Most kids learn about the triboelectric effect well before they become familiar with the term. If you’ve ever rubbed a balloon on your hair and witnessed the effect of the static electricity – pulling your hair toward the balloon and potentially being strong enough to stick the balloon to your head – then you understand the basics of the triboelectric effect.
It’s basically a form of “contact electrification,” where electric charge, in the form of electrons, moves from one object to another, leading to a buildup of negative charge on one object and a deficit on the other. A rubber balloon and human hair are just two examples of objects that show this fairly common phenomenon.
Learning the details about the triboelectric effect, how it works, what causes it, and what you can find out from the triboelectric series helps you understand and predict what will happen in situations involving the transfer of electric charge.
What Is the Triboelectric Effect?
The triboelectric effect has been known to humans since at least 600 B.C., when Thales, a Greek philosopher found that you could rub amber and make it attract fluff, paper and other small, light objects. The term triboelectric effect comes from the Greek for “rubbing” and “amber,” owing to this history of the discovery of the effect. Of course, scientists today have a much better understanding of the causes of the triboelectric effect and the nature of electric charge in general.
The triboelectric effect is called contact electrification because it’s the process of objects making contact – especially rubbing against each other, like the rubber balloon against human hair or your feet across a carpet, that leads to the build up of surface charge that creates the effect.
The electric charge – in the form of electrons, the negative charge-carrying components of atoms – is transferred from one object to another during the rubbing process. The charge transfer that takes place means that one object gains electrons and thereby a net negative charge, while the other loses electrons and therefore ends up with a net positive charge.
This buildup of electrons leaves a net charge on both objects, and from this point onward they behave like any two charged objects: Like charges will repel each other, and unlike charges (like the two used to create the effect) will attract one another. The extent to which this happens depends on the materials themselves and ultimately the total charges on each object after the rubbing takes place.
Causes of the Triboelectric Effect
Ultimately, the phenomenon of triboelectricity is caused by friction: When one material is rubbed against another, electrons are effectively “stripped” from one object, and the other ends up with an abundance of electric charge.
However, to truly understand the phenomenon and what causes it, you need to think about the structure of atoms. A small, densely-packed nucleus contains positively charged protons and charge-free neutrons, with a “cloud” of negatively-charged electrons around it, usually balancing out the positive charge from the nucleus. The friction leads to the charge transfer, taking some of the negatively-charged electrons from one material.
The degree to which a material will take electrons from another material is called it’s electron affinity or charge affinity. If the atoms of one material have a higher electron affinity than the other material, then it will tend to take electrons (and thereby build up a negative charge) from the other material (which then has a deficit of electrons and develops a net positive charge). As well as a rubber balloon and human hair, feet and a carpet and amber and a cloth, another classic example of the phenomenon is provided by Teflon and rabbit fur.
In short, the amount of triboelectricity materials display differs for different materials, as a result of their specific electron or charge affinity. This is why scientists have created a list of materials ranked by their tendency to gain or lose electrons, called the triboelectric series.
The Triboelectric Series
The triboelectric series is a list of objects ranked by their propensity to acquire a net positive charge or a net negative charge when brought into contact with each other.
Materials toward the top of the triboelectric series are more likely to give up electrons on contact (and develop a net positive charge), and materials toward the bottom are more likely to gain electrons (and so negative charge).
In ideal conditions – if everything is dry – objects placed higher in the triboelectric series will tend to give up electrons to items further down in the list, and they will become positively charged. The more distance between two different materials in the triboelectric series, the greater the triboelectric effect when they are rubbed together.
Triboelectric Series Chart
You can find a great example of a triboelectric series chart here, which was based on tests performed by Bill Lee at AlphaLab, inc. This table gives details about how the materials were tested as well as limitations of the measurements.
The values in the table are in nC/J, which stands for nanocoulombs per joule, with a Coulomb being the standard unit of charge, and Joules being the unit for the energy associated with the friction. The positive or negative sign represents their likelihood of picking up positive or negative charges, respectively.
So for example, latex rubber picks up 105 nC of charge per joule of energy invested in the rubbing process, and the minus sign tells you that it picks up a net negative charge. On the other hand, dry skin has a value of +30 nC/J, meaning it will lose electrons, so it ends up with a positive charge of 30 nC per Joule of energy that goes into the rubbing process.
Finally, you will note that most of the different materials on the list (for example, silicone rubber and PVC) are insulators, so they can’t carry an electric current in normal situations. This is an important reminder that triboelectricity works completely differently than ordinary electricity, and generally, insulators are better than conductors at holding this type of static charge.
Van De Graaff Generators
Van de Graaff generators are a well-known piece of equipment that makes use of the triboelectric effect to produce a buildup or store of charge that you can measure as a potential difference using a voltmeter.
In most Van de Graaff generators, a rubber belt is rubbed against a metallic “comb” at the bottom, which draws electrons from the belt and leaves it with a net positive charge. This is then picked up by a matching comb at the top to spread the charge to the metallic dome at the top of the generator.
Of course, electrons are the mobile charge carriers, so the belt loses electrons at the bottom and then picks up electrons from the comb and dome at the top, leaving them with a deficit of electrons and so a net positive charge.
The massive potential difference created by this process can exceed 100,000 volts and is often used in a classic classroom display where somebody in contact with the generator has their hair stand on end. This is because the strands of hair all gain a matching (positive) charge and therefore start to repel each other.