Welcome to PY212 General Physics II

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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.

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Welcome to PY212General Physics II
Prof. Meenakshi Narain
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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.

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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

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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

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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.

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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.

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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.

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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.

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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

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Lightning
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Static Electricity
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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)

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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

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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
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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

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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.

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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.

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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.

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Forces between charges Coulombs Law
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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

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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

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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

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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.

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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.

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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.

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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

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Franklins Kite
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Matter anti-Matter interaction

• bubble-chamber picture

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Electric Shocks
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