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Static electricity is common on cold dry days. Kids at the playground go down the slide with their hair standing on end. Clothes come out of the dryer with that pesky "static cling"; socks have to be peeled off of the sheets. You walk across a vinyl floor in your wool socks and get an electric shock when you touch the doorknob.

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Fig. 1: Rubbing a balloon on hair transfers negative electrons from the hair to the balloon. As the balloon is removed, it pulls on the hair with an attractive electric force.

 

Some common materials in the triboelectric series.

(positive +)

dry skin

rabbit fur

glass

human hair

nylon

wool

cat fur

silk

paper

cotton

wood

rubber balloon

rayon

polyester

orlon acrylic

styrene (styrofoam)

plastic wrap

polyethylene (tape)

polypropylene

polyvinyl chloride (PVC)

silicon

teflon

silicone rubber

(negative -)

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Fig. 2: A balloon is brought near a neutral pith ball. Electrons are pushed to the far side of the ball, leaving behind the positive remainders of the atoms. Since the positive charges are closer to the negative balloon than the negatively charged electrons, the positive charges dominate and the ball is attracted to the balloon.

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Fig. 3: Negatively charged pith ball being repelled by a negatively charged balloon. Even though the positive charges on the pith ball are closer to the balloon, there are enough negative charges overall to overcome the attractive force due to charge separation and push the ball away from the balloon.

activity - balloon on the ceiling
On a dry day, blow up a rubber balloon and rub it back and forth over your hair. You might be able to hear the crackle of static electricity as you do so. After about 10  seconds of rubbing, pull the balloon away from your hair. Your hair should be pulled up along with the balloon (figure 1). Take the side of the balloon that was rubbed and press it against a plasterboard ceiling. (Other ceiling materials may also work.) If the atmospheric humidity is low enough, the balloon should stick to the ceiling.

how does it work

Under some circumstances, it is possible for neutral atoms or molecules, which normally contain equal numbers of positive and negative charges (protons and electrons), to acquire extra electrons and turn into negative ions, or to give up electrons and turn into positive ions. Some materials have a stronger attraction to electrons than other materials. When two materials with dissimilar attraction to electrons come in contact, electrons may be transferred from one material to another, thereby making one material positively charged and the other negatively charged. This electron transfer is usually facilitated by rubbing. In essence, electrons are rubbed off of one material, and stick to the other.

In chemistry the measure of how easily an atom or molecule acquires an extra electron is related to "electron affinity".1 Materials are ordered in the "triboelectric series" (from the Greek $\tau\rho\iota\beta\omega\sigma$ "rubbing") from those that have the strongest electron affinity at the bottom to those that have the weakest electron affinity at the top. The first triboelectric list of materials was published by  the Swedish Johan Carl Wilck in 1757,2 who not coincidentally lived in a cold dry climate. A modern version is shown at the right.

Rubbing together two materials that are close together in the series transfers little or no charge, while rubbing together materials that are far apart in the sequence transfers a large charge. When a rubber balloon is rubbed against human hair, electrons are transferred from the hair to the rubber, giving the balloon a net negative charge, and leaving the hair with a net positive charge. As the balloon is pulled away, the opposite charge on the hair causes it to be attracted to the balloon. Even after the balloon is removed, the positive charge on the individual hairs causes each hair to repel the other hairs, and can result in some hairs standing on end, pushed up by the other hairs.

When the balloon is brought near the ceiling, another interesting phenomenon occurs. Although the molecules in the ceiling have net zero charge with equal numbers of electrons and protons, the electrons are slightly mobile and can move from one side of the molecule to the other. As the negatively charged balloon comes near, electrons in the ceiling material are pushed to the far side of their molecules, and protons in the nuclei are pulled closer. As the balloon touches the ceiling, it is being attracted to the positive charges in the ceiling and repelled from the negative charges, but since the negative electrons are farther away on average, the repulsion is weaker and the net force is attractive.

In metals, some electrons are highly mobile, and can hop from one atom to the next with very little resistance. Therefore, as a charged object is brought near a metal, electrons in the metal will move a relatively long distance within the metal. The large separation of charge in the metal produces a strong attractive force between the metal and the charged object. If the object is negatively charged, electrons in the metal are pushed away and the nearby positively charged protons are strongly attracted; if the object is positively charged, electrons are drawn near and are strongly attracted.

activity - pith ball
The "pith ball" is a device that uses a metal foil on a light weight core to show electrostatic attraction and repulsion. You can easily make one yourself by wrapping a styrofoam ball in aluminum foil and tying a thread on the ball. Attach the other end of the thread to a convienient overhang, perhaps on a ruler hanging over the edge of a table. When you bring a charged balloon near the pith ball, the electrons run to the far side of the ball, leaving behind the remaining positive parts of the aluminum atoms. The resulting charge separation leads to an attractive force. You should see the ball swing towards the balloon (figure 2).

