Electrostatic induction, also known as "electrostatic influence" or simply "influence" in Europe and Latin America, is a redistribution of electric charge in an object that is caused by the influence of nearby charges.[1] In the presence of a charged body, an insulated conductor develops a positive charge on one end and a negative charge on the other end.[1] Induction was discovered by British scientist John Canton in 1753 and Swedish professor Johan Carl Wilcke in 1762.[2]Electrostatic generators, such as the Wimshurst machine, the Van de Graaff generator and the electrophorus, use this principle. See also Stephen Gray in this context. Due to induction, the electrostatic potential (voltage) is constant at any point throughout a conductor.[3] Electrostatic induction is also responsible for the attraction of light nonconductive objects, such as balloons, paper or styrofoam scraps, to static electric charges. Electrostatic induction laws apply in dynamic situations as far as the quasistatic approximation is valid.
Demonstration of induction, in the 1870s. The positive terminal of an electrostatic machine(right) is placed near an uncharged brass cylinder (left), causing the cylinder's near end to acquire a negative charge and the far end to acquire a positive charge. The small pith ball electroscopes hanging from the bottom show that the charge is concentrated at the ends.
Styrofoam peanuts clinging to a cat's fur. The static electricity that builds up on the fur causes a polarization of the molecules of the styrofoam due to electrostatic induction, resulting in a slight attraction of the styrofoam to the charged fur.
A normal uncharged piece of matter has equal numbers of positive and negative electric charges in each part of it, located close together, so no part of it has a net electric charge.[4]:p.711–712 The positive charges are the atoms' nuclei which are bound into the structure of matter and are not free to move. The negative charges are the atoms' electrons. In electrically conductive objects such as metals, some of the electrons are able to move freely about in the object.
When a charged object is brought near an uncharged, electrically conducting object, such as a piece of metal, the force of the nearby charge due to Coulomb's law causes a separation of these internal charges.[4]:p.712 For example, if a positive charge is brought near the object (see picture of cylindrical electrode near electrostatic machine), the electrons in the metal will be attracted toward it and move to the side of the object facing it. When the electrons move out of an area, they leave an unbalanced positive charge due to the nuclei. 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 process is just a redistribution of the charges that were already in the object, it doesn't change the total charge on the object; it still has no net charge. This induction effect is reversible; if the nearby charge is removed, the attraction between the positive and negative internal charges causes them to intermingle again.
Charging an object by induction
Gold-leaf electroscope, showing induction (labelled polarity of charges), before the terminal is grounded.Using an electroscope to show electrostatic induction. The device has leaves/needle that become charged when introducing a charged rod to it. The leaves bend the leave/needle, and the stronger the static introduced, the more bending occurs.
However, the induction effect can also be used to put a net charge on an object.[4]:p.711–713 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 separation 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 charged object 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. This charge neutralizes the charge in the gold leaves, so the leaves come together again. 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. The charge is held in the top of the electroscope terminal by the attraction of the inducing charge. But when the inducing charge is moved away, the charge is released and spreads throughout the electroscope terminal to the leaves, so the gold leaves move apart again.
The sign of the charge left on the electroscope after grounding is always opposite in sign to the external inducing charge.[5] The two rules of induction are:[5][6]
If the object is not grounded, the nearby charge will induce equal and opposite charges in the object.
If any part of the object is momentarily grounded while the inducing charge is near, a charge opposite in polarity to the inducing charge will be attracted from ground into the object, and it will be left with a charge opposite to the inducing charge.
The electrostatic field inside a conductive object is zero
Surface charges induced in metal objects by a nearby charge. The electrostatic field(lines with arrows) of a nearby positive charge (+) causes the mobile charges in metal objects to separate. Negative charges (blue) are attracted and move to the surface of the object facing the external charge. Positive charges (red) are repelled and move to the surface facing away. These induced surface charges create an opposing electric field that exactly cancels the field of the external charge throughout the interior of the metal. Therefore electrostatic induction ensures that the electric field everywhere inside a conductive object is zero.
A remaining question is how large the induced charges are. The movement of charges is caused by the force exerted on them by the electric field of the external charged object, by Coulomb's law. 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.[3] 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 and shape to cancel the external electric field throughout the interior of the metal object.[3][7] 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.[3]
Induced charge resides on the surface
Since the mobile charges (electrons) in the interior of a metal object are free to move in any direction, there can never be a static concentration of charge inside the metal; if there was, it would disperse due to its mutual repulsion.[3] Therefore in induction, the mobile charges move through the metal under the influence of the external charge in such a way that they maintain local electrostatic neutrality; in any interior region the negative charge of the electrons balances the positive charge of the nuclei. The electrons move until they reach the surface of the metal and collect there, where they are constrained from moving by the boundary.[3] The surface is the only location where a net electric charge can exist.[4]:p.754
This establishes the principle that electrostatic charges on conductive objects reside on the surface of the object.[3][7] External electric fields induce surface charges on metal objects that exactly cancel the field within.[3]
The voltage throughout a conductive object is