Contact electrification
Encyclopedia
Contact electrification is an obsolete scientific theory from the Enlightenment
that attempted to account for all the sources of electric charge known at the time. In the late 18th century, scientists developed sensitive instruments for detecting 'electrification', otherwise known as electrostatic charge
imbalance. The phenomenon of electrification by contact, or contact tension, was quickly discovered. When two objects were touched together, sometimes the objects became spontaneously charged. One object developed a net negative charge, while the other developed an equal and opposite positive charge. Then it was discovered that 'piles' of dissimilar metal disks separated by acid-soaked cloth, Voltaic pile
s, could also produce charge differences. Although it was later found that these effects were caused by different physical processes - triboelectricity, the Volta effect, differing work function
s of metals, and others - at the time they were all thought to be caused by a common 'contact electrification' process.
The contact electrification phenomenon allowed the construction of so-called 'frictional' electrostatic generator
s such as Ramsden's or Winter's machines, but it also led directly to the development of much modern electrical technology such as batteries
, fuel cell
s, electroplating
, thermocouple
s, and semiconductor junction devices including radio detector diodes
, photocell
s, LED
s, and thermoelectric cells.
as then understood. It was eventually replaced by the current theory of electrochemistry
, namely, that electricity is generated by the action of chemistry
and the exchange of electron
s between atom
s making up the battery. An important fact leading to the rejection of the theory of contact tension was the observation that corrosion
, that is, the chemical degradation of the battery, seemed unavoidable with its use, and that the more electricity was drawn from the battery, the faster the corrosion proceeded.
In fact, the Volta effect does correspond to a weak electric potential
difference developed by the contact of different metals. Nowadays, this is often known as a contact potential difference. This effect was first discovered by Alessandro Volta
, and can be measured using a capacitance electroscope comprising different metals. However, this effect does not, by itself, account for the action of electric batteries.
A number of high voltage
dry piles were invented between the early 19th century and the 1830s in an attempt to determine the answer to this question, and specifically to support Volta’s hypothesis of contact tension. The Oxford Electric Bell
is one example.
is produced. This so-called "tribo" or "rubbing" effect is not well understood. It may be caused by electron-stealing via quantum tunneling, or by transfer of surface ions. Friction is not required, although in many situations it greatly increases the phenomenon. Certain phenomena related to frictionally generated electrostatic charges have been known since antiquity, though of course the modern theory of electricity was developed after the Scientific Revolution
.
material, the metal will spontaneously become charged, while the electrolyte will acquire an equal and opposite charge. Upon first contact, a chemical reaction called a 'half-cell reaction' occurs on the metal surface. As metal ions are transferred to or from the electrolyte, and as the metal and electrolyte become oppositely charged, the increasing voltage at the thin insulating layer between metal and electrolyte will oppose the motion of the flowing ions, causing the chemical reaction to come to a stop. If a second piece of a different type of metal is placed in the same electrolyte bath, it will charge up and rise to a different voltage. If the first metal piece is touched against the second, the voltage on each metal piece will be forced to a different level, and the chemical reactions will run constantly. In this way the 'contact electrification' becomes continuous. At the same time, an electric current will appear, with the path forming a closed loop which lead from one metal part to the other, and leading out through the chemical reactions on the metal surface, through the electrolyte, then back into the chemical reactions on the second metal surface. In this way, contact electrification leads to the invention of the Galvanic cell
or battery
. See also: Dry pile
s are touched together, one steals electrons from the other, and the opposite net charges grow larger and larger; this is the Volta effect. The process is halted when the difference in electric potential
(electrostatic potential) between the two metals reaches a particular value (the difference in work function values - usually less than one volt.) At this point, the Fermi levels for the two metals are equal, and there is no voltage difference between them. [If there were a voltage difference between them, then a current would flow between them: so "zero current" means "zero voltage difference".]
or rectifier
and triggers the revolution in Semiconductor electronics
and physics.
In materials with a direct band gap, if bright light is aimed at one part of the contact area between the two semiconductors, the voltage at that spot will rise, and an electric current will appear. When light meets contact electrification, the light energy is changed directly into electrical energy, allowing creation of solar cell
s. Later it was found that the same process can be reversed, and if a current is forced backwards across the contact region between the semiconductors, sometimes light will be emitted, allowing creation of the LED
.
