electrostatics or, electric forces when nothing is moving.

Transcription

1 electrostatics or, electric forces when nothing is moving.

2 Summarizing the properties of charge: 1. Charge is quantized in units of e = C 2. Electrons carry one unit of negative charge, e 3. Protons carry one unit positive charge, e 4. Objects become charged be gaining or losing electrons, not protons 5. Electric charge is always conserved Table 3.1: Properties of electrons, protons, and neutrons Particle Charge [C] [e] Mass [kg] electron (e ) proton (p ) neutron (n 0 )

3 a) before charged rubber rod metal sphere b) contact c) after

4 Little pieces of tissue paper (or light grains of sawdust) are attracted by a glass rod rubbed with a silk handkerchief (or by a piece of sealing wax or a rubber comb rubbed with flannel). - from a random 1902 science book

5 neutral metal sphere b) charged rubber rod d)

6 F r12 q2 r12 F q2 ^r12 - F F q1 q1

7

8 2.0 m 2. Three point charges lie along the x axis, as shown at left. A positive charge q 1 = 15 µc is at x = 2 m, and a positive charge of q 2 = 6 µc is at the origin. Where must a negative charge q 3 be placed on the x-axis between the two r m - r 23 positive charges such that the resulting electric force on it is zero? E23 E13 q2 q3 q1 - x

9 2.0 m 2. Three point charges lie along the x axis, as shown at left. A positive charge q 1 = 15 µc is at x = 2 m, and a positive charge of q 2 = 6 µc is at the origin. Where must a negative charge q 3 be placed on the x-axis between the two r m - r 23 positive charges such that the resulting electric force on it is zero? E23 E13 q2 q3 q1 - x ~ 0.77m from q2 or ~ 1.23m from q1

10 q - -q (a) (b)

11 equal charges B A q C q field: A > B > C

12 opposite charges dipole q -q e.g., LiF & HF

13 unequal like q1 q2

14 unequal unlike q1 q2

15 (a) (b) 9. Which set of electric field lines could represent the electric field near two charges of the same sign, but different magnitudes? (c) (d) a b c d

16 (a) (b) 9. Which set of electric field lines could represent the electric field near two charges of the same sign, but different magnitudes? (c) (d) a b c d

17 (a) (b) (c) (d) 10. Referring again to the figure above, which set of electric field lines could represent the electric field near two charges of opposite sign and different magnitudes? a b c d

18 (a) (b) (c) (d) 10. Referring again to the figure above, which set of electric field lines could represent the electric field near two charges of opposite sign and different magnitudes? a b c d

19 amoeba conductor

20

21

22 q

23 q conductor E=0

24 conducting dome B A belt insulating stand ground

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26

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28 A normal!! E Area = A = A cos!

29 (a) (b) q q S3 S1 S2 S3 S1 S2

30 (a) (b) R R 1 R P 2 P 1

31 (a) (b)

32 (a) Q 3 (b) 1 Q 2 E = Q 1 R 1 R 2

33 (a) (b) E