Lecture 2.1 :! Electric Field

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1 Lecture 2.1 :! Electric Field Lecture Outline:! Electric Field! Electric Field of Point Charges! Electric Field of Continuous Distribution of Charge! Textbook Reading:! Ch Jan. 20,

2 Announcements Quiz in class Thu. (Jan. 22) covers Ch. 25 material. You are responsible for bringing your own calculator if you think you ll need one. You will have 15 minutes to complete the quiz.! HW1 was due this morning at 9am. HW2 due next Tue. (1/27) at 9am. REMINDER: No late homework accepted in this course!! Please don t wait until the night before an assignment is due to start on it.! I highly recommend you work out all the homework problems on paper first, before attempting the Mastering version.! If you are having trouble with clickers, Mastering Physics, or textbooks please let me know ASAP. 2

3 Last Lecture... We introduced the concept of Electric Force (Coulomb s Law) and Electric Field. on = on = A source charge creates an Electric Field everywhere in space. A probe charge placed in this field will feel a force. 3

4 Clicker Question #1 An electron and a proton are deposited at the same location in an electric field. Which experiences the larger force? A. The electron! B. The proton! C. They experience equal force.! D. Need more information. 4

5 Clicker Question #1 An electron and a proton are deposited at the same location in an electric field. Which experiences the larger force? A. The electron! B. The proton! C. They experience equal force.! D. Need more information. 4

6 Clicker Question #1 An electron and a proton are deposited at the same location in an electric field. Which experiences the larger force? A. The electron! B. The proton! C. They experience equal force.! D. Need more information. What else can we say about the forces/accelerations for the electron and proton? 4

7 Clicker Question #2 At which point (a, b, c, d) is the Electric field weakest? 5

8 Clicker Question #2 At which point (a, b, c, d) is the Electric field weakest? 5

9 Electric Field Electric Field exists everywhere in space, regardless of whether there is any probe charge present to feel it. Electric field points away from a positive source charge. Electric field points towards a negative source charge. 6

10 Electric Field of Point Charges Net electric force on charge q due to a group of point charges: F net on q = F 1onq + F 2onq + Net electric field due to a group of point charges: E net = F net on q q = F 1onq q + F 2onq q + = E 1 + E 2 + = i E i Superposition! 7

11 Electric Field of Point Charges E-Field DEMO! Electric Field of a Dipole! (positive and negative charge separated by distance s). 8

12 Electric Field of Point Charges Electric Field of a Dipole! (positive and negative charge separated by distance s). j + +q s i - -q 9

13 Electric Field of Point Charges Electric Field of a Dipole 10

14 Clicker Question #3 In the diagram shown, with a dipole along the y-axis, in what direction does the electric field point at the spot labeled P? + A. To the left! B. To the right! C. Upwards! D. Downwards - P 11

15 Clicker Question #3 In the diagram shown, with a dipole along the y-axis, in what direction does the electric field point at the spot labeled P? + A. To the left! B. To the right! C. Upwards! D. Downwards - P 11

16 Electric Field of Point Charges Electric Field Vectors of a Dipole 12

17 Electric Field of Point Charges Electric Field Lines of a Dipole 13

Electric Field of Point Charges Water molecules are permanent dipoles. Dipole moment = 6.

18 Electric Field of Point Charges Water molecules are permanent dipoles. Dipole moment = Cm Which way does the dipole moment point in this picture? (Balloon has been negatively charged). 14

19 Electric Field of Point Charges Compare field of one charge versus that of dipole.! Think about whether the behavior as r increases make sense. 15

20 Electric Field of Continuous Distribution How do we calculate the electric field of continuous distributions of charge? Charge density of this 2-D surface is labeled η a, and is equal to the total charge on the object divided by its total area. 16

21 Clicker Question #4 A uniformly charged object is broken into two pieces. Rank the surface charge densities (η) from largest to smallest. A. η a > η b > η c! B. η a > η c > η b! C. η b > η c > η a! D. η a = η b = η c 17

22 Clicker Question #4 A uniformly charged object is broken into two pieces. Rank the surface charge densities (η) from largest to smallest. A. η a > η b > η c! B. η a > η c > η b! C. η b > η c > η a! D. η a = η b = η c 17

23 What about a uniformly charged line of charge? 18

24 Electric Field of Continuous Distribution 19

25 Electric Field of Continuous Distribution 20

26 Electric Field of Continuous Distribution What happens to charge density as length becomes infinitely long? 20

27 Reminders! Quiz on Thursday!! Homework #2 is posted. Don t wait too long to start on this.! Finish reading through Ch this week.! Please take advantage of office hours (Tuesdays, 3-5pm). me if you would like to set up a different meeting time. 21

Lecture 1.2 :! Electric Force and Electric Field

Lecture 1.2 :! Electric Force and Electric Field Lecture Outline:! Charging Objects! Coulomb s Law! Electric Field! Textbook Reading:! Ch. 25.3-25.5 Jan. 15, 2015 1 Announcements Quiz in class next Thu.