PHYS 10163 • Instructor: T. WaldekZerda • office: 142 SWR or 307 SWR • campus phone: 817-257-7124 • email: email@example.com • office hours: M.Tu.W. 2-4 PM; or by appointment • eCollege contains syllabus, lecture notes and homework solutions • Lecture notes can also be found: • http://personal.tcu.edu/zerda/10164/ch**_lecture.ppt • Lab instructions can be found: • http://personal.tcu.edu/zerda/manual/lab**.pdf
Homework Working problems is the only way to learn physics. You are encouraged to consult your classmates or TCU physics graduate students in solving the homework problems. DO NOT COPY the answers of your fellow students. I reserve rights to reject a homework that I suspect was copied from another student, a web site, a solution manual, or any other source. Homework must be turn in on time, usually by 10 AM on the day following the day we completely covered a chapter of the textbook. No late homework will be accepted because I will post the homework solution on the class web page immediately after the deadline. Each homework assignment is worth 100 points. By the end of the semester I will calculate the average of all assignments and it will count for up to 16% of your final grade.
Exams There will be 3 partial tests and a final. Each will count for 16% of the final grade. If you are unable to make it to a scheduled exam, please contact me in advance to make alternate arrangements. Missing the final exam will result in an incomplete in the course. You will have to retake the final at a later date. During the test all books and notebooks must be closed. You will be allowed to bring ONE 3”x 5” index card with hand written note, equations, etc. and a calculator. The index card may not be photocopied, reduced or computer generated. Violation of this rule will result in failure of the examination. I will provide relevant constants.
Exams, cont. Each exam is a partial test and will cover material discussed between the tests. The final will be structured in a similar way as the partial exams, but may contain additional review questions/problems. There will be about five questions and about five problems on each exam, a total of ten questions/problems. The problems will be similar to those solved in class, solved examples in the textbook, and homework problems. Questions will be based on conceptual questions, quick quizzes, and additional questions discussed in class. You have ONE week from the day the exam was returned to you to appeal the grade. After the seventh day the grade becomes permanent.
Class participation I encourage you to make every effort to attend class. Due to the nature of this short course, I may have to make changes/announcements and the only way you can learn about them is to attend the class. I will not read the class roll or check students attendance, except for the first two or three lectures, when I will try to memorize your names. Remember that you must bring homework assignments to class by 10 AM.
Make-up work Make-ups for missed homework and exams can be granted only under exceptional circumstances (medical emergency, official university business, wedding, etc.). If you know of such impeding circumstances, please contact me in advance so alternative arrangements can be prepared. If it is impossible to notify me in advance, please send an email as soon afterward as possible.
Final grade The final grade will be determined by adding scores for the partial exams, the final, and the average homework. Each of these components will count for 16% of the final grade, 5*16%=80%. The lab will count to the remaining 20% 94 - 100 = A • 90 - 93 = A- • 87 - 90 = B+ • 84 - 86 = B • 80 - 83 = B- • 77 - 80 = C+ • 74 - 76 = C • 70 - 73 = C- • 66 - 70 = D+ • 60 - 65 = D • 55 - 60 = D- • Below 55 F
Homework • Ch.15:3,10,17,31 • Ch.16:3,13,29,39 • Ch.17:15,19,36,47 • Ch.18:2,14,39,45 • Ch.19:7,20,31,45 • Ch.20:10,17,22,37 • Ch.21:5,7,23,35 • Ch.22:7,17,30,45 • Ch.23:6,13,32,39 • Ch.24:3,15,23,39 • Ch.25:9,12,13,19 • Ch.26:5,9,13,19 • Ch.27:6,13,23,43 • Ch.28:7,15,17,38 • Ch.29:1,9,15,30
Chapter 15 Conceptual questions: 2,4,7,9,12,14 Quizzes: 1,2,3,4,5,6 Problems: 11,21,28, 48 Examples: 1,3,6,7,8 Electric Forces and Electric Fields
Electric Charges • Two types of charges exist; positive and negative • Like charges repel and unlike charges attract one another • Nature’s basic carrier of positive charge is the proton and of negative charge is the electron • Protons do not move from one material to another because they are held firmly in the nucleus • Negative charges are primarily carried by electrons • Gaining or losing electrons is how an object becomes charged