Age of Enlightenment
The Age of Enlightenment was an elite cultural movement of intellectuals in 18th century Europe that sought to mobilize the power of reason in order to reform society and advance knowledge. It promoted intellectual interchange and opposed intolerance and abuses in church and state...
that attempted to account for all the sources of electric charge known at the time. In the late 18th century, scientists developed sensitive instruments for detecting 'electrification', otherwise known as electrostatic charge
Electrostatics
Electrostatics is the branch of physics that deals with the phenomena and properties of stationary or slow-moving electric charges....
imbalance. The phenomenon of electrification by contact, or contact tension, was quickly discovered. When two objects were touched together, sometimes the objects became spontaneously charged. One object developed a net negative charge, while the other developed an equal and opposite positive charge. Then it was discovered that 'piles' of dissimilar metal disks separated by acid-soaked cloth, Voltaic pile
Voltaic pile
A voltaic pile is a set of individual Galvanic cells placed in series. The voltaic pile, invented by Alessandro Volta in 1800, was the first electric battery...
s, could also produce charge differences. Although it was later found that these effects were caused by different physical processes - triboelectricity, the Volta effect, differing work function
Work function
In solid-state physics, the work function is the minimum energy needed to remove an electron from a solid to a point immediately outside the solid surface...
s of metals, and others - at the time they were all thought to be caused by a common 'contact electrification' process.
The contact electrification phenomenon allowed the construction of so-called 'frictional' electrostatic generator
Electrostatic generator
An electrostatic generator, or electrostatic machine, is a mechanical device that produces static electricity, or electricity at high voltage and low continuous current...
s such as Ramsden's or Winter's machines, but it also led directly to the development of much modern electrical technology such as batteries
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
, fuel cell
Fuel cell
A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used...
s, electroplating
Electroplating
Electroplating is a plating process in which metal ions in a solution are moved by an electric field to coat an electrode. The process uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal...
, thermocouple
Thermocouple
A thermocouple is a device consisting of two different conductors that produce a voltage proportional to a temperature difference between either end of the pair of conductors. Thermocouples are a widely used type of temperature sensor for measurement and control and can also be used to convert a...
s, and semiconductor junction devices including radio detector diodes
Crystal radio receiver
thumb|Boy listening to a modern crystal radioA crystal radio receiver, also called a crystal set or cat's whisker receiver, is a very simple radio receiver, popular in the early days of radio. It needs no battery or power source and runs on the power received from radio waves by a long wire antenna...
, photocell
Solar cell
A solar cell is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect....
s, LED
Light-emitting diode
A light-emitting diode is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting...
s, and thermoelectric cells.
History
The theory held that static electricity was generated by means of contact between dissimilar materials, and was in close agreement with the principles of static electricityStatic electricity
Static electricity refers to the build-up of electric charge on the surface of objects. The static charges remain on an object until they either bleed off to ground or are quickly neutralized by a discharge. Static electricity can be contrasted with current electricity, which can be delivered...
as then understood. It was eventually replaced by the current theory of electrochemistry
Electrochemistry
Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...
, namely, that electricity is generated by the action of chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
and the exchange of electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s between atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s making up the battery. An important fact leading to the rejection of the theory of contact tension was the observation that corrosion
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
, that is, the chemical degradation of the battery, seemed unavoidable with its use, and that the more electricity was drawn from the battery, the faster the corrosion proceeded.
In fact, the Volta effect does correspond to a weak electric potential
Electric potential
In classical electromagnetism, the electric potential at a point within a defined space is equal to the electric potential energy at that location divided by the charge there...
difference developed by the contact of different metals. Nowadays, this is often known as a contact potential difference. This effect was first discovered by Alessandro Volta
Alessandro Volta
Count Alessandro Giuseppe Antonio Anastasio Gerolamo Umberto Volta was a Lombard physicist known especially for the invention of the battery in 1800.-Early life and works:...
, and can be measured using a capacitance electroscope comprising different metals. However, this effect does not, by itself, account for the action of electric batteries.
A number of high voltage
High voltage
The term high voltage characterizes electrical circuits in which the voltage used is the cause of particular safety concerns and insulation requirements...
dry piles were invented between the early 19th century and the 1830s in an attempt to determine the answer to this question, and specifically to support Volta’s hypothesis of contact tension. The Oxford Electric Bell
Oxford Electric Bell
The Oxford Electric Bell or Clarendon Dry Pile is an experimental electric bell that was set up in 1840 and which has rung almost continuously ever since. It is located in the foyer of the Clarendon Laboratory at the University of Oxford, England...
is one example.
