Week 2

Labor day on Monday, so no class Monday.

Read: Conservation of electric charge (Chap. 3) and Mueschenbroeck's wonderful bottle (Chap. 4)

PHY 202 Lecture: Vector algebra, dot products, work, cross products, torque, Lorentz force law
Quiz: None; labor day.

Homework:
  1. Charged spheres (ASGv3 Ex. 3.1),
  2. Mueschenbroek bottle (ASGv3 Ex. 4.1),
  3. Potential of N charged spheres (ASGv3 Ex. 4.3)
  4. PHY 202: Vector multiplication (ASGv3 Ex. A.3, Ex. A.4)


Supplementary website: This website explains some interesting modern applications of electrostatics like laser and ink-jet printing, electrostatic painting, and air cleaning.

Chapter 3 (4 videos):









Chapter 4 (6 videos):














Laboratory: Capacitance, charge, and electric potential (Ex. 4.2). Be sure to read the introductory paragraphs in your textbook about the capacitance of a sphere and of parallel plates. Here are a few highlights of the four experiments you'll be performing:

  1. Measuring charge: in this experiment you will attach the electrometer leads to the faraday ice pail and then insert various charged objects (rubbed silk, plastic, and the blue and white charge producers) into the inner pail so as to measure the relative magnitude and sign (positive or negative) of the charged object. You might try inserting one at a time, and even multiple objects together. What happens if you insert and object and touch it to the interior ice pail before removing it? Make a detailed table of your results and try to make sense of your results. To prevent unwanted drift in voltage measurements on your electrometer, be sure to have the electrometer grounded; this can be done by connecting the jack on the side of the electrometer to the third (ground) prong inlet of a wall outlet using a patch cord with banana plugs on the ends.
  2. Charge distribution on a sphere: here, you will keep the leads of the electrometer attached to the faraday ice pail. On the other side of your desk, you will attach a 3000 volt electrostatic power supply to a metal sphere (using a spade connector) so as to maintain the sphere at a constant electric potential (voltage). (a) Now first, touch your proof plane (it looks like a metal coin on the end of a wand) to the electrified sphere and dip it into the ice pail. You should get a positive reading on your electrometer, indicating that the electrified sphere has positive charge. Do this a few times at different locations on the electrified sphere to make sure the charge is distributed uniformly over its surface. (b) Next, move a second (initially uncharged) sphere into the vicinity of the electrified sphere—perhaps a centimeter or two from the charged sphere. Don't touch the spheres together! You can now use a small metal proof plane to explore the charge distribution on the second sphere. To do this, just touch the proof plane to various location on the second sphere and then dip it in the faraday ice pail. To get the best results, be sure that you have the electrometer attached to earth ground (the ground prong inlet of a wall outlet). Make a drawing in your notebook that ``maps out," so to speak, the charge distribution (if any) on the second sphere. Be as quantitative as possible. Is there a uniform charge distribution on your second sphere? (C) Now move the second sphere far away from the firsts and measure the charge distribution on its surface. Is it the same as when it was near the first (electrified) sphere? (c) Finally, move the second sphere back close to the first sphere and then momentarily touch the second sphere with your finger while it is near the first sphere. Once again, map out the charge distribution on the second sphere. Is it the same as previously? Explain what is going on.
  3. Parallel plates with constant spacing: For this experiment, keep the metal sphere electrified using the power supply (like in the last experiment.) Place the capacitor plates very near one another (perhaps a few millimeters apart). Now attach your electrometer to the parallel plate capacitor instead of the faraday ice pail. This can be done using the low capacitance test leads; attach the ground clip to the stationary plate and the other (red) clip to the movable plate. Next, use the proof plane to gradually ``scoop" electrical charge from the charged sphere over to the -movable- (ungrounded) plate of the capacitor. With each scoop, observe the reading of the electrometer that is hooked up to the capacitor plates. Make a data table (and eventually, a plot!) to show how the voltage reading on the electrometer increases with each ``scoop." Repeat this experiment for several capacitor plate separations. Carefully explain your results. Does the capacitance of the capacitor vary with plate separation? If so, how? And how do you know? Make a plot of voltage reading across the plates versus the number of scoops you used.
  4. Parallel plates with constant voltage: For this final experiment, remove the power supply from the sphere and instead attach it to the parallel plate capacitor. Be sure it is set to the 30 volt setting, and not one of the kilovolt settings for the parallel plate capacitor experiments! The power supply will maintain the plates at a constant voltage difference. The stationary plate should be attached to the ground terminal of the power supply. Now, attach the electrometer to the faraday ice pail. Use the proof plane to measure the charge at various locations on the inner face of the moveable capacitor plate by touching the proof plane to the parallel plate and then inserting it into the ice pail. How does the charge that you measure on the plate depend on the separation of the capacitor plates? Be as quantitative as possible, and explain your results. Make a plot of the charge at a particular location versus the plate separation.
Physics 2