21 Electric Field 1 Covering all sections

2 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

3 Like repel, Unlike attact Relative electron affinity

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

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

6 Charging by Induction (1 of 3)

7 Charging by Induction (2 of 3)

8 Charging by Induction (3 of 3)

9 attraction of an uncharged conductor

10 “Ground” = source or sink of charge

11 Charge re-distributes quickly after rod moves far away

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13 Example in 2D

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

15 Direction of E

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

17 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)

18 Components of Total Electric Field

19 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

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

21 Electric Field around a Dipole

22 Effect of point charge on oil soaked threads

23 Effect of two like point charges on oil soaked threads

24 Effect of two unlike point charges on oil soaked threads

25 EFL Conventions

26 Force on – charge is opposite to E.

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