1. Phys. 122: Tuesday, 22 Sept. Mast. Phys.: assign. 3 due this evening. Assign. 4 due one week from Thursday (01 Oct.). Written HW 5: due in one week (note: Tuesday!) Reading: Finish ch. 27. Begin ch. 28 afterward and finish by next Thursday. Exam 1: will be over material we've covered in chapters 23, 20, 21, and 22, on Thursday. Study guide and sample formula sheet are available from our class web page. Sample exam problems also available (but skip electric force problems there, and add some Doppler shift ones!). Remember: only “dumb” calculators allowed. First 10-15 min. of class will be review; bring questions. Don't forget your calculator and a formula sheet!!

2. Clickers: Long ago, Poisson criticized Fresnel's theory describing the wavelike behavior of light. Can you guess why? a) Because it predicted laser beams could exist. b) Because it predicted bright spots in the center of circular shadows. c) Because ocean waves don't have colors which depend on their frequency! d) Because we should feel the pressure from the waves, if they existed. e) Because it meant tiny surfers could ride on light waves using light surfboards.

3. This should remind you of the gravitational field! The main difference is that “charge” replaces “mass” here. Do not try to use the electric field DUE TO charge q in this definition! Charges only feel forces due to other charges. Useful to remember: Electric fields point in the force direction that a positive charge would feel.

4. Since total force is the vector sum of the forces, so is the total electric field the sum of the electric fields.

5. Example: E for an infinite charged rod

6. Use many infinite rods to do the infinite plane of charge:

7. Electric Field Lines... Formed by “connecting the dots” with the E field.... Lose information about how long each E vector is. Some of that information is re-encoded as the density of field lines: stronger E fields occur where lines are closer together. Important Property: Groups of field lines only begin on positive charges and only end on negative charges. They're a good way to show what the entire E field looks like.

9. Clickers: Can electric field lines ever cross each other? (a) Yes!! (b) No!!

11. Clickers: The total charge in the picture is 4e. What are the individual charge values at the three points indicated? A B C a) +2e -2e +4e b) -4e +8e -8e c) + e - e +4e d) +2e -4e +4e e) +4e -8e +8e

12. The quantity on the left is called the Flux of the Electric Field through a closed surface, and it's usually given by the symbol (Closed) Ф E. Interpretation: The electric flux represents the overall number of field lines coming OUT of the surface. However, the units are those of electric field times area. Closed surfaces are the boundary of some volume, while open surfaces are not. Gauss' Law applies only to closed surfaces!!

13. Any closed surface which lies entirely on the interior of a conductor must have zero total charge inside! Also, cavities (holes) inside conductors are shielded from outside electric fields! This is really what protects you in a car in a lightning storm. The constant ε ₀ is that which showed up in the constant k of Coulomb's law: k = 1/(4 πε ₀ ). Explicitly, ε ₀ = 8.854 x 10 ˉ¹² C ² /(N m ² ).