Electrostatic induction
Encyclopedia
Electrostatic induction is a redistribution of electrical charge in an object, caused by the influence of nearby charges. Induction was discovered by British scientist John Canton
in 1753 and Swedish professor Johan Carl Wilcke in 1762. Electrostatic generator
s, such as the Wimshurst machine
, the Van de Graaff generator
and the electrophorus
, use this principle. Electrostatic induction should not be confused with electromagnetic induction
; both are often referred to as 'induction'.
A normal uncharged piece of matter has equal numbers of positive and negative electrical charges in each part of it, located close together, so no part of it has a net electric charge. When a charged object is brought near an uncharged, electrically conducting
object, such as a piece of metal, the force of the nearby charge causes a separation of these charges. For example, if a positive charge is brought near the object (see picture at right), the negative charges in the metal will be attracted toward it and move to the side of the object facing it, while the positive charges are repelled and move to the side of the object away from it. This results in a region of negative charge on the object nearest to the external charge, and a region of positive charge on the part away from it. These are called induced charges. If the external charge is negative, the polarity of the charged regions will be reversed. Since this is just a redistribution of the charges that were already in the object, the object has no net charge. This induction effect is reversible; if the nearby charge is removed, the attraction between the positive and negative internal charges cause them to intermingle again.
A minor correction to the above explanation is that only the negative charges in conductive objects, the electron
s, are free to move; the positive charges, the atoms' nuclei, are bound into the structure of solid matter. So all motion of charges is a result of the motion of electrons only. In the above example, the electrons move from the left side of the object to the right. However, when a number of electrons move out of an area, they leave an unbalanced positive charge due to the nuclei. So the movement of electrons creates both the positively and negatively charged regions described above.
However, the induction effect can also be used to put a net charge on an object. If, while it is close to the positive charge, the above object is momentarily connected through a conductive
path to electrical ground
, which is a large reservoir of both positive and negative charges, some of the negative charges in the ground will flow into the object, under the attraction of the nearby positive charge. When the contact with ground is broken, the object is left with a net negative charge.
This method can be demonstrated using a gold-leaf electroscope, which is an instrument for detecting electric charge. The electroscope is first discharged, and a charged object is then brought close to the instrument's top terminal. Induction causes a redistribution of the charges inside the electroscope
's metal rod, so that the top terminal gains a net charge of opposite polarity to that of the object, while the gold leaves gain a charge of the same polarity. Since both leaves have the same charge, they repel each other and spread apart. The electroscope has not acquired a net charge: the charge within it has merely been redistributed, so if the charge were to be moved away from the electroscope the leaves will come together again.
But if an electrical contact is now briefly made between the electroscope terminal and ground
, for example by touching the terminal with a finger, this causes charge to flow from ground to the terminal, attracted by the charge on the object close to the terminal. The electroscope now contains a net charge opposite in polarity to that of the charged object. When the electrical contact to earth is broken, e.g. by lifting the finger, the extra charge that has just flowed into the electroscope cannot escape, and the instrument retains a net charge. So the gold leaves remain separated even after the nearby charged object is moved away.
The sign of the charge left on the object after grounding is always opposite in sign from the external inducing charge.
of the external charged object. As the charges in the metal object continue to separate, the resulting positive and negative regions create their own electric field, which opposes the field of the external charge. This process continues until very quickly (within a fraction of a second) an equilibrium is reached in which the induced charges are exactly the right size to cancel the external electric field throughout the interior of the metal object. Then the remaining mobile charges (electrons) in the interior of the metal no longer feel a force and the net motion of the charges stops. In any conductive object there is a very large number of mobile charge carrier
s (electrons), enough to cancel out extremely large external electric fields.
of the electrostatic potential, another way of saying this is that in electrostatics, the potential (voltage
) throughout a conductive object is constant.
) objects, and is responsible for the attraction of small light nonconductive objects, like scraps of paper or Styrofoam
, to static electric charges
. In nonconductors, the electron
s are bound to atom
s and are not free to move about the object; however they can move a little within the atoms. If a positive charge is brought near a nonconductive object, the electrons in each atom are attracted toward it, and move to the side of the atom facing the charge, while the positive nucleus is repelled and moves slightly to the opposite side of the atom. This is called polarization. Since the negative charges are now closer to the external charge than the positive charges, their attraction is greater than the repulsion of the positive charges, resulting in a small net attraction toward the charge. This effect is microscopic, but since there are so many atoms, it adds up to enough force to move a light object like Styrofoam. This is the principle of operation of a pith-ball electroscope.
John Canton
John Canton FRS was an English physicist.Canton was born in Middle Street Stroud, Gloucestershire, the son of a weaver John Canton and Esther He had only a common education, after which he was put apprentice to a broadcloth weaver, but his leisure hours were devoted to mathematical studies, and...
in 1753 and Swedish professor Johan Carl Wilcke in 1762. 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 the Wimshurst machine
Wimshurst machine
The Wimshurst influence machine is an electrostatic generator, a machine for generating high voltages developed between 1880 and 1883 by British inventor James Wimshurst ....
, the Van de Graaff generator
Van de Graaff generator
A Van de Graaff generator is an electrostatic generator which uses a moving belt to accumulate very high voltages on a hollow metal globe on the top of the stand. It was invented in 1929 by American physicist Robert J. Van de Graaff. The potential differences achieved in modern Van de Graaff...
and the electrophorus
Electrophorus
An electrophorus is a capacitive generator used to produce electrostatic charge via the process of electrostatic induction. A first version of it was invented in 1762 by Swedish professor Johan Carl Wilcke,...
