Materials

• a clear glass Snapple bottle (or similar size/shape glass bottle)
  • empty and cleaned
  • label and most glue soaked off
• poster putty (e.g. UHU tac) or plumber's putty
• 1 paperclip
• small sheet of aluminum foil (6" x 6" is more than enough)
• 1 coffee straw or other extra-thin straw
• scissors
• glue
• roll of Scotch tape

Introduction It's a cold, dry day, and you wore a fleece sweatshirt to school. Inside the classroom you pull the sweatshirt off... and suddenly strands of hair are sticking every which way out of your head. What is happening? The fleece sweatshirt ripped many tiny particles called electrons off of the atoms and molecules on your skin and hair. Electrons have a negative electric charge, so your hair and body are now positively charged. Have you heard that "opposites attract?" Well, opposite electric charges do attract, and similar electric charges repel each other. Your positively charged strands of hair repel each other, making your hair stick up and out until something can discharge you. Moisture in the air is good at carrying extra electrons where they need to go to discharge the static -- but it's a dry day, so the static stays around for an extra-long time. Congratulations! You have just used your hair and body as a basic electroscope. In this activity, each student or group will make and use an electroscope -- one that doesn't involve your hair. The finished electroscope will look something like the picture shown here. While you are building your electrscope and after it is finished, you will be charging it by ripping electrons off your desk with Scotch tape and placing them on the electroscope rod. The electroscope "leaves" should stick up (like your hair) when the scope is charged. You'll investigate some of the many ways in which the electrons can leak off to eventually discharge your electroscope.

Instructions

• Students may work individually or in groups. Each student (or group) should start by unbending their paperclip until only the innermost (smallest) bend remains. The paperclip should now look like a long, straight rod with a single hook at the end. This paperclip will be your electroscope "rod."

• Cut two identical pieces of aluminum foil, about 1cm wide and 6cm long. These are called "leaves" of foil.

• Push one foil leaf onto the paperclip hook so that the paperclip punches a hole in the foil about 1/2 cm from one end.

• Push the other foil leaf onto the paperclip hook in the same way.

• Both leaves should now hang from the bottom of the paperclip hook. Smooth out the leaves so that they hang straight and next to each other, but do not try to press them into each other. The leaves must be free to move apart without catching or tugging on each other.

• Keep the leaves hanging from the hook and lower the assembly into the mouth of the Snapple bottle until only about 2-3cm of the paperclip is above the rim. Make sure the paperclip is centered and vertical, so that the foil leaves hang in the center of the bottle, as far as possible from all the inside walls.

• Press poster putty or plumber's putty into the mouth of the bottle to hold the paperclip assembly in place. Check the position and tilt when you are finished. Now you have an electroscope with a short rod sticking out the top. If you charge the rod, electrons will flow along the rod until they charge the foil leaves as well.

• Cut your coffee straw to a length of 8 to 10 centimeters. Pry a small amount of putty away from the lip of the bottle and insert one end of the straw into the opening. Replace putty firmly around it, and check the position of your rod and leaves to make sure they are still correct.

• Take a new piece of Scotch tape from your roll and tape it onto your desk or worktable. Hold one end of the piece and rub the rest firmly onto the table. Then quickly rip the tape off the table; some extra electrons will come off on it, though they are much too small to see. Quickly bring the piece of tape up to the electroscope and touch it to the electroscope rod. Observe what happens to the foil leaves when you do this. [Note: If nothing happens, first make sure that only the tape (and not your finger) is touching the rod.]

• Adjust the paperclip rod, foil leaves, and putty until the charging the electroscope clearly makes the leaves fly apart from each other, preferably without hitting the side of the bottle. Each time you want to charge the electroscope, you will have to rip more electrons off the table with the tape. You can use the same piece of tape until it begins to lose its stickiness.

• Try charging the electroscope with a piece of tape fresh off the roll without doing the tape-to-table-and-rip-off step. The foil leaves should not fly apart this time, since you aren't actually giving the electroscope any charge.

• Once you are satisfied with the electroscope's operation and the positions of all the pieces, carefully spread or pour glue over the putty at the mouth of the bottle. After the glue sets your electroscope pieces will be firmly held in place.

• You can use your finished electroscope to investigate many questions:
  • How far can you make the electroscope leaves fly?
  • Can you make the foil leaves stay apart for several seconds?
  • What happens when you touch the rod with your fingers while charging the rod or after it is already charged? Why?
  • Use the straw to suck some of the air out of the electroscope. Cover the opening with your finger or with more putty to keep the air out (at least mostly). Try charging the electroscope again and observing it. Does it behave the same way as it did before, or are their any differences?
  • What other materials (besides tape) are good at accumulating charge and charging up the electroscope?
  • Does your electroscope behave the same way on dry days and on rainy days? Why or why not?