If you let the balloon and the pith ball touch, you may see the pith ball suddenly jump away. When the balloon touches the pith ball, some excess electrons on the balloon will be transferred to the aluminum foil. This electron transfer is very quick, and may even result in a spark. Now the balloon and the ball will each have a net negative charge, which will produce a repulsive force. If enough charge is transferred to the ball, this repulsive force can overcome the attractive force due to the charge separation (which still occurs) and push the ball away (figure 3).

Other pairs of materials can produce even larger static charges. To get the biggest charge transfers, use a material at the top of the triboelectric list (like a rabbit) with one at the bottom of the list (like a PVC pipe).

humidity

Humidity reduces the build up and retention of static charges. On humid days, most materials will absorb a certain amount of water. Some materials will even develop a thin layer of water across the surface. Since water is conductive, the presence of water can reduce both surface resistivity and volume resistivity, allowing electrons to travel more freely and quickly dissipate any static charge across the surface or into the bulk of the material.

If a static charge is produced on an object, water vapor in the air (which includes H⁺ and OH^- ions), can be attracted to and neutralize the static charge.

questions to ponder

Sometimes when we transfer charge to the pith ball, the pith ball remains stuck to the charged object instead of bouncing away. Why is that?
Although some charge is transferred to the pith ball, giving the pith ball the same charge as the object that touches it, there may not be enough to overcome the attractive force due to charge separation. Try repeatedly charging the object and repeatedly touching the pith ball. Roll the object back and forth across the pith ball to move charge from all over the surface of the object.

Although the triboelectric series suggests that a material should get a certain charge when rubbed with another material, when we test that hypothesis by bringing it near a pith ball with that same charge, the material and the pith ball attract instead of repel. Why is that?
There are three possible reasons for seeing an attraction where you expect a repulsion. 1) There may not be enough charge on the pith ball to overcome the attractive force due to charge separation. 2) Oil or dirt on the surface of any of the materials can often change the result. Make sure everything is clean when you do your tests. 3) The published series may be wrong. It's very hard to get the order correct for materials with very similar electron affinities. Published lists often disagree about the order of nearby materials. See further resources below.

What causes the electric shock when I touch a doorknob?
When you walk over a vinyl floor (or any other electronegative material) in your socks, electrons are transferred from your socks to the floor, leaving your socks with a net positive charge. Since your socks are touching your feet, electrons from your body will flow onto the socks to make up the difference. You will be left with a net positive charge that can sometimes correspond to very high voltage (up to 35,000 Volts).3 When you subsequently touch a metal conductor such as a doorknob, electrons from the metal jump to your hand to neutralize your static charge. Fortunately, this discharge involves very little current, not enough to do damage to the human body, though it can reek havoc with sensitive electronics.

teaching notes

If there is time in class, it's worthwhile to let students construct their own table of triboelectric materials. Start with materials far apart on the triboelectric series to get a large transfer of charge. Human hair and PVC pipe are good materials to start with. Then transfer electrons from the PVC pipe to the pith ball to give the pith ball a known negative charge. Now test other materials against that known charge, for example:

• after rubbing wool on a rubber balloon, the balloon should repel a negative pith ball - wool is above a rubber balloon on the list
• after rubbing hair on lucite, the lucite should repel a negative pith ball - hair is above lucite on the list
• after rubbing wool on PVC, the PVC should repel a negative pith ball - wool is above PVC on the list
• after rubbing wool on styrofoam, the styrofoam should repel a negative pith ball - wool is above styrene on the list
• after rubbing polypropylene on glass, the glass should attract a negative pith ball - glass is above polypropylene on the list
• after rubbing wool on lucite, the lucite should attract a negative pith ball - lucite is above wool on the list

Make sure students keep detailed notes on the results of their tests, and the conclusions they make.

Some tips for good measurements:

• It's better to base conclusions over observed repulsion rather than observed attraction. Attraction can be caused either by opposite charge attraction or by charge separation overcoming like charge repulsion. In order to reduce the effects of charge separation in the pith ball, use a small pith ball. Most any light weight conductor will do. Even an aluminum pull tab off of a soda can works well.
• Make sure that you ground objects before rubbing them to remove any residual charge.
• Avoid handling objects too much. Oil and dirt from your hands can change the results.

further resources

The folks at Buffalos State University of New York have posted some clever static electricity experiments using sticky tape and straws.

Not all triboelectric series lists are the same. A great many have been published since Johan Wilck's first version in 1757. They all show the same general trends but sometimes differ on the order of nearby materials. Here's a small sample for reference:

• http://www.regentsprep.org/regents/physics/phys03/atribo/default.htm
• http://www.siliconfareast.com/tribo_series.htm
• http://en.wikipedia.org/wiki/Triboelectric_effect
• http://physicsed.buffalostate.edu/SeatExpts/resource/tribelec.htm

• 1. To be precise, "electron affinity" measures the energy produced when an electron is captured by an atom or molecule. Materials that produce more energy hold onto the captured electrons more tightly and have higher electron affinity.
• 2. Johan Carl Wilck. Disputatio physica experimentalis de electricitatibus contrariis (Rostock, 1757)
• 3. https://web.archive.org/web/20151030173241/http://www.reade.com/Safety/e...