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 ofthe 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 formidterm grades. Please check your email to see whether you wereaffected by this (and how to fix it).

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), notethat I only wanted the E field due to thebackground V, not due to the chargementioned in part (I).]#8 (= second “6,” on last page) is NOTeligible – answer will be given soon.Bring corrections to OSL tutors; return themto me with your original test by next Thurs.

Two example problems:

1: Show that only half the energydrawn from a battery in charging acapacitor in an RC circuit ends up in thecapacitor. What happens to the rest?

2: Show that a battery delivers themost power when the load resistanceacross its terminals equals its internalresistance.

Clickers: a charged particle withspeed v suddenly enters a region witha 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

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 andat right angles to the magnetic field.

The full motion generally forms a helix:

Currents are charges in motion! So, currentscan also feel the magnetic force.

(Actually, this is only true for a straight wire.For curved wires, the correct expression isan integral (sum) of the force on each bit of thewire.)

NEARLY ALL ELECTRICAL DEVICESWHICH PRODUCE MOTION FROMELECTRICAL POWER RELY ON THISFORCE RULE!

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).

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

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 thedeterminant of a matrix, for those who have seenmatrix algebra before.)

Motion perpendicular to a uniform B fieldis circular:

The full motion generally forms a helix:

Clickers: The magnetic force has deflectedthese initially straight wires apart, as shown.Which way is current flowing?

a) Left in bothb) Right in bothc) Clockwised) Counterclockwisee) Who knows? I'm already ready for the weekend....

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 leftB.To the right

Example: Find the net force.

A simple DC electric motor

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 basisof electromagnetic induction, which iswhere EMF that isn't from batteriesoriginates.