Physics 241: Electricity and Optics

1. Lecturer in charge: Prof. X. Wei Lecture 9:30 am-10:30 am Office: Room 246, Physics building Phone: 494-8743 Email: wxie@purdue.edu Physics 241: Electricity and Optics Lecturer: Prof. R. P. Scharenberg CHIP Administrator: Dr. V. K. Saxena Lecture: 10:30 am-11:20 am Office: Room 176, Physics Building Lecture: 11:30 am-12:20 am Phone: 494-9575 Office: Room 245, Physics Building Email: saxena@physics.purdue.edu Phone: 494-5393 Email: schrnbrg@purdue.edu Office Hours for Saxena, Scharenberg, Wei by appointment

2. Physics 241 Electricity and Optics • TEXTBOOK • Physics for Scientists and Engineers, 6th edition, Volume 2 • Paul A. Tipler and Gene Mosca (W. H. Freeman and company) I-CLICKER • Audience response device, ISBN 0-7167-7939-0 • Publishers Bedford, Freeman and Worth To activate your I-Clicker you will need to place the ID-code • from the device into CHIPS the first time you login. You should • do this as soon as possible as the I-Clicker be used during the first • week of class, both in lecture and recitation REVIEW QUESTION There will a clicker graded question about the assigned reading material at the beginning of each lecture. PHYSICS 241 WEB SITE: http://www.physics.purdue.edu/physics241 .

3. An Old (and Fundamental!) Question:What is the Structure of Matter in the Universe?

4. Wood Iron Universe Ancient Man ~1m Size Scales in the Universe Scale ~ 1026 m Gravitation Leptons Quarks Gluons Basics Constituents Scale ~ 10-10 m Electromagnetism Strong Nuclear Force Nuclei ~ 10-15 m Atoms

5. Reading Quiz 1 • An electron with charge -eannihilates with an anti-electron +e(positron) and produces two gamma rays. Does this violate charge conservation ? • (A) yes, charge is destroyed. • (B) No, net charge is conserved. • (C) No, gamma rays have charge. • (D) Yes, physical laws work on the large scale not at the individual quantum scale.

6. Concept of Charge -- Charging by rubbing

7. Charging by induction polarization by induction grounding

8. friction can cause electrons to move from one object to another.

9. 2 x 4 ELECTROSCOPE

10. Fundamental unit: elementary charge e Quantization of Charge • An electron carries a charge of –e ; a proton carries a charge of +e It is typically the electrons that move between objects. • Coulomb (C): one coulomb is the amount of charge that is transferred through the cross section of a wire in 1second when there is a current of 1 ampere in the wire.

11. Conductors: material in which electric charges can move around “freely” Metals, tap water, human body, … Insulators: material in which electric charges are “frozen” in place Air, glass, plastic, … Semi-conductor: material in which electric charges can move around but not as freely as in conductors Silicon, germanium, … Conductor vs. Insulator Cu 1029 / m3 Ge 1019 / m3

12. Conservation of Charge The net electric charge is conserved in any physical process. But … • Charge can be transferred from one object to another. • Individual charges can also, in fact, be “destroyed” or “created”, but not net charges ( annihilation) ( pair production)

13. POPULAR SCIENCE ATOM

14. Charges with the same sign repel each other, and charges with opposite signs attract each other. The electrostatic force between two particles is proportional to the amount of electric charge that each possesses and is inversely proportional to the distance between the two squared. q1 q2 where e0 is called the permittivity constant. r Coulomb’s Law by 1 on 2 1,2 1,2 1,2 • Coulomb constant:

15. Electron and proton in a hydrogen atom How strong are Coulomb forces? • Compare electric and gravitational forces electron and proton me = 9.11x10-31 kg, mp =1.67x10-27 kg

16. Quiz Question 2 • A Human weighs 120 lb, which of the • following statements are correct? • A large fraction of the weight come from the attraction force between the electric charges on human body and earth. • All the weight comes the attraction force between the electric charges on the human body and the earth. • All the weight come from the gravitational forces. The electric forces are negligible.

17. q1 q3 F13 F12 q2 F1 Principle of Superposition • Add by components or • Magnitude and direction separately by using trigonometry

18. Warm-up Quiz 3 • One known charge Q1 = Q > 0 and the other unknown positive charge Q2 > 0 are held fixed at a separation d = R as shown. • Another (non-zero) charge Q3 is introduced somewhere along the line connecting Q1 and Q2. • Which of the following statements is true? A) The force on Q3 can be zero only if Q3 is placed to the left of Q1. B) The force on Q3 can be zero only if Q3 is placed between Q1 and Q2. C) The answer to above depends on the sign of Q3. D) The answer to above depends on the magnitudes of Q1andQ2 . E) The force on Q3 can never be zero, no matter what the (non-zero!) charge Q3 is.

19. 2Q - Q d Physics 241 – 10:30 Quiz 3 – Jan. 12, 2010 Two point charges are separated by distance d as shown. Where can you put a third charge of +1 C so that there is no net electric force acting on it? (Take Q > 0.) • to the right of charge –Q • to the left of charge 2Q • C) between the two charges • D) some other place • E) nowhere e = 1.610-19 C k = 8.99109 Nm2/C2

20. Q - 2Q d Physics 241 – 11:30 Quiz 3 – Jan. 12, 2010 Two point charges are separated by distance d as shown. Where can you put a third charge of +1 C so that there is no net electric force acting on it? (Take Q > 0.) • to the right of charge -2Q • to the left of charge Q • C) between the two charges • D) some other place • E) nowhere e = 1.610-19 C k = 8.99109 Nm2/C2

21. 2Q Q d Physics 241 – Sample Quiz C – Jan. 8, 2008 Two point charges are separated by distance d as shown. Where can you put a third charge of 1 C so that there is no net electric force acting on it? (Take Q > 0.) to the right of charge Q to the left of charge 2Q between the two charges some other place nowhere e = 1.610-19 C k = 8.99109 Nm2/C2