Triboelectric contact
If two different insulators are touched together, such as when a piece of rubber is touched against a piece of glass, then the surface of the rubber will acquire an excess negative charge, and the glass will acquire an equal positive charge. If the surfaces are then pulled apart, a very high voltageHigh voltage
The term high voltage characterizes electrical circuits in which the voltage used is the cause of particular safety concerns and insulation requirements...
is produced. This so-called "tribo" or "rubbing" effect is not well understood. It may be caused by electron-stealing via quantum tunneling, or by transfer of surface ions. Friction is not required, although in many situations it greatly increases the phenomenon. Certain phenomena related to frictionally generated electrostatic charges have been known since antiquity, though of course the modern theory of electricity was developed after the Scientific Revolution
Scientific revolution
The Scientific Revolution is an era associated primarily with the 16th and 17th centuries during which new ideas and knowledge in physics, astronomy, biology, medicine and chemistry transformed medieval and ancient views of nature and laid the foundations for modern science...
.
Electrolytic-metallic contact
If a piece of metal is touched against an electrolyticElectrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
material, the metal will spontaneously become charged, while the electrolyte will acquire an equal and opposite charge. Upon first contact, a chemical reaction called a 'half-cell reaction' occurs on the metal surface. As metal ions are transferred to or from the electrolyte, and as the metal and electrolyte become oppositely charged, the increasing voltage at the thin insulating layer between metal and electrolyte will oppose the motion of the flowing ions, causing the chemical reaction to come to a stop. If a second piece of a different type of metal is placed in the same electrolyte bath, it will charge up and rise to a different voltage. If the first metal piece is touched against the second, the voltage on each metal piece will be forced to a different level, and the chemical reactions will run constantly. In this way the 'contact electrification' becomes continuous. At the same time, an electric current will appear, with the path forming a closed loop which lead from one metal part to the other, and leading out through the chemical reactions on the metal surface, through the electrolyte, then back into the chemical reactions on the second metal surface. In this way, contact electrification leads to the invention of the Galvanic cell
Galvanic cell
A Galvanic cell, or Voltaic cell, named after Luigi Galvani, or Alessandro Volta respectively, is an electrochemical cell that derives electrical energy from spontaneous redox reaction taking place within the cell...
or battery
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
. See also: Dry pile
Metallic contact
If two metals having differing work functionWork function
In solid-state physics, the work function is the minimum energy needed to remove an electron from a solid to a point immediately outside the solid surface...
s are touched together, one steals electrons from the other, and the opposite net charges grow larger and larger; this is the Volta effect. The process is halted when the difference in electric potential
Electric potential
In classical electromagnetism, the electric potential at a point within a defined space is equal to the electric potential energy at that location divided by the charge there...
(electrostatic potential) between the two metals reaches a particular value (the difference in work function values - usually less than one volt.) At this point, the Fermi levels for the two metals are equal, and there is no voltage difference between them. [If there were a voltage difference between them, then a current would flow between them: so "zero current" means "zero voltage difference".]
Semiconductor contact
If metal touches a semiconductive material, or if two different semiconductors are placed into contact, one becomes charged slightly positive and the other slightly negative. It is found that if this junction between semiconductors is connected to a power supply, and if the power supply is set to a voltage slightly higher than the natural voltage appearing because of contact electrification, then for one polarity of voltage there will be a current between the two semiconductor parts, but if the polarity is reversed, the current stops. Thus contact electrification leads to the invention of the diodeDiode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
or rectifier
Rectifier
A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
and triggers the revolution in Semiconductor electronics
Semiconductor device
Semiconductor devices are electronic components that exploit the electronic properties of semiconductor materials, principally silicon, germanium, and gallium arsenide, as well as organic semiconductors. Semiconductor devices have replaced thermionic devices in most applications...
and physics.
In materials with a direct band gap, if bright light is aimed at one part of the contact area between the two semiconductors, the voltage at that spot will rise, and an electric current will appear. When light meets contact electrification, the light energy is changed directly into electrical energy, allowing creation of solar cell
Solar cell
A solar cell is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect....
s. Later it was found that the same process can be reversed, and if a current is forced backwards across the contact region between the semiconductors, sometimes light will be emitted, allowing creation of the LED
Light-emitting diode
A light-emitting diode is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting...
.