Introduction

When Styrofoam is rubbed with wool it acquires a negative charge. The Styrofoam is an insulator which holds this charge. The electric forces produced by the charged plate can be felt and used to move large objects. The charged Styrofoam plate can be used to charge a metal plate. The conducting metal plate can then create sparks, and make neon and fluorescent lamps glow.

Material

Aluminum pie plate 20 cm (8”) in diameter or larger, but not larger than the Styrofoam.
Styrofoam, Thicker Styrofoam works better. (Home insulation that is not white or blue may be conducting.) Try:

a slab of packing material,
a stack of inverted picnic plates
or blue home insulation.

wool or felt, for example clean wool socks
Styrofoam cup
Hot melt glue, or water based glue like Elmer’s Glue All, or tape
Neon glow tube (Available at Radio Shack.)
A tape electroscope (see activity)
Optional a small fluorescent tube.

Assembly

Glue or tape a Styrofoam cup into the middle of a aluminum pie plate. Caution, solvent based glues will dissolve Styrofoam, if you use glue, choose hot melt glue, or water based glue. Or use tape.

Attach a Styrofoam cup to the center of the aluminum plate.

To Do and Notice

Charge the Styrofoam.

Rub the Styrofoam with the wool. Rub firmly for at least 15 seconds.

Rub a Styrofoam block with a wool cloth.

Test if the plate is charged by using formication

That is, move it side-to-side an inch or so from your bare arm or face. You should feel a strange sensation as the hairs on your body are moved by electrical forces from the charges on the Styrofoam. This sensation is called formication, which is technically the sensation of ants crawling over your skin. It indicates that the plate is charged. If you feel nothing have someone else test the plate.
If they detect no charge, try rubbing the plate with the wool for another minute. If this fails to charge the plate try another piece of Styrofoam.
You probably will not feel a spark. (The Styrofoam holds its charge.)

Finding the sign of the charge on the plate.

Use the tape electroscope to determine the sign of the charge on the Styrofoam. (See the Tape Electroscope activity)

Briefly,
Pull off a 10 cm (4”) length of Scotch Magic tape and smooth it down onto a table or other firm surface. Call this a BASE tape. Pull off one 10 cm length of tape and stick it down to the BASE tape, make a handle by rolling one end. Call this one a BOTTOM tape and print a B on one end.
Pull off another 10 cm length and stick it down on top of the BOTTOM tape. Leave a handle. Call this a TOP tape. Print a T on one end.
Pull up both TOP and BOTTOM tapes. Keep them together. Stroke them gently between your fingers a few times until they no longer attract your hand. The pair of tapes will then have no charge.
Pull The tapes apart rapidly and hold one in each hand. The B tape will be negatively charged. The Styrofoam plate will repel the negative B tape showing that the Styrofoam is negative after it has been rubbed with wool.

Move objects with the Styrofoam

The charged Styrofoam can be used to move other objects. Place an empty dry aluminum can on a smooth surface. The Styrofoam will attract the neutral can and make the can move without any contact. Try an aluminum can race to see who can move a can the length of a table fastest.

You can also attract dry spices such as pepper or dill weed. When they jump up to the Styrofoam they impact with a click that sounds like the random noise from a Geiger counter. Sometimes charge will transfer from the Styrofoam or balloon to the spice particles. The particles will then be repelled.

What’s Going On?

When you rub the Styrofoam with the wool, charge is transferred from one to the other. The net result is that the Styrofoam becomes negative. We cannot say whether the negative charge moved from the wool to the Styrofoam or whether the positive charge moved from the Styrofoam to the wool. The net result of both of these is the same, the Styrofoam plate is negative.

The charged plate will attract uncharged objects like the hairs on your arm or an aluminum can because the charged plate will polarize the uncharged objects. An uncharged object has an equal number of positive and negative charges. The negatively charged plate attracts the positives and repels the negatives making the near side of the neutral object positive and the far side negative and thus electrically polarized. The electrical attraction for the nearby positives exceeds the repulsion for the more distant negatives the result is that the charged Styrofoam attracts the uncharged hair or aluminum.

