## PHYSICS 220/230 Lab 2: Electrostatics

We will perform two exercises designed to test your conceptual understanding of electric charge and electric force. Your lab notebook for this experiment should contain two separate back-to-back short reports.

Exercise 1: The Electroscope

These operations are referred to as "Sticky Tape Electrostatics" since you will be using Scotch Magic Tape to construct a low budget electroscope and to perform experiments that provide evidence for the following statements:

• There are two types of charge.
• Like charges repel.
• Unlike charges attract.
• The electric force between two charged objects depends strongly on the distance between them.

You can find a description of suggested operations in another version of these experiments which can be found "on line" at the WWW server for the Science Exploratorium. Follow the instructions that are given there, but omit using the bent soda straws and supporting stands to hold the tape. Simply attach the tape "handles" to your fingertips.

For the instructions, point your browser at: Snackbook Experiments

Exercise 2: Electric Field Hockey

We will now try to develop a qualitative understanding of the Coulomb force and the superposition of forces.

ELECTRIC FIELD HOCKEY is a computer-simulated game played with a charged puck (or "ball") on a frictionless surface. The object of the simulation, as in a traditional field hockey game, is to score a goal by propelling the puck into a net. However, here the puck is charged and moves only as a result of the influence of other charged particles which, once you put them where you want them, are "glued" down on the playing surface.

Since the game is a computer program, you can't run it on the WWW. You have to return to your computer's desktop and double-click on the shortcut icon: "Py Software". Then double-click on the folder called "Hockey", and finally double-click on "Hockey.bat".

Read the instructions about how the game works.  Then do the following:

• Score a goal at level 1.
• Score a goal at level 3.
• Score a goal at level 5. When you do this, you want to print a copy of the screen of your goal showing the puck's trajectory.

NOTE: To print your screen solution, first push the "Print Screen" key on the keyboard while the charge placements and trajectory are in view. This brings up the PrintKey menu.  Click on the cropping tool (rectangular boxes). Click and drag over the region you wish to print.  This copies the selected region to the clipboard.   You can either print this region directly, or you can open the Microsoft Word processor, and "Paste" the contents of the clipboard into a document. Then print the document using the printer from the word processor.

• Use the fact that the dots marking the trajectory are separated by equal intervals of time to estimate at which point the magnitude of the velocity is the greatest. Label this point on the printout of the screen.
• Return to the game screen, which should still show your goal. This is accomplished by holding down the Alt key and using the Tab key until the MSDos icon is chosen. Then release those keys. (Alternatively, you can click on the C:WINNT\… bar on the Taskbar.) From the "Display" menu, choose "Show Force Vector", then click on "Repeat", then "Go" at the bottom right of the screen. Now you can see how the force on the puck varies along its trajectory. Print this screen using the same method previously described. Label the printout to show that the magnitude of the Coulomb force is greatest when charged objects are closest together.
• Choose "Level 0" from the "Level" menu. This level is special in that there's no net, but you get to choose where the puck starts. This allows you to set up your own electrostatics simulations. For your first one, use 3 charges to "trap" the puck (so it keeps moving but never leaves the screen). Do the same using 4 charges. Sketch and comment on the arrangement of the charges in each case.
• Now use two charges (one positive and one negative) to get the puck to spiral into one of the charges. The puck should move around for several seconds until it finally crashes into the charge. Print the final screen and comment.