Welcome to Physics 104Spring 2013 January 23rd, 2013 All course information is available on Learn@UWplease get into the habit of working on it!
Physics 104: The Cast • Lectures (Faculty) • Pupa Gilbert, firstname.lastname@example.org • Wesley Smith, email@example.com • (Profs. Gilbert & Smith Office Hours after class W 3:45 – 4:45) • Laboratory & Discussion (Teaching Assistants) • AbhishekAggarwalaaggarwal5@wisc.edu • Laura Dodd firstname.lastname@example.org • Jungha Kim email@example.com • Greg Lau firstname.lastname@example.org • Subin Lee email@example.com • James Osborne firstname.lastname@example.org • Pankhuripankhuri@wisc.edu • HirenPatel email@example.com • Nitin Rameshnramesh2@wisc.edu • Nate Woods firstname.lastname@example.org
Your Teaching Assistants (TA) AbhishekAggarwal Laura Dodd Jungha Kim Greg Lau Subin Lee James Osborne Pankhuri Nitin Ramesh Hiren Patel Nate Woods
Hours per week • ~2 hours • 2 hours • 2 hours • 2 hours • ~4 hours • ~2 hours Basic Course Philosophy • read about it (text) • untangle it (reading quiz, lectures) • collaborative learning(discussion) • experiment with it (labs) • challenge yourself (homework) • close the loop (office hours, reading) We offer different formats in which you can learn physics. Please make the best use of all of them, find the one that works best for you, and use it to enlighten the others. We don’t “teach” - we help you learn. YOU will do the learning.
Lectures • Come prepared • Read material assigned from textbook • See “Syllabus” • Do Reading Quiz on the web • Available 5 days prior to lecture, due at noon, day of lecture • Lecture itself includes • Mostly Concepts • Demos • I will solve 1 problem in each lecture, handwritten • DO NOT expect to learn by passively sitting and listening in the lecture - please participate – ask questions! • Lectures are NOT complete. Homework, discussions and labs are needed to learn physics completely in this course • Lecture slides are placed on the web after lecture • No need for extensive note-taking! Participation: you learn by doing!
Discussion and Lab Discussion – about 8% of grade (via the weekly quiz) • Work Problems -- collaboratively in groups & with TA • Tuesday, Thursday • You can consult your book • Discussion sections are mandatory • There is no way to makeup a discussion session Lab – about 8% of grade (via the weekly quiz) • Also led by your TA • Work in groups of 3 • Complete the pre-lab questions before attending (TA will check) • Labs are mandatory (receive no quiz credit w/o attending) • Must do at least 9 of 11 labs in order to pass this course • Make up missed labs during week of lab or next midterm exam week • Requires TA consent(given for documented emergency or academic conflict) TA Help Desk Hours: 10 per week, in room 3320 Chamberlin See Learn@UW for Help Desk hours and TAs.
Exams, Homework & Quizzes • Exams (70% of course grade: 3x15+ 25) • Multiple-choice, bring formula sheet(s): 1, hand-written on both sides for each mid-term, 4 sheets (same sheets!) for final • YOU MUST CONTACT PROF. GILBERT OR SMITH BEFORE FEBRUARY 1st IF YOU CANNOT ATTEND ALL EXAMS (i.e. you have a course that meets or has exams on Thurs. Feb. 21, Mar. 21 and Apr. 25 at 5:45-7:00 PM, or conflicts with final at 2:45 – 4:45 PM on Tue. May 14.) • There are no make-up exams. • Take the practice exams under time constraint with your formula sheet • Homework (8% of grade via weekly quiz) • Important for learning, maybe most important! • Problems selected from text book – listed on learn@uw • Weekly Quizzes (25% of course grade) • Available Thurs. 6 PM, due by Mon. at noon: each WQ contains questions on HW, Lab, Discussions (3 hours allowed) • Reading Quizzes (5% of course grade, graded on participation, not correctness) • Available 5 days prior, due Mon. and Wed. at noon before lectures
Content Physics 103 Physics 104 • Kinematics • Forces • Energy • Fluids • Waves&Sound • Thermodynamics Macroscopic Microscopic • Electromagnetism (E&M) • electricity • magnetism • Radiation • circuits • Optics • Modern Physics • atomic-nuclear • Relativity
Let’s begin! Chapter 17: Electric forces and fields. • Concept of charge • Unit of charge ( C ) • Coulomb’s Law • Electric Field • Gauss’s Law
Concept of Charge • Charge is an intrinsic property of matter • Two types: positive and negative
Conductors and Insulators Electrons are free to move in a conductor Electrons stay with their atom in an insulator Most materials are in between perfect conductors and insulators
Separating Charge • Triboelectric – friction • Conduction – contact • Induction • Proximity/ground
Charging by friction Pith balls
Charging by friction Van de Graaff generator Toepler-Holtz machine
Charging by Induction • Bring a charged rod near • Polarizes conductors in its proximity Positive charged rod results in positive leaves.