Properties of Charge • Electric charge is always conserved • Charge is not created, only exchanged • Objects become charged because negative charge is transferred from one object to another • All charge is a multiple of a fundamental unit of charge, symbolized by e • Electrons have a charge of –e • Protons have a charge of +e • The SI unit of charge is the Coulomb (C) • e = 1.6 x 10-19 C
Insulators • Insulators are materials in which electric charges do not move freely • Glass and rubber are examples of insulators • When insulators are charged by rubbing, only the rubbed area becomes charged. The charges do not move. Conductors Electrons move freely. Metals are examples of conducting materials
Charging by Conduction • A charged object (the rod) is placed in contact with another object (the sphere) • Some electrons on the rod can move to the sphere • When the rod is removed, the sphere is left with a charge • The object being charged is always left with a charge having the same sign as the object doing the charging
Charging by Induction • A negatively charged rubber rod is brought near an uncharged sphere • The charges in the sphere are redistributed. Some electrons in the sphere are repelled from the electrons in the rod. After the sphere is grounded they leave the sphere. • The positive charge on the sphere is evenly distributed due to the repulsion between the positive charges • Charging by induction requires no contact with the object inducing the charge
Examples of Polarization • The charged object (on the left) induces charge on the surface of the insulator • A charged comb attracts bits of paper due to polarization of the paper
QUICK QUIZ 15.1 If a suspended object A is attracted to object B, which is charged, we can conclude that (a) object A is uncharged, (b) object A is charged, (c) object B is positively charged, or (d) object A may be either charged or uncharged.
Conceptual questions: 2. Operating room personnel must wear special conducting shoes while working around oxygen. Why? 7. When a metal object receives a positive charge, does its mass increase, decrease or remain the same? 14. Would life be different if the electrons were positively charged and the protons were negatively charged?Does the choice of signs have any bearings on physical and chemical interactions?
Coulomb’s Law • Coulomb’s electrical force has the following properties: • It is inversely proportional to the square of the separation between the two particles and is along the line joining them • It is proportional to the product of the magnitudes of the charges q1 and q2 on the two particles • It is attractive if the charges are of opposite signs and repulsive if the charges have the same signs
Coulomb’s Law • ke is called the Coulomb Constant • ke = 8.99 x 109 N m2/C2
Vector Nature of Electric Forces • Two point charges are separated by a distance r • The like charges produce a repulsive force between them • The force on q1 is equal in magnitude and opposite in direction to the force on q2
Object A has a charge of +2 µC, and object B has a charge of +6 µC. Which statement is true:(a) FAB = –3FBA(b) FAB = –FBA(c) 3FAB = –FBA QUICK QUIZ 15.2
Superposition Principle Example • The resultant force on any one charge equals the vector sum of the forces exerted by the other individual charges that are present. • Remember to add the forces vectorially • The force exerted by q1 on q3 is F13 • The force exerted by q2 on q3 is F23 • The total force exerted on q3 is the vector sum of F13 and F23
Electric Field • A charged particle, with charge Q, produces an electric field in the region of space around it • A small test charge, qo, placed in the field, will experience a force
Electric Field • Use this Eq. for the magnitude of the field • The electric field is a vector quantity • The direction of the field is defined to be the direction of the electric force that would be exerted on a small positive test charge placed at that point Electric field of a point charge is given by
Direction of Electric Field • The electric field produced by a negative charge is directed toward the charge • A positive test charge would be attracted to the negative source charge The test charge is a VERY small positive charge
QUICK QUIZ 15.3 A test charge of +3 µC is at a point P where the electric field due to other charges is directed to the right and has a magnitude of 4 106 N/C. If the test charge is replaced with a –3 µC charge, the electric field at P(a) has the same magnitude but changes direction, (b) increases in magnitude and changes direction, (c) remains the same,(d) decreases in magnitude and changes direction.
