1. 1

2. 2 PHYSICS 632 SUMMER 2010 9:00 – 10:50 Room 203 Electricity, Magnetism and Light Richard A. Lindgren, Office Room 302/22 Text: Halliday, Resnick, and Walker, 7’th edition, Extended Version, Starting with Chapter 21 on Charge.

3. 3 Your Goals and Interest Get a Masters degree Crossover teaching Fill in knowledge gaps Review, learn new teaching ideas Peer learning Modeling Inquiry learning Group learning New experiments and demos Computer technology Solidify concepts Learn how to do problems - practice - practice - practice.

4. 4 Assignments and Exams See syllabus for daily homework assignments using WebAssign. –Graded Problems on webassign –assignments are up –Ungraded Problems - not on WebAssign –Warm up Problems - also ungraded Work on problems in recitation after lecture. Polling questions in class. Discuss with TA’s and others in the apartments at night. Due at 8:00 am next day. How do you improve problem-solving skills. Lots of practice and more practice. Quizzes on second two Wednesdays and one Thursday There will also be 4 homework assignments due in August and September. Final exam at the beginning of September on WebAssign.

5. Grading Homework Problems – 25% Polling in Class questions – 5% Quizzes (3) – 2 hours each – 35% Final exam -3 hours – 35% 5

6. 6 Class Organization Cartoon Lecture and discussion Demonstrations Hand-worked problems on Elmo Clicker / Polling / Conceptual problems

7. 7 References and other Texts Check out Appendices and and back of covers of your text. Physics for Scientists and Engineers, –Serway, Jewett Physics for Scientists and Engineers, –Fishbane, Gasiorowicz, Thornton Physics for Scientists & Engineers, –Giancoli Principals of Electricity –Page, Adams Calculus-Based Physics II (download) –Schnick –http://www.anselm.edu/internet/physics/cbphysics/downloadsII.htmlhttp://www.anselm.edu/internet/physics/cbphysics/downloadsII.html

8. 8 Lecture 1 Charge Ch. 21 Electrostatics: study of electricity when the charges are not in motion Math Review and Tooling up Topics –What is electric charge? –Methods of charging objects. –Electrostatic kit –Instruments to measure charge –Quantization of charge and conservation of charge –Coulombs Law and examples –Principle of superposition and examples Demos Elmo Some Examples Polling

9. 9 “In the matter of physics, the first lessons should contain nothing but what is experimental and interesting to see. A pretty experiment is in itself more valuable than 20 formulae.” Albert Einstein Student charged up to 300,000 Volts

10. 10 Charge What is charge? How do we visualize it. We only know charge exists because in experiments electric forces cause objects to move. Charge is analogous to mass in mechanics. We know how it behaves, but we don’t know what it really is. The same is true for charge.. Need some experiments to get the point across

11. 11 3 protons, 3 neutrons

12. 12 Properties of Elementary Particles. Electron: charge -1.6 x 10 -19 Coulombs (SI units) mass m e = 9.11 x 10 - 31 kg radius < 10 – 18 m Proton: charge = +1.6 x 10 -19 Coulombs m p = 1.67 x 10 -27 kg radius = 0.805 +/-0.011 x 10 -15 m Neutron: charge = 0 Pions: Three types: charge = +, -, 0 radius = 0.66 +/- 0.01 x 10 -15 m Quarks: Point objects. Confined to the proton and neutron, – Not free – Proton (uud) charge = 2/3e + 2/3e -1/3e = +e – Neutron (udd) charge = 2/3e -1/3e -1/3e = 0

13. 13 Normally atoms are in the lowest energy state. This means that the material is electrically neutral. You have the same number of electrons as protons in the material. How do we change this? How do we add more electrons than protons or remove electrons? There are several different ways. How do materials get a net charge?