Part 2 Charging an aluminum pie plate

To do and notice

Use thick Styrofoam, or a Styrofoam food plate turned upside down on a table. (If a thin, charged Styrofoam plate is placed on an uncharged table it will polarize the table, the polarization of the table will reduce the effects of the charged Styrofoam so that this experiment will not work.)

Place the aluminum pan on top of the charged Styrofoam plate. Use the Styrofoam handle and be careful to avoid touching, or even coming near any part of the pan with any part of your body.
Bring the tip of your finger close to the pan. When you are closer than a centimeter you should: hear a snap, feel a shock and &emdash; in dim light &emdash; see a spark.

Place the aluminum pan on the charged Styrofoam plate, bring your hand near the plate and feel the spark!

Lift the pan two feet above the Styrofoam using the handle. Be careful not to touch the pan or even come near it. Bring the tip of a finger near the pan, another spark will jump.
You can repeat the process over and over again:
Always using the handle to move the aluminum pan.
Place the aluminum plate on the Styrofoam, then touch it to get a spark,
lift the aluminum from the Styrofoam, then touch it to get another spark.

Charge the pan and then determine the sign of the charge with a tape electroscope. The pan will repel the positive T tape, so the aluminum pan is positive. Notice that when you lift the charged aluminum pan from the Styrofoam that the pan attracts the Styrofoam.

Notice that if you lift the pan only a short distance from the Styrofoam the electric spark is smaller than if you lift it a larger distance.

What’s going on

The Styrofoam is an electrical insulator which means that charge cannot move through it. The negative charge stays on the Styrofoam plate until it slowly leaks away through the air.
The aluminum pan is a conductor, charge can move through the pan, as well as onto it and off it when it touches another conductor.
When the initially uncharged pan is placed on the Styrofoam, the negative Styrofoam repels the negatives on the pan and attracts the positives. The negative charges on the metal are electrons which can move through the metal. (See the Etc on the experiment showing electrons are mobile). However, electrons cannot leave the metal pan and move into the surrounding air, because air is an insulator.
Human bodies are conductors, when a finger touches the pan the negative charges can move off the pan and into the finger. (More about the spark below.) When the negatives leave the initially neutral pan, it becomes positively charged. When the metal pan is picked up by its insulating handle the positively charged pan can be moved around without gaining or losing charge. It will attract the negative charges in nearby objects such as the Styrofoam plate. When touched by a human finger, negative charges will be attracted onto the pan through another spark making the pan neutral again.
Notice that in all of this charging and discharging the negative charge that was originally on the Styrofoam plate did not move. So the Styrofoam plate is still negative and the process can be repeated over and over again.

Part 3 Using a neon glow tube

To do and Notice

Bring a neon glow tube near the charged plate. The tube will glow when it is near the plate, particularly if you hold the lead on the side of the tube opposite the plate.

The electrode of a neon tube nearest the metal plate will glow.

There are two metal electrodes inside the neon glow tube, notice which electrode glows. The glow is always around the electrode nearest the negative charges. Therefore glow should be around the electrode nearest to the Styrofoam plate.

Charge up the aluminum pan as in part 2. Hold it in the air. Bring the neon tube toward the pan. Notice that the electrode furthest from the pan glows, showing that the pan is positive. Bring the neon tube closer to the pan. Eventually the tube will flash as a larger amount of electric current passes through it. The aluminum pan will be uncharged after the neon tube flashes. Test it with the the tape electroscope. The uncharged pan will repel neither of the tapes. (Remember that the only thing that tells you the sign of a charge is repulsion. The uncharged pan may attract both tapes.)

When you charge or discharge the aluminum plate with a finger, observe the length of the spark. Is one end of the spark brighter than another. Is it the same end on charging and discharging? Does the spark appear to travel in one direction or the other?
Notice the length of the spark. On humid days we see 1/2 cm long sparks on dry days, 1 cm or longer.

