Announcements. I clickers. Expected to come prepared to participate in I clickers

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1 Announcements Required pre-survey: Take TODAY and save confirmation page. Deadline Jan 26 midnight. Makeup for required pre-test: Tuesday, January 30. Details to follow. I clickers Expected to come prepared to participate in I clickers First I clickers TODAY 4% of your grade; 4 lowest scores dropped Please register your I clicker as soon as possible, if not yet. 1

2 Study Groups register Friday starting Noon What: Optional FREE, non-credit study groups for Physics 227 Spring Group meets every Wednesday 1:40 to 3:00 pm in SEC104 starting January 24. If you sign up, you are expected to attend each session. If you miss more than 2 sessions, you will be removed from the session and your spot will be given to a student on the waiting list. Study groups begin Wednesday, January 24 and end Wednesday April 25, When: Registration begins at noon, Friday, January 19 first come-first served. To register, go to Sign in with your net ID and password there is only one 227 Physics session. You must be enrolled in Physics 227 to participate. When you register, you are committing to attend the study group for the full semester. For more information or any questions, contact learningassistants@echo.rutgers.edu. 2

3 Lecture 1 January 18, 2018 Electric Charge, Coulomb's Law 3

4 Introduction to Electricity Today we begin our study of electromagnetism, one of the four fundamental forces. We start with electric charge and look at electric fields. WHY?? A large fraction of the phenomena in our lives and the basis of the devices we use all of the time are based on electro-magnetic phenomena 2016 Pearson Education Inc.

5 Learning Goals for Chapter 21 How objects become electrically charged. How we know that electric charge is conserved. How to use Coulomb s law to calculate the electric force between charges. The distinction between electric force and electric field. How to use the idea of electric field lines to visualize and interpret electric fields. How to calculate the properties of electric charge distributions, including dipoles Pearson Education Inc.

6 Properties of electric charge Atom made of electrons going around the nucleus. Orbit of electron m Radius of nucleus m Two types of charge Negative, e.g., electron Positive, e.g., proton in atom s nucleus Electric charge of electron is negative unit of charge e All known charges are integer multiples of e Charge on proton = charge on electron e= Coulombs Ø Coulomb of charge is a lot of charge! Electric charge is conserved Net charge of universe does not change If take away an electron from atom, same amount of positive charge left behind 2016 Pearson Education Inc.

7 PHET Also fun 7

8 Types of materials: Conductors and insulators Conductors Example: Metals such as Copper Most electrons bound to their parent nuclei except for one electron is free bound to the conductor as a whole, can wander through the material Insulators Example: transparent materials such as Nylon Demo materials: could add/subtract charges by rubbing 2016 Pearson Education Inc.

9 Properties of electric charges Like charges REPEL Unlike charges ATTRACT Demonstrated in PHET with balloon(s) and sweater Charging by induction In PHET saw how charges close to charged balloon moved Negatively charge balloon INDUCED separation of charges on the wall Negative charges moved away from balloon More examples in Textbook Figures 21.6, 21.7 and 21.8

10 Quantitative: Force between charged particles? Remember force of gravity 10

11 Newton s law of gravitation= force of gravity Law of gravitation: Every particle of matter attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them, where G=constant F g = Gm 1 m 2 r Pearson Education Inc.

12 Coulomb s Law: Electrical force between charged objects Coulomb s Law: The magnitude of the electric force between two point charges is directly proportional to the product of their charges and inversely proportional to the square of the distance between them, where k=constant = 1/4πε 0 F = kq 1q 2 E r 2 ; k = 1 4πε 0 Charges can be positive or negative Charges of same sign REPEL Charges of opposite sign ATTRACT 1 = k = N-m 2 4πε 0 C Pearson Education Inc.

13 I clicker question Three point charges lie on the x-axis. All three charges have the same magnitude +1C. The positions of the charges are shown on the plot. Find the force exerted on #3 by #1 and # m 0m 1m x A. F = k 2 N B. F = +2k N C. F = + k 2 N D. F = + 5k 4 N E. F = 3k 4 N 13

14 I clicker answer Three point charges lie on the x-axis. All three charges have the same magnitude +1C. The positions of the charges are shown on the plot. Find the force exerted on #3 by #1 and # m 0m 1m x A. F = k 2 N B. F = +2k N C. F = + k 2 N D. F = + 5k 4 N E. F = 3k 4 N 14