Force between charges: Coulomb’s Law • Coulomb’s law gives a quantitative description of the electrical forcebetween charged particles • Unit of charge is Coulomb (C) • An electron has a charge of -1.6 × 10-19 C • Coulomb’s Law gives the magnitude and the direction of the force between chargesq1and q2separated by a distance r. • Magnitude: • direction: along the line joining the charges • Likecharges repel, opposite chargesattract • εo = permittivity of free space • εo= 8.85 × 10-12 C2/Nm2 • k= 1/(4πεo) = 8.99 × 109 Nm2/C2
Electric Field • What is it? • Field lines around charges • Superposition of electric field vectors • Field lines near a conductor • Flux & Gauss’s Law (read in text)
Electric Field A charged particle creates an electric field. The direction of this field is the direction of the force on a test charge q(test positive charge) EElectric Field (independent of test charge) F= qE proton electron Qp=1.6x10-19 C Qe=-1.6x10-19 C E + - r = 1x10-10 m (to the right)
Direction of Electric Field • A positive test charge would be attracted to the negative source charge • A positive test charge would be repelled by the positive source charge
Electric Field of a Point Charge 0.8x1011 N/C 32x 1011 N/C 25 x1011 N/C 2.9x1011 N/C + E
Electric Field • Density gives strength • # lines proportional to Q • lines never cross! • Arrow gives direction • Start on +, end on -
Electric Field Patterns Dipole: 2 equal and opposite charges
Charges in a conductor • The charges move apart until net force on each vanishes • So… excess charge moves to the surface • The amount of charge per unit area is smaller at the flat end
E inside conductor • In a conductor the conduction electron are free to move • electrons exposed to an electric field feel a force and move • If E = 0 electrons do not move
Conductors in Equilibrium • Consider a conductor with an excess of positive charge • All of the charge resides at the surface • E = 0 inside the conductor • The electric field just outside the conductor is perpendicular to the surface
E outside a conductor • On an irregularly shaped conductor, the charge accumulates at locations where the radius of curvature of the surface is smallest (that is, at sharp points)
Electric Field Patterns Dipole: 2 equal and opposite charges
Electric Field Patterns 2 unequal and opposite charges Note that two lines leave the +2q charge for each line that terminates on -q
Electric Field Patterns 2 equal and like charges • At a great distance from the charges, the field would be approximately that of a single charge of 2q • The bulging out of the field lines between the charges indicates the repulsion between the charges • The low density of field lines between the charges indicates a weak field in this region
Electric Flux • Field lines penetrating an area A. • The flux, Φ is: ΦE= E A cosθ • The normal to the area A is at an angle θ to the field • If the area encloses a volume: • for E-field lines that go into the volume, the flux is negative • for E-field lines coming out of the volume the flux is positive
Gauss’sLaw • Gauss’sLaw states that the electric flux through any closed surface is equal to the net charge Q inside the surface divided by εo • εois the permittivity of free spaceand it equalsεo= 8.85 x 10-12 C2/Nm2 • The area used for Φis an imaginary surface, a Gaussian surface, it does not have to coincide with the surface of a physical object
Electric Field of a Charged Thin Spherical Shell • The field outside the shell is identical to that of a point charge • The electric field inside the shell is zero • The charge inside is also zero
Electric Field of a Nonconducting Plane Sheet of Charge • The sheet has a charge per unit area of σ = charge/area • Use a cylindrical Gaussian surface with ends of area A • Flux through each of two ends is EA, and there are no field lines going through the curved surface of the cylinder (cosq=0). • The total charge is Q = σA • Note, the field is uniform everywhere on the sheet or other planes parallel to it, and does not depend on the distance from the surface