PHYSICS 220/230 Lab 1: Electrostatics

We will perform two exercises designed to test your conceptual understanding of electric charges and electric force. 

Exercise 1: Sticky Tape Electrostatics

Introduction: 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.

Procedures:  Tear off a 20 cm piece of tape and press it firmly to a tabletop or other flat surface, leaving one end of the tape sticking up as a handle. Quickly pull the tape from the table, suspending it in the air. While holding the tape, bring your finger near to the tape without touching it.  Describe the interaction between your finger and the tape and discuss why the interaction occurred.  

This interaction is an indication that the tape has acquired a "charge".  Why would ripping tape from a table leave the tape charged?  

Now charge another piece of tape in an identical way.  How do the two charged tapes interact?

Tear off two more pieces of tape and press the sticky side of one (top tape) against the smooth side of the other (bottom tape), leaving one end of each tape sticking out as a handle. Quickly pull the tapes apart and  then bring the tapes close together. How do these two tapes interact?  Bring your finger near each tape.  Describe the interaction between your finger and the top and bottom tape and discuss why the interaction occurred.

Do the experiments support the four statements given in the introduction?  If so explain your reasoning.  If not, design and carry out further experiments to support or refute the statements.  

It is known that if you rub glass and silk the glass will become positively charged and that if you rub rubber and fur the rubber will become negatively charged.  Use this information to determine the sign of charge on the top and bottom tapes and the original tape pulled from the table.  Explain the process you used.

Suppose you were given an object with unknown charge.  Describe a procedure you could follow, using the top and bottom tapes, to determine if the object has a positive or negative charge or is neutral.

Exercise 2: Physlet Field Hockey

Introduction: We will now try to develop a qualitative understanding of the Coulomb force and the superposition of forces.  Physlet Field Hockey is a computer-simulated game played with a charged puck (or "ball") on a frictionless horizontal 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 positively 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.  Hitting a gray obstacle will end the simulation.

Procedures: Take some practice shots by selecting Practice, adding a fixed charge, and pressing the play button.  If you miss, select the game again, add and reposition the fixed charge, and try again.  Note that you can set the sign and magnitude q when adding a fixed charge.  When you have scored a goal and have a good feel for the playing field, move on to Games 1 and 2.  Score a goal in each of these configurations and, for Game 2, print a copy of the screen, with Field Vectors off, showing the winning puck's trajectory.

NOTE: To print your scoring screen, first click the MWSnap icon on the desktop while the fixed charge positions and puck trajectory are in view. This should open the MWSnap window.  Click on the Rectangle tool with the red cross and then highlight the field hockey playing field.   Clicking the left mouse button will select the part you want to print.  Then choose Edit/Copy to copy the selected portion of the image onto the clipboard.  Now run Microsoft Word and paste the contents of the clipboard into the blank document.  Print the document.

Use the fact that the plus marks showing the trajectory are separated by equal intervals of time to estimate where the magnitude of the velocity is the greatest.  Label this point on the printout of the screen.  Now return to the browser screen, which should still show your winning charge arrangement.  Play the animation again paying careful attention to how the force vector varies along the puck's trajectory.  Label the printout to show where the magnitude of the Coulomb force is greatest.  Why is the force greatest at these locations?

Now select the Trapping configuration. This level is special in that there's no net and the puck starts in the center of the playing field.  This arrangement allows you to set up your own electrostatic simulations. For your first one, use 3 charges to "trap" the puck (so it keeps moving but never leaves the screen).  You may find that turning the Field Vectors on facilitates this exercise.  The puck should stay on the screen for at least 10 seconds.  Repeat the simulation using 4 charges.  Sketch the arrangement of the charges for the 3-charge trap and print the screen showing the 4-charge trap. Why do these arrangements work?

Discussion:

1) Suppose you changed the sign of the charges on the puck and on the fixed charges (turned all positive charges into negative and vice versa) for a situation where you scored a goal.  What would be the effect on the trajectory of the puck?  Explain.

2) How do the results of your trapping simulations depend on the TWO DIMENSIONAL nature of Physlet Field Hockey? In other words, would it be possible to trap the puck, using only the fixed electric charges on the screen, if you moved the puck perpendicularly out of the plane of the screen? Would the configurations that were able to trap the puck on the screen work in real life?

EField version 4.0 was written by Wolfgang Christian.