14. 14 3

15. 15 Charging using the UVa Electrostatic Kit

16. 16 - - - Two teflon rods on spinner

17. 17 Detect Charge using UVa Electroscope

18. 18 Summary Comments Silk(+) on teflon(-) – Leads to Conservation of Charge Charged objects always attract neutral objects – Leads to electric dipoles Show Triboelectric series –In air everything gets more negative Not only chemical composition important, structure of surface is important - monolayer of molecules involved, quantum effect. (nanotechnology)

19. 19 Triboelectric series http://www.sciencejoywagon.com/physicszone/lesson/07elecst/static/triboele.htm

20. 20 More on our Model of electricity 1cm long and a radius of 0,005 cm 1cm long and a radius of 0,005 cm Copper atom: Z=29(protons), N= 34(neutrons), 29 Electrons Carbon or diamond - poor conductor Copper (Face Centered Cube) - good conductor Consider solid material like a piece of copper wire. The proton core is fixed in position in a lattice like structure. In a conductor, the valence electrons are free to move about. How many electrons are free to move about?

21. 21 Electrostatics is based on 4 four empirical facts Conservation of charge Quantization of charge (Lets get more quantitative) Coulombs Law The principle of superposition

22. 22 Conservation of charge Rubbing does not create charge, it is transferred from object to another Teflon negative - silk positive Acrylic positive - silk negative Demo: Show electronic electroscope with cage: gives magnitude and sign of charge. Use teflon and silk to show + and -. Electromagnetic reactions p + + e - = p + +pi 0 + e - Radioactive decay 238 U 92 = 234 Th 90 + 4 He 2 High energy particle reactions p + + n 0 = d + + γ 0

23. 23 What is meant by quantization of charge? Discovered in 1911 by Robert A. Millikan in the oil drop experiment. He determined that the difference in charges on different oil drops was always a multiple of The unit of charge is so tiny that we will never notice it comes in indivisible lumps. The elementary unit is defined as Electron charge is - e Proton charge is + e Example: Suppose in a typical experiment we charge an object up with a nanoCoulomb of charge (Q = 10 -9 C). How many elementary units, N, of charge is this?

24. 24 Coulombs Law In 1785 Charles Augustin Coulomb reported in the Royal Academy Memoires using a torsion balance two charged mulberry pithballs repelled each other with a force that is inversely proportional to the distance. q2q2 q1q1 r Lab Experiment Spheres same as points Point charges Electrical Permittivity of Vacuum Coulomb constant

25. 25 Coulombs Law q2q2 q1q1 r Repulsion Repulsion Attraction + + + - - - Charges can be positive or negative In solving problems you have to take account of whether the force is attractive or repulsive

26. 26 Coulombs Law Two Positive Charges +1 nC 1 cm q2q2 q1q1 r What is the magnitude of the force between two positive charges, each 1 nanoCoulomb, and 1cm apart in a typical demo? Repulsion What is the direction of the force? +1 nC 1 cm What is the direction of the force F 1 on charge q 1 and F 2 on charge q 2 in unit vector notation in rectangular coordinates?

27. 27 1 nC 1 cm What is the direction of the force F 1 on charge q 1 and F 2 on charge q 2 in unit vector notation rectangular coordinates? x y 1 2

28. 28 Demo: Show uniformity of charge around sphere using electrometer. Demo: Show charging spheres by induction using electrometer

29. 29 Show induction using two conducting spheres and Electrometer

30. 30

31. 31 Use Principle of Superposition Assume you can use the Coulomb Laws for each pair of charges and add up the results algebraically. This is true for any number of charges. Coulombs Law does not change when you add a 3rd charge near the first two charges. Consider the following example. Three charges lie on the x axis: q 1 =+25 nC at the origin, q 2 = -12 nC at x = 2m, q 3 =+18 nC at x=3 m. What is the net force on q 1 and the direction of the force? Method 1 We simply compute the magnitude of the force for each charge and put in the sign according to the direction of the force using common sense and add them up algebraically. Choose some direction to be positive. Solve without using unit vector notation. 1 2 x 3 q 1 =+25 nC q 3 =+18 nC q 2 = -12 nC Principle of Superposition

32. 32 ( Directed to the right in the positive x direction ) Attraction positive x direction Repulsion negative x direction 1 2 x 3 q 1 =+25 nC q 3 =+18 nC q 2 = -12 nC is the magnitude of the force acting on particle 1 by particle 2. is the magnitude of the force acting on particle 1 by particle 3.