What’s going on?

There is low pressure neon gas inside the neon glow tube.
It takes a voltage of 80 volts or more across the electrodes of the neon glow tube to make it glow. When you see a neon glow tube glowing you know the voltages across it is 80 volts or more. It takes very little current to make the neon tube glow visibly, about 10^-12 ampere. When the tube glows, this small current is being conducted through the air. Even though air is a good insulator, it is not a perfect insulator. Some electric current can travel through air. This is the current that makes the glow tube glow.

Electrons accelerate away from the terminal nearest the negatively charged object making the neon glow. These moving electrons collide with neutral neon molecules giving energy to the electrons that are in the neon atoms. When these electrons drop back to lower energy states they emit the classic, orange light of a neon tube.

It takes voltages of 3000 volts to make a spark one centimeter long in air with a pressure of 1 atmosphere. If the spark is 0.5 cm long, as it often is, then the voltage difference between you and the plate is 1,500 volts. The fat end of the spark is at the negative end. The spark travels too fast for you to see it move, however the eye “sees” bright light slightly quicker than it sees dim light (The Pulfrich effect.) so you may perceive the spark moving from the bright end toward the dim end.

Etc.

You can attach several neon tubes in a row. If 5 of them flash then there was at least a voltage difference of 5 x 80 or 400 volts between the aluminum plate and your hand.

Etc.

You may also make a fluorescent lamp glow. The small camping fluorescents work best. Touch the electrode at one end of the lamp with your finger and bring the Styrofoam plate or aluminum pan near the electrode at the other end or near the glass at the middle of the tube. Small electric currents will flow through the tube exciting the mercury gas to emit ultraviolet light causing the phosphor on the walls of the tube to glow.
You may produce a bright flash by touching the far electrode with the charged aluminum pan.

Etc

Styrofoam is an excellent insulator and works well in this experiment.
Please use recycled slabs of Styrofoam packing material when possible or buy blue Styrofoam house insulation and use it over and over. Do not use other colors of home insulation as they are more electrically conductive than the blue. Please save Styrofoam plates for reuse.

Other notes on the use of Styrofoam.
Styrofoam plates can be recycled but in most places they are not.
Compare this to wax or plastic coated paper plates which are not recyclable.
In the 1980’s Styrofoam products were inflated with chlorofluorocarbons which contribute to the destruction of the ozone layer. Today almost all Styrofoam products are inflated with nitrogen which is a harmless major component of the atmosphere.
Many people are surprised to learn that Styrofoam plates contribute less total pollution than paper plates. (But people who live near paper mills are not surprised.) The total pollution tends to be proportional to the mass of the plate, and the Styrofoam plate is much less massive than the paper plate.

Take a second piece of Styrofoam. Do not rub it. This Styrofoam will be used only as an insulator.

Place a metal pie pan upside down on the Styrofoam. At first the metal pie pan is uncharged.

A pieplate inverted on Styrofoam.

Rub another piece of Styrofoam with the wool cloth, the Styrofoam will have a negative charge, and hold it close to the pie plate without touching the pie plat, a few centimeters away. While holding the Styrofoam near the pie plate, touch the pie plate with your finger.

Rub Styrofoam with wool, then hold it over the pie plate. Touch the pie plate with your finger.

Notice the spark between the pie plate and your finger. The negative charges on the Styrofoam have repelled the negative charges on the pie plate and caused them to jump to your finger. The pie plate started out with no charge, it lost some negative charges, so it ends up positively charged.

This technique for charging objects is called charging by induction.

Try to figure out how to charge a pie pan negatively by induction.

To charge the pie pan negatively you need to hold a positive object above it.

Charge a pie pan with a Styrofoam handle positive as show at the start of this exploration. Then hold this positive pie pan by its handle over the new pie pan. Touch the new pie pan and it will gain a negative charge.

Return to electrostatics

Scientific Explorations with Paul Doherty

© 1999

30 May 2000