15-21 A Styrofoam ball covered with a conducting paint has a mass of 5.0 ´ 10-3 kg and has a charge of 4.0 C. What electric field directed upward will produce an electric force on the ball that will balance the weight of the ball?
QUICK QUIZ 15.4 A circular ring of radius b has a total charge q uniformly distributed around it. The magnitude of the electric field at the center of the ring is (a) 0 (b) keq/b2(c) keq2/b2(d) keq2/b(e) none of these.
QUICK QUIZ 15.5 A "free" electron and "free" proton are placed in an identical electric field. Which of the following statements are true?(a) Each particle experiences the same electric force and the same acceleration. (b) The electric force on the proton is greater in magnitude than the force on the electron but in the opposite direction. (c) The electric force on the proton is equal in magnitude to the force on the electron, but in the opposite direction. (d) The magnitude of the acceleration of the electron is greater than that of the proton. (e) Both particles experience the same acceleration.
Problem 15-24 Positive charges are situated at three corners of a rectangle of size 0.200m by 0.600m. Find the electric field at the fourth corner. 6.00 nC 3.00 nC 5.00 nC
Electric Field Lines • The field lines are related to the field by • The electric field vector, E, is tangent to the electric field lines at each point • The number of lines per unit area through a surface perpendicular to the lines is proportional to the strength of the electric field in a given region
Electric Field Line Patterns • Point charge • The lines radiate equally in all directions • For a positive source charge, the lines will radiate outward
Electric Field Line Patterns • For a negative source charge, the lines will point inward
Electric Field Lines of a Dipole • An electric dipole consists of two equal and opposite charges • The high density of lines between the charges indicates the strong electric field in this region
Electric Field Patterns • Unequal and unlike charges • Note that two lines leave the +2q charge for each line that terminates on -q
Rules for Drawing Electric Field Lines • The lines for a group of charges must begin on positive charges and end on negative charges • The number of lines drawn leaving a positive charge or ending on a negative charge is proportional to the magnitude of the charge • No two field lines can cross each other
QUICK QUIZ 15.6 Rank the magnitudes of the electric field at points A, B, and C in the figure below, largest magnitude first.
15.28 Fig. 15.28 Figure P15.28 shows the electric field lines for two point charges separated by a small distance. (a) Determine the ratio q1/q2. (b) What are the signs of q1 and q2?
Conductors in Electrostatic Equilibrium • The electric field is zero everywhere inside the conducting material • Any excess charge on an isolated conductor resides entirely on its surface • The electric field just outside a charged conductor is perpendicular to the conductor’s surface • 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)
Two charged parallel plates Electric field in between the plates is uniform and it magnitude is given by E=s/eo - - - + + + s = Q/A (surface charge density) ke=1/4peo eo =8.85 x 10-12 C2/Nm2 permittivity of the empty space
Conceptual questions 9. In fair weather there is an electric field at the surface of the Earth, pointing down into the ground. What is the electric charge on the ground in this situation? 12. Is it possible for the electric field to exist in empty space? 18. Why is it a bad idea to seek shelter under a tree during a lightning storm?
Review problem based on Example 15.1 • In the Bohr theory of the hydrogen atom, an • electron moves in a circular orbit about a proton, where the • radius of the orbit is 0.53 × 10–10 m. • Find the electrostatic force acting on each particle. 8.2 10-8N (b) If this force causes the centripetal acceleration of the electron, what is the speed of the electron? 2.2 106 m/s
MCAT questions • Which electric charge is possible? • a. 6.02 x 1023 C • b. 3.2 x 10-19 C • c. 2.4 x 10-19 C • d. 8.0 x 10-20 C • A sphere with a net charge of +3.0 x 10-3 C is touched by a second identical sphere B, which has a net charge of –9.0 x 10-3C. The two spheres then separate. The net charge on sphere A is now: • a. 3.0 x 10-3 C • b. -3.0 x 10-3 C • c. -6.0 x 10-3 C • d. -9.0 x 10-3 C