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.

Welcome to PY212General Physics II

Prof. Meenakshi Narain

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.

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

Exams and Grades

• Exams:– Midterm 1: Monday Feb 10th, from 6-7:30pm– Midterm 2: Monday March 31st, from 6-7:30pm– Final exam date/time to be determined

• Grades:– 20% discussion: homework+conceptual excercises

• Require at least 50% on the homework grades– 14% Laboratory section. – 7% lecture: quizzes– 17% exam 1– 17% exam 2– 25% Final exam

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.

Homeworks• Require at least 50% on the homework grades to pass the course• Due Tuesdays at 11:59pm.• 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.

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.

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.

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…

Lightning

Static Electricity

Chapter 21Electric charge and Electric Field

Outline:• Historical Timeline • Static Electricity, Electric Charge and conservation• Insulators, Conductors• Process of Charging:

– Conduction

– Induction

• Coulomb’s law– Describes the nature of force between charges

– Examples using point charges and collection of charges (discrete and continuous)

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

Conservation of ChargePositron interacts with an electron in the “bubble chamber”, turns

into a photon with zero charge.

Photon decays back into an electron and a positron

Original positron track

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

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.

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.

Forces between charges: Coulomb’s Law

• Which factors affect the magnitude of this force?

• Coulomb’s 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.

Forces between charges: Coulomb’s Law

Electrostatic forcesTwo 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:

7. None of the above

1 2 3

4 5 6

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

Forces between charges: Coulomb’s 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

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.

-Q +Q +Q

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.

-Q +Q +Q

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.

1. F1 = F2 > F3 2. F1 > F2 > F3 3. F2 > F1 = F3 4. F2 > F1 > F3 5. None of the above.

-Q +Q +Q

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?

1) 0 4) 3

2) 1 5) 4

3) 2 6) Either 0 or more than 4

• Think of the signs of the equal-magnitude charges occupying each corner…

1

4 3

2

5

Franklin’s Kite

Matter anti-Matter interaction

• “bubble-chamber” picture

Electric Shocks

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