Lab #1 – Invisible Tension

Purpose:  Electrons, electricity, electronic and other words that begin with “electr...” all originate from the Greek word “elektor,” meaning “beaming sun.” In Greek, “electron” is the word for the substance we call amber.  Amber is a very pretty goldish brown "stone" that sparkles orange and yellow in sunlight. Amber is actually fossilized tree sap! It's the stuff they got the dinosaur DNA from in the movie “Jurassic Park”.  Greeks were fascinated by it. In their mythology, amber was made from the tears of a nymph as they dropped into water.  They discovered that amber behaved oddly - like attracting feathers - when rubbed by fur or other objects. They didn't know what it was that caused this phenomenon.  The Latin word, electricus, means to "produce from amber by friction." So, we get our English word electricity from Greek and Latin words that were about amber.

In this lab we will be exploring some of the same effects that the Greeks saw over 2500 years ago!  Part 3 of this lab will be a formal lab writeup and you will need to follow the instructions very carefully, and you must take notes as you perform each stage of the experiment.  You will include in the introduction of the lab writeup a brief summary of what you found in parts 1 and 2 of this lab, as well as questions that may have arisen from these investigations.  Answer all of the questions at the end as best you can after the in-class discussion – these answers should go into your analysis section of the lab.  For the procedure section, include this handout, as well as indicating in your own words any deviations or additions that you made to the procedure so that your procedure actually corresponds with what you did.  Record all your observations carefully and clearly.

Glass Rod, Plastic Rod, Rubber Rod, Wool, Fur, Polyester, Silk, Newspaper, Pith Ball

Part 1.  What materials respond to the effect?
Tear a sheet of newspaper into small bits approximately one-half centimeter in diameter.  Place the paper on your desk and:

  1. Bring the plastic rod near the bits of paper.
  2. Rub the rod with wool, then bring it near the bits of newspaper.
  3. Try rubbing just a small corner of the wool on the rod, then bring the wool near the paper.
  4. See what other kinds of light materials respond to the rubbed rod.  Try to get materials of the same mass – i.e. very light.  Try aluminum foil, pieces of plastic wrappers, saran wrap, hair, bits of lint, whatever you can find!

Part 2.  Pith Balls
Have one person suspend a single pith ball in the air by lightly wrapping the end of the string around one of the screws of the arm-holder that is nearest to the vertical part of the stand so that the pith ball hangs within a few inches of the metal stand.

  1. Rub a plastic rod with wool, and move it towards the pith ball without touching the ball.
  2. Move the rod until it touches the pith ball.
  3. Remove the rod (making sure not to touch the pith ball), then move your hand toward the pith ball.
  4. Now touch the pith ball.
  5. After having touched the pith ball, one person ‘prepares’ a plastic rod by rubbing it with fur or wool.  The second person then suspends a pith ball.  Slowly bring the rod towards the pith ball from one side while simultaneously moving your palm towards the pith ball from the opposite side, until the pith ball touches the rod.  See if you can produce any ‘jumps’ – see how many you can get.  Write down questions that may help organize your observations and lead to some hypotheses.
  6. After touching the pith ball with your hand, bring a rubbed plastic rod near the pith ball until it touches the pith ball and is repelled by the rod.  Now rub a glass rod with silk and bring it near the pith ball.  Try the same experiment but start first with the glass rod this time.
  7. Next you can suspend two pith balls next to each other, and bring a ‘prepared’ rod towards them.  Play around with the rod and pith balls and see what kinds of behaviors occur.  Note anything interesting!

Part 3.  Swinging Rods (TO BE WRITTEN UP)
Have one person tie a string around the exact center of a plastic rod, and suspend the rod in the air by the string so that it hangs parallel to the ground and can rotate freely and won’t hit the physics stand.  Use a piece of tape to mark one end of the rod (the end which will not be rubbed). 

  1. Now you will systematically try to collect data using all the available materials.  First we must perform a ‘control’ experiment on an unprepared rod (which will be the suspended plastic rod) by examining the effects of all the possible ways of ‘preparing’ each type of rod. Each ‘run’ of the experiment will take the same format, only with different materials:
    1. Ex. Prepare any rod by rubbing it with any fabric, then bring the rod near, but not touching, the hanging rod.  Eventually you will try all possible combinations of rods and fabrics and record your data on the sheet provided.
      1. Note whether the rod is ATTRACTED (A), REPELLED (R), or NEUTRAL (N).
      2. Note the relative strength of the effect using the following descriptors: NO EFFECT (NE), LIGHT (L), MEDIUM (M), STRONG (S), or VERY STRONG (VS)
    1. Take the rod away from the hanging rod and touch it (without rubbing it), all around with your hand – this helps lessen any residual effects before using the same rod with another fabric.
    2. Once you are done with a single ‘run’, you may stabilize the hanging rod by touching only the marked side if necessary. 
  1. Now rub the unmarked side (only) of the hanging rod with the yellow polyester cloth, then try every possible combination of the materials to ‘prepare’ rods in different ways.  Bring the ‘prepared’ rods near (but not in contact with) the hanging plastic rod that has been rubbed with polyester (your ‘base’ rod to which all further results will be compared).  Record your data on the included data sheet.  After a few runs you may need to re-prepare the hanging rod again.
  2. Now see if you can get a response not from the rods, but from the fabrics that you use to rub the rods.  In other words, instead of testing the prepared rods, now test the prepared fabrics.  Rub the fabric with the rod and try to bring the part of the fabric rubbed with the rod near (but not touching) the ‘base’ prepared rod.  Note any effects and record on the data sheet.

Questions to consider in your MLB lab writeup:
Part One: (This will help you write your introduction)

  1. What is happening when you rub a plastic rod with wool or animal fur, or to a glass rod rubbed with silk?  Can you ‘tell’ anything is different about the materials in a ‘before’ and ‘after’ comparison or not?  What senses can you use?
  2. Why do the newspaper bits behave the way they do when you bring a ‘prepared’ rod near them?

Part Two: (This will help you write your introduction)

  1. Why is the pith ball initially attracted to the ‘prepared’ rod?
  2. Why does the pith ball, when it is actually touched by the rod, sometimes immediately change from being attracted to the rod to being repelled by the rod?
  3. Why is the pith ball, after having been touched by a charged rod, sometimes attracted to your hand?
  4. What might be going on in #9?
  5. What conclusion can you draw from #10?

Part Three: (All of the answers to these questions go in your Analysis section)

  1. Explain what is happening when you bring a ‘prepared’ rod near the unrubbed rod in the control experiment, and why it behaves the way it does.
  1. Why is a suspended rod that has been rubbed with a given material repelled by an equivalent rod prepared with the same material?
  1. What is suggested by your results with #14?
  1. What do your results, taken as a whole, seem to be suggesting?  Can you form any hypotheses?
  1. What are some sources of error in this experiment?
  1. What could be done to improve this experiment to get better or more reliable results?