Week 2 (Sept. 3, 4, 5)

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 (ASGv3Ex. 4.1),
  3. Potential of N charged spheres (ASGv3Ex. 4.3)
  4. Vector addition (ASGv3Ex. A.2*)
  5. Vector multiplication (ASGv3Ex. A.3*, Ex. A.4*)

Laboratory: Capacitance, charge, and electric potential (Ex. 4.2). Be sure to read the introductory paragraphs 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 etc.) into the inner pail so as to measure the relative magnitude and sign (positive or negative) of the charged object. 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). Next, move a second (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" (it looks like a metal coin on the end of a wand) 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 both the electrometer and the COM port of the electrostatic power supply 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. Is there a uniform charge distribution? Does the second sphere have a net charge? After measuring the charge distribution on the second sphere, just for fun, you might want to momentarily touch the second sphere with your finger while it is near the first sphere. Now remap your charge distribution. Is it the same as before? Why or why not?
  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 millimeter or two). Now attach your electrometer to the parallel plate capacitor. 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?
  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. This will maintain the plates at a constant voltage difference. The stationary plate should be attached to the ground terminal of the power supply. Also, attach the electrometer to the faraday ice pail. Now use the proof plane to measure the charge at various locations on the inner face of the moveable capacitor plate. How does the charge that you measure on the plate depend on the separation of the capacitor plates? Explain your results.

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):












Physics 2