15 I clicker solution - solving Three point charges lie on the x-axis. All three charges have the same magnitude +1C. The positions of the charges are shown on the plot. Find the force exerted on #3 by #1 and # m 0m 1m x 15

16 I clicker question 1 2 α 1 α 2 Consider two point-like charges of the same mass: #1 has a charge +q, and #2 has a charge Q = +10q. You hang them from threads near each other. The angle between the thread supporting #1 and the vertical is α 1, the angle between the thread supporting #2 and the vertical is α 2. Choose the statement with which you agree: Α. α 1 > α 2 Β. α 1 < α 2 C. α 1 = α 2 D. You need to know the mass to answer this question. 16

17 I clicker answer 1 2 α 1 α 2 Consider two point-like charges of the same mass: #1 has a charge +q, and #2 has a charge Q = +10q. You hang them from threads near each other. The angle between the thread supporting #1 and the vertical is α 1, the angle between the thread supporting #2 and the vertical is α 2. Chose the statement with which you agree: Α. α 1 > α 2 Β. α 1 < α 2 C. α 1 = α 2 D. You need to know the mass to answer this question. 17

18 I clicker solving 1 2 α 1 α 2 Consider two point-like charges of the same mass: #1 has a charge +q, and #2 has a charge Q = +10q. You hang them from threads near each other. The angle between the thread supporting #1 and the vertical is α 1, the angle between the thread supporting #2 and the vertical is α 2. Chose the statement with which you agree: 18

19 I clicker F = 0 q 1 q 2 x Two charges are placed a certain distance apart. The probe charge is at some point on a line connecting the charges but not between them. The force exerted on the probe charge is 0. What is the most general thing we can say about the charges? A. They have the same sign and magnitude. B. They have opposite signs but the same magnitude. C. They have the same sign and they must have different magnitudes. D. They have opposite signs and they must have different magnitudes. E. Both charges must have zero magnitude. 19

20 I clicker answer F = 0 q 1 q 2 x Two charges are placed a certain distance apart. The probe charge is at some point on a line connecting the charges but not between them. The force exerted on the probe charge is 0. What is the most general thing we can say about the charges? A. They have the same sign and magnitude. B. They have opposite signs but the same magnitude. C. They have the same sign and they must have different magnitudes. D. They have opposite signs and they must have different magnitudes. E. Both charges must have zero magnitude. 20

21 I clicker solving F = 0 q 1 q 2 x Two charges are placed a certain distance apart. The probe charge is at some point on a line connecting the charges but not between them. The force exerted on the probe charge is 0. What is the most general thing we can say about the charges? A. They have the same sign and magnitude. B. They have opposite signs but the same magnitude. C. They have the same sign and they must have different magnitudes. D. They have opposite signs and they must have different magnitudes. E. Both charges must have zero magnitude. 21

22 Electric fields & dipoles 22

23 Gravitational field Gravitational field: Gravitational force on a test mass due to other masses For example: gravitational field g on the surface of the earth tells us about the force on a test object of mass m F g = m Gm earth 2 r earth = mfield g = mg Field g = F g m The gravitational field = Force per unit mass In general, don t need to know where the Field comes from to calculate the Force on test mass F g = mfield g 2016 Pearson Education Inc.

24 Electrical field Electrical force on a positive test charge q 0 due to other charges Q Example: Force between 2 charges to determine field E from charge Q F E = q 0 kq r 2 E = F E = q 0 E Electric field E = Force per unit positive test charge Units: Newton/Coulomb In general, don t need to know where the Field comes from to calculate the Force on test mass q 0 F E = q 0 E 2016 Pearson Education Inc.

25 Electric field in pictures Mutual repulsion of two positively charged bodies A and B Pearson Education Inc.

26 Electric field in pictures Next consider body A on its own. Body A modifies the properties of the space at point P Pearson Education Inc.

27 Electric field in pictures Can measure the electric field produced by A with a positive test charge. Remember: Electric field (force) is a vector it has magnitude AND direction 2016 Pearson Education Inc.

28 PHET Properties of electric field lines Start on positive charges End on negative charges Direction of field line gives direction of E field at that point (force on positive test charge) Closeness of lines can be measure of strength of the electric field (force) If diverge => weaker electric field If converge => stronger electric field Always continuous, never cross 28

29 Electric field lines An electric field line is an imaginary line or curve whose tangent at any point is the direction of the electric field vector at that point Pearson Education Inc.

30 Next lecture Demo Field lines from Van de Graaff 30