, use this principle. Electrostatic induction should not be confused with electromagnetic induction
Electromagnetic induction
Electromagnetic induction is the production of an electric current across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, electric motors, synchronous motors, and solenoids....
; both are often referred to as 'induction'.
Explanation
Electrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...
object, such as a piece of metal, the force of the nearby charge causes a separation of these charges. For example, if a positive charge is brought near the object (see picture at right), the negative charges in the metal will be attracted toward it and move to the side of the object facing it, while the positive charges are repelled and move to the side of the object away from it. This results in a region of negative charge on the object nearest to the external charge, and a region of positive charge on the part away from it. These are called induced charges. If the external charge is negative, the polarity of the charged regions will be reversed. Since this is just a redistribution of the charges that were already in the object, the object has no net charge. This induction effect is reversible; if the nearby charge is removed, the attraction between the positive and negative internal charges cause them to intermingle again.
A minor correction to the above explanation is that only the negative charges in conductive objects, the 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, are free to move; the positive charges, the atoms' nuclei, are bound into the structure of solid matter. So all motion of charges is a result of the motion of electrons only. In the above example, the electrons move from the left side of the object to the right. However, when a number of electrons move out of an area, they leave an unbalanced positive charge due to the nuclei. So the movement of electrons creates both the positively and negatively charged regions described above.
Charging an object by induction
Electrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...
path to electrical ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
, which is a large reservoir of both positive and negative charges, some of the negative charges in the ground will flow into the object, under the attraction of the nearby positive charge. When the contact with ground is broken, the object is left with a net negative charge.
This method can be demonstrated using a gold-leaf electroscope, which is an instrument for detecting electric charge. The electroscope is first discharged, and a charged object is then brought close to the instrument's top terminal. Induction causes a redistribution of the charges inside the electroscope
Electroscope
An electroscope is an early scientific instrument that is used to detect the presence and magnitude of electric charge on a body. It was the first electrical measuring instrument. The first electroscope, a pivoted needle called the versorium, was invented by British physician William Gilbert...
's metal rod, so that the top terminal gains a net charge of opposite polarity to that of the object, while the gold leaves gain a charge of the same polarity. Since both leaves have the same charge, they repel each other and spread apart. The electroscope has not acquired a net charge: the charge within it has merely been redistributed, so if the charge were to be moved away from the electroscope the leaves will come together again.
But if an electrical contact is now briefly made between the electroscope terminal and ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
, for example by touching the terminal with a finger, this causes charge to flow from ground to the terminal, attracted by the charge on the object close to the terminal. The electroscope now contains a net charge opposite in polarity to that of the charged object. When the electrical contact to earth is broken, e.g. by lifting the finger, the extra charge that has just flowed into the electroscope cannot escape, and the instrument retains a net charge. So the gold leaves remain separated even after the nearby charged object is moved away.
The sign of the charge left on the object after grounding is always opposite in sign from the external inducing charge.
The electrostatic field inside a conductive object is zero
A remaining question is how large the induced charges are. The movement of charge is caused by the force exerted by the electric fieldElectric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
of the external charged object. As the charges in the metal object continue to separate, the resulting positive and negative regions create their own electric field, which opposes the field of the external charge. This process continues until very quickly (within a fraction of a second) an equilibrium is reached in which the induced charges are exactly the right size to cancel the external electric field throughout the interior of the metal object. Then the remaining mobile charges (electrons) in the interior of the metal no longer feel a force and the net motion of the charges stops. In any conductive object there is a very large number of mobile charge carrier
Charge carrier
In physics, a charge carrier is a free particle carrying an electric charge, especially the particles that carry electric currents in electrical conductors. Examples are electrons and ions...
s (electrons), enough to cancel out extremely large external electric fields.
Induced charge resides on the surface
Since the mobile charges in the interior of a metal object are free to move in any direction, there can never be a concentration of charge inside the metal; if there was, it would attract opposite polarity charge to neutralize it. Therefore the induced charges are located on the surface of the metal object, where they are constrained from moving by the boundary. This establishes the important principle that electrostatic charges on conductive objects reside on the surface of the object. External electric fields induce surface charges on metal objects that exactly cancel the field within. Since the field is the gradientGradient
In vector calculus, the gradient of a scalar field is a vector field that points in the direction of the greatest rate of increase of the scalar field, and whose magnitude is the greatest rate of change....
of the electrostatic potential, another way of saying this is that in electrostatics, the potential (voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
) throughout a conductive object is constant.
Induction in dielectric objects
A similar induction effect occurs in nonconductive (dielectricDielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
) objects, and is responsible for the attraction of small light nonconductive objects, like scraps of paper or Styrofoam
Styrofoam
Styrofoam is a trademark of The Dow Chemical Company for closed-cell currently made for thermal insulation and craft applications. In 1941, researchers in Dow's Chemical Physics Lab found a way to make foamed polystyrene...
, to static electric charges
Static 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...
. In nonconductors, the 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 are bound to 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 and are not free to move about the object; however they can move a little within the atoms. If a positive charge is brought near a nonconductive object, the electrons in each atom are attracted toward it, and move to the side of the atom facing the charge, while the positive nucleus is repelled and moves slightly to the opposite side of the atom. This is called polarization. Since the negative charges are now closer to the external charge than the positive charges, their attraction is greater than the repulsion of the positive charges, resulting in a small net attraction toward the charge. This effect is microscopic, but since there are so many atoms, it adds up to enough force to move a light object like Styrofoam. This is the principle of operation of a pith-ball electroscope.