Friday, June 23, 2006

Announcements:

• As of 7:00 am this morning, only 1/2 of the class has submitted the Intro to WebAssign and HW19a.  The deadline for these assignments has been extended one day to allow more time for the rest of the class to login and submit these assignments. Contact your instructor if you are having difficulties logging in to WebAssign (a few students had corrupted passwords that needed to be reset).
• Remember that there is no charge to use WebAssign until July 6 (one day after the drop date), so everyone is encouraged to at least try out this system during the free trial period. Since the average score for WebAssign is usually much higher than the exam average (typically 90% versus 70%), use of this homework system generally makes a significant improvement in students’ final grades. Remember also that using WebAssign can only benefit your overall score!
• For essay questions in WebAssign, as long as you make a reasonable attempt to answer the question, you will receive full credit.
• Answers to essay questions in WebAssign can be viewed after the assignment is due.
• The number of submissions allowed for a particular question in WebAssign depends on the type of question.  Pay attention to the box on the left side of the screen that shows the number of points (pts) awarded for a particular question and the number of attempts you have made out of the total number of submissions (subs) allowed.

Assignments:

• Submit HW19b via WebAssign (due Sunday at midnight).
• Read Chapter 20 and submit HW20a before class on Monday.

Ch. 19 - Electric Charges, Forces, and Fields

19-1: Electric Charge:  e = 1.6 x 10^-19 C
Charges are either positive or negative (arbitrarily assigned by Benjamin Franklin).
Charges are quantized in "bits" of e, the magnitude of the fundamental charge on an electron or proton.
 - Why is the charge on an electron such a small unit?
Charges are transferred, but not created or destroyed (conservation of charge)
Demo - electrostatic charging (rods, fur, balloon), triboelectric series
Ponderable - Why was amber the first material that the Greeks discovered to be "electric"?
Ponderable - Amber becomes negatively charged when rubbed by animal fur.  Is it possible for amber to have a positive charge?
What charge should be on the dome of a Van de Graaff generator?  How could you confirm your prediction?
Polarization of charge - A conductor or insulator can become charged by induction (without touching) due to a re-alignment of charges within the object.
Demo - charged balloon and stream of water.

19-2:  Insulators and Conductors
An insulator does not allow charges to move, while a conductor allows excess charges to move freely.  Excess charges reside on the outer surface of the object due to mutual repulsion.  Semiconductors can act like conductors or insulators depending on their composition (purity, doping).  Photoconductive materials are useful in many modern devices (laser printers, photocopiers, light sensors).

19-3:  Coulomb's Law
Coulomb's law is similar to the gravitational force, except that Fg is always attractive, and their relative sizes are quite different. (Prob. 68)
Conceptual Checkpoint 19-2 - An electron and proton are suspended in space 1 m apart and 1 m above the ground (assume ground is neutral).  If both of these particles are released from rest at the same time, where should we expect to find them 1.0 s later?  1.0 min later?

19-4:  Electric Field
Electric field is defined as E = F/q (force per unit charge, similar to g (force per unit mass).
The direction of the electric field is defined in terms of a small positive test charge.
Electric fields are 3D (the 2D images in your book are "slices" of this 3D field.)
Electric field mapping (Lab 1) - applications
Field lines are perpendicular to equipotential surfaces (includes conductors but not insulators).

19-5:  Electric Field Lines
Prob. 41 (HW19b) - Electric field lines
Review of vectors and superposition.
CC 19-4:  What is the direction of the electric field along the midline of a dipole?

19-6:  Shielding and Charging by Induction
Electrostatic shielding - applications:  coaxial cables, Faraday cage, cars hit by lightning
Charging by induction - example

19-7:  Electric Flux and Gauss's Law
Electric flux - Electric field "flow" through a surface.
Gauss's law - Positive charges are "sources" of electric field lines, while negative charges are "drains". 
Prob. 51 - What is the flux through the top, bottom, and sides of an empty box in a uniform electric field?  What if the box had a positive or negative charge in it?

Sample Problems:
P19.6 (similar to HWP19.5) - What length of tape must be pulled to transfer 1.8e13 electrons if lambda = 0.14 uC/cm? [21 cm]
P19.24 (similar to HWP19.21) - Find the electrostatic force on q1 given three charges that form an equilateral triangle.
P19.52 - What is the charge density on the inner and outer walls of a nerve cell membrane that is 0.10 microns thick and has an electric field of 7e5 N/C? [6.2e-6 C/m^2]
P19.66 - Suppose the cell membrane of a nerve cell has a thickness of 0.12 microns.  If the surface charge density is 5.9e-6 C/m2, what is the electric field within the membrane?  What if the thickness of the membrane were doubled? [E = 6.7e6 N/C, independent of thickness]
P19.73 & 68 - The Earth produces a nearly uniform electric field at ground level of 110 N/C pointing toward the center of the Earth.  (a) Find the surface charge density on the surface of the Earth. [-9.7e-10 C/m^2]  (b) Find the total electric charge on the Earth. [-5.0e5 C]  (c)  If the Moon had the same charge distributed uniformly over its surface, how would its electric field compare? [E would be greater on the surface of the moon.]
P19.68 - Calculate the electric force between the Earth and Moon with charge Q (from prob. 73), and compare with the gravitational force between them. [Fe = 1.53e4 N, Fg = 2.0e20 N, Fg/Fe = 1.3e6]

Demonstrations:

- Rods and fur

- Van de Graaff generator
- Franklin's be

Minute Paper: In addition to your answers to the in-class questions, answer the following questions for class participation (write your name on your paper):
1)  What is the most important thing you learned in class today?
2)  What questions do you still have?l