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1 Physics for Scientists and Engineers Chapter 21: The Electric Field I: Discrete Charge Distributions Copyright © 2004 by W. H. Freeman & Company Paul.

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Presentation on theme: "1 Physics for Scientists and Engineers Chapter 21: The Electric Field I: Discrete Charge Distributions Copyright © 2004 by W. H. Freeman & Company Paul."— Presentation transcript:

1 1 Physics for Scientists and Engineers Chapter 21: The Electric Field I: Discrete Charge Distributions Copyright © 2004 by W. H. Freeman & Company Paul A. Tipler Gene Mosca Fifth Edition

2 Electric Charge

3 3 Electric Charge Q = ± Ne, (N = integer) Charge of Matter q proton = +e q neutron = 0 q electron = -e Examples: He = (2p, 2n, 2e - ) charge = 0 He + = (2p, 2n, 1e - ) = +1e

4 4 Charge Interaction Like charges repel Unlike charges attract

5 5 Relative affinity of materials in attracting electrons.

6 Conductors and Insulators

7 7 Conductors “free” electrons electron motion = “conduction”

8 8 Evidence of electron conduction away from central metallic element (gold leaf repulsion)

9 9 Charging by Induction (1 of 3)

10 10 Charging by Induction (2 of 3)

11 11 Charging by Induction (3 of 3)

12 12 attraction of an uncharged conductor

13 13 “Ground” = source or sink of charge

14 14 Charge re-distributes quickly after rod moves far away

15 Coulomb’s Law

16 16

17 17 Example in 2D

18 18 Electric Field force per charge-size [N/C]

19 19

20 20 Direction of E

21 21 Electric Field of a Point Charge Electric Field due to Multiple Point Charges

22 22 Example: Approximating E above Uniform Line of Charge Given: 6 m long uniform strip of charge Q lies along x axis 0, 6 m. Estimate E at P = (6, 2)m by breaking strip into 3 discrete point charges q = Q/3 located at 1, 3, and 5 m along the x-axis. r1/r1 = {(6, 2) – (1, 0)}/  (29). = (5,2)/  (29) r2/r2 = (3, 2)/  (13) r3/r3 = (1, 2)/  (5)

23 23 Components of Total Electric Field

24 24 The net electric field at (6,2) is E = E1 + E2 + E3. E = kq(0.1855, ) angle of E = tan-1(.2344/.1855) = 51.6 . magnitude of E = kq  ( ) = kq= kQ/3 = kQ The exact value of E = kQ/12 = kQ

25 25 Dipole – two equal size opposite sign charges at small separation distance

26 26 Electric Field around a Dipole

27 Electric Field Lines

28 28 Effect of point charge on oil soaked threads

29 29 Electric Field Lines (EFLs) inferred from experiment

30 30 Effect of two like point charges on oil soaked threads

31 31 Electric Field Lines (EFLs) inferred from experiment

32 32 Effect of two unlike point charges on oil soaked threads

33 33 Electric Field Lines (EFLs) inferred from experiment

34 34 EFL Conventions

35 35 EFLs  lines for +q as distance from charges increas

36 36 Relative charge = #lines leaving – #lines entering Example: Each sphere has count of +8

37 Motion of Point Charges in Electric Fields

38 38

39 39 Force on – charge is opposite to E.

40 40

41 Electric Dipoles in Electric Fields

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43 43 Problems

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