33. 33 Some texts use the following notation. Thornton et al. The positive sign means the force on q 2 is in the same direction as the unit vector. q1q1. q2q2 r

34. 34 Method 2: Now use unit vector notation. Same problem of three charges in a line. What is the net force on q 1 and the direction of the force? When you have more than one charge, it is customary to use a different notation. This also allows you to generalize when the charges are not all in a line. (Directed to the right) 1 2 x 3 Note put in sign of charge

35. 35 Force on one charge q 1 due to many charges lying in a plane - q 3 +q 2 +q 1 r 13 r 12 -q 3 +q 2 +q 1

36. 36 q 1 = + 1 nCq 2 = + 1 nC +y +x 2 cm 1 cm q 3 = - 2 nC Example Consider three point charges in a plane. What is the magnitude of the net force on q 1 and in what direction?

37. 37 Find magnitude of and the angle relative to x axis. Method 3: Find x and y components of force on q 1 due to q 2 and q 3 and add them up. q 1 = + 1 nCq 2 = + 1 nC +y +x 2 cm 1 cm q 3 = - 2 nC

38. 38 Magnitude of Force due to q 3 F 13y F 13x  x and y Components of force due to q 3 Find x and y Components of force due to q 2 Example Cont q 1 = + 1 nCq 2 = + 1 nC +y +x 2 cm 1 cm q 3 = - 2 nC  negative x direction

39. 39 Example Cont. Total force along the x-axis Total force along the y-axis  q 1 = + 1 nC +y +x1 cm q 3 = - 2 nC  q 2 = + 1 nC Magnitude Direction

40. 40 Example Two Pennies without electrons What is the force between two 3 gm pennies one meter apart if we remove all the electrons from the copper atoms? Can we neglect the gravitational force between the pennies?

41. 41 In an atom can we neglect the gravitational force between the electrons and protons? What is the ratio of Coulomb’s electric force to Newton’s gravity force for 2 electrons separated by a distance r ? q q r m m r Huge number - pure number - no units

42. 42 Why are neutral objects always attracted to positive or negative charged objects. For example: Rubbed balloon is attracted to wall Comb is attracted to small bits of paper Clothes in the dryer stick together. Demo: Put 2 x 4 on glass and move it using charged teflon rod. What is the explanation of all of these phenomena?

43. 43 Explanation: The neutral objects atoms and molecules orient themselves in the following way so that the Coulomb forces due to attraction are greater than those due to repulsion because the latter are further away. (Inverse square Law) ++++++++++++++ Acrylic Rod Wooden block ++++++++++++++ ------------ ++++++++++++ Attraction Repulsion Attractive forces >> Repulsive Forces

44. 44 Demonstration : Hanging charged pith ball – First attraction by induction, – Then contact, then conduction of charge, then repulsion –Two equally charged pith balls Problem for Recitation: Two equally charged hanging pith balls Two equally charged pith balls mass 0.2 mg are in equilibrium as shown below. If d=2.0 cm and L= 20 cm, what is the charge q on each ball? L d

45. 45 Force on a charge due to an infinitely long wire. Charge Q is uniformly distributed on the infinite wire with charge density  We choose the Y-axis through the line charge and the X-axis through the point charge. Now, because the line charge is infinite and straight (on Y-axis), we can assume the X-axis is the mid-point of the infinite wire. Then by Coulomb’s law for the small line segment dy Resolving the forces into its components: As it is can be seen that the y-component of the force exerted by the top half of the infinite wire will cancel the y-component force exerted by the bottom half of the infinite wire. +L -L 0 x y r

46. 46 Force on a charge due to an infinitely long wire cont. You can also use polar coordinates for this integration. -L 0 x y r +L

47. Integration 47 L=0.1 m x=.05 m Area under the curve xy

48. 48 Am J. Phys. 65,846(1977) How is charge distributed along a conducting rod? No such thing as a uniformly charged metal rod. Click on C1_21_Charge

49. 49 +++ mg F c = kq 2 /r 2 q q q q Pivot point How Does the Electroscope Work? Consist of two electrically connected conductors with pivot point above cm.

50. 50 Charged Hair Van de Graaff Demo How does the Van de Graaff produce the spark? What upward forces are keeping your hair up? How are these forces produced? Why do the hair strands spread out from each other? Why do they spread out radially from the head? Is hair a conductor or insulator? How can we find out? Does it depend if is wet or dry. Need a female teacher to come forward.

51. 51 I-clickers and Polling