.
  • Semiconductors complicated conduction properties
    (in between)
  • Methods of charging an object
  • Conduction
  • Induction
  • Polarization

18
Using the Personal Response System (PRS)
  • Hit the power button so the green light comes on.
  • To choose response 4, for example, for a
    multiple-choice question, aim your transmitter at
    one of the receivers (look for the red light) and
    hit 4.
  • Look for your transmitter number on the screen to
    confirm that your response was received.
  • If you are sure of your answer, hit H for "high
    confidence" before sending your response.
  • If you are basically guessing, hit L for "low
    confidence" before sending your response.
  • Part of your 7 lecture grade will come from your
    responses entered in this way. Most of it will
    come from simply being in class and taking part.
    A small amount will come from getting the correct
    answer.
  • You should always use only the transmitter
    assigned to you, and be sure to return it after
    every class because students in other sections
    use these transmitters, too!
  • Entering responses for other students is not
    allowed.

19
Charges
  • Three pithballs are suspended from thin threads.
    Various objects are then rubbed against other
    objects (nylon against silk, glass against
    polyester, etc.). All or some of the pithballs
    are charged by touching them with one of these
    objects.
  • It is found that pithballs 1 and 2 attract each
    other and
  • that pithballs 2 and 3 repel
    each other.
  • From this we can conclude that
  • 1. 1 and 3 carry charges of opposite sign.
  • 2. 1 and 3 carry charges of equal sign.
  • 3. all three carry the charges of the same sign.
  • 4. one of the objects carries no charge.
  • 5. we need to do more experiments to determine
    the sign of the charges.

20
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21
Forces between charges Coulombs Law
  • Which factors affect the magnitude of this force?
  • Coulombs Law Force on Q1 due to Q2
  • the constant k 8.99 x 109 N m2 / C2.
  • Unit Newtons
  • Force is a vector.
  • The force between charges is similar to the
    gravitational force between interacting masses.
    The equations are very similar, and in both cases
    the force goes as 1 / r2.

22
Forces between charges Coulombs Law
23
Electrostatic forces
  • Two uniformly charged spheres are firmly fastened
    to and electrically insulated from frictionless
    pucks on an air table.
  • The charge on sphere 2 is three times the charge
    on sphere 1.
  • Which force diagram correctly shows the magnitude
    and direction of the electrostatic forces

1
2
3
4
5
6
24
Comparing Gravity and Electric Forces
  • A hydrogen atom is composed of a nucleus
    containing a single proton, about which a single
    electron orbits.The electric force between the
    two particles is 2.3 x 1039 greater than the
    gravitational force!
  • If we can adjust the distance between the two
    particles, can we find a separation at which the
    electric and gravitational forces are equal?
  • 1. Yes, we must move the particles farther apart.
  • 2. Yes, we must move the particles closer
    together.
  • 3. no, at any distance

25
Forces between charges Coulombs Law
  • How do we find compute the force on a charge due
    to a collection of charges?
  • Since force is a vector, when more than one
    charge exerts a force on another charge, the net
    force is the vector sum of the individual forces.
  • PRINCIPLE OF SUPERPOSITION

26
Forces (1D example)
  • Three charges are equally spaced along a line.
    The distance between neighboring charges is a.
    From left to right the charges are
  • q1 -Q, q2 Q, q3 Q
  • What is the magnitude of the force experienced by
    q2?
  • Let's define positive to the right.
  • The net force on q2 is the vector sum of the
    forces from q1 and q3.
  • F2 F21 F23
  • The force has a magnitude of 2kQ2/a2 and points
    to the left.
  • Signs Handling the signs correctly is a critical
    part of any vector addition problem. The negative
    signs in each of the terms above come from the
    direction of each of the forces (both to the
    left) and not from the signs of the charges. I
    generally drop the signs of the charges and get
    any signs off the diagram by drawing in the
    forces.

27
Forces (1D example)
  • Three charges are equally spaced along a line.
    The distance between neighboring charges is a.
    From left to right the charges are
  • q1 -Q, q2 Q, q3 Q
  • What is the magnitude of the force experienced by
    q2?
  • Let's define positive to the right.
  • The net force on q2 is the vector sum of the
    forces from q1 and q3.
  • F2 F21 F23
  • The force has a magnitude of 2kQ2/a2 and points
    to the left.
  • Signs critical part of any vector addition
    problem.
  • The negative signs in each of the terms above
    come from the direction of each of the forces
    (both to the left) and not from the signs of the
    charges.
  • I generally drop the signs of the charges and get
    any signs off the diagram by drawing in the
    forces.

28
Forces (1D example)
  • Three charges are equally spaced along a line.
    The distance between neighboring charges is a.
    From left to right the charges are
  • q1 -Q, q2 Q, q3 Q
  • Order the charges according to the magnitude of
    the net force they experience, from largest to
    smallest.
  • F1 F2 gt F3
  • F1 gt F2 gt F3
  • F2 gt F1 F3
  • F2 gt F1 gt F3
  • None of the above.

29
Charge configurations (3D example)vector nature
of forces!
  • Consider a situation where the net force which
    the positive charge at the center of a square
    experiences, because of equal-magnitude charges
    placed at each corner of the square, is toward
    the top-right.
  • How many possible configurations can you come up
    with that will produce the desired force?
  • 0 4) 3
  • 1 5) 4
  • 2 6) Either 0 or more than 4
  • Think of the signs of the equal-magnitude
    charges occupying each corner

1
2
5
4
3
30
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31
Franklins Kite
32
Matter anti-Matter interaction
  • bubble-chamber picture

33
Electric Shocks
34
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