1. Phys. 122: Thursday, 05 Nov. Written HW 10: due by 2:00 pm. Written HW 11: ch. 32, problems 6, 14, 22, 36, 48, and 72. Due in one week. Mast. Phys.: Assign. 7 due Tuesday after next.. Reading: Begin ch. 33 and finish by next Thursday. (You may skip or skim section 33.10: we won't cover LR circuits.) Exam 2: average was 13.25/20; there were several scores in the 18s; congratulations! We will allow “test corrections” on most of the non-multiple choice questions [all but 6(i) – see next slide]. Clicker registrations: for some of you have not gone through; for those students an average of unclaimed clicker scores was used for midterm grades. Please check your email to see whether you were affected by this (and how to fix it).

2. Eligible for “test corrections”: #1 (i) - IF you missed more than 0.2 pts. #3 - IF you missed more than 0.2 pts. #7 [both parts (i) and (ii)]. [For (ii), note that I only wanted the E field due to the background V, not due to the charge mentioned in part (I).] #8 (= second “6,” on last page) is NOT eligible – answer will be given soon. Bring corrections to OSL tutors; return them to me with your original test by next Thurs.

3. Two example problems: 1: Show that only half the energy drawn from a battery in charging a capacitor in an RC circuit ends up in the capacitor. What happens to the rest? 2: Show that a battery delivers the most power when the load resistance across its terminals equals its internal resistance.

4. Clickers: a charged particle with speed v suddenly enters a region with a magnetic field, at 45 ° to its velocity. Its speed will... a) Increase b) Decrease c) First increase, then decrease d) First decrease, then increase e) Stay the same

5. Magnetic Force on Charged Particles (chapter 32, section 7) Units of magnetic field (B) are Tesla (T). Force is at right angles to the velocity and at right angles to the magnetic field.

6. The full motion generally forms a helix:

7. Currents are charges in motion! So, currents can also feel the magnetic force. (Actually, this is only true for a straight wire. For curved wires, the correct expression is an integral (sum) of the force on each bit of the wire.) NEARLY ALL ELECTRICAL DEVICES WHICH PRODUCE MOTION FROM ELECTRICAL POWER RELY ON THIS FORCE RULE!

8. The Vector Product (Cross Product) A x B Spits out a vector (not like the dot product!). Magnitude of the cross product = |A| |B| sin θ Direction given by the right-hand rule: - Put R.H. Fingers in direction of A - Curl them toward B along smallest angle - R. H. thumb now points along A x B (perp. to both A and B).

9. Cross Product Direction: follows the “right-hand rule.”

10. Component Formula for A x B: (A x B)_x = A_y B_z – A_z B_y (A x B)_y = A_z B_x – A_x B_z (A x B)_z = A_x B_y – A_y B_x (These formulas can be written as one, using the determinant of a matrix, for those who have seen matrix algebra before.)‏

11. Motion perpendicular to a uniform B field is circular:

12. The full motion generally forms a helix:

14. Clickers: The magnetic force has deflected these initially straight wires apart, as shown. Which way is current flowing? a) Left in both b) Right in both c) Clockwise d) Counterclockwise e) Who knows? I'm already ready for the weekend....

15. Clicker Question 2 The figure shows a flexible conducting wire passing through a magnetic field that points out of the page. The wire is deflected upward, as shown. In which direction is current flowing in the wire? A.To the left B.To the right

16. Example: Find the net force.

17. Unnumbered Figure 26.7 A simple DC electric motor

19. Conductors in equilibrium have a vanishing force per charge. In magnetic fields, this modifies the “electrostatics” rule E = 0: A sneak peek ahead: this rule is at the basis of electromagnetic induction, which is where EMF that isn't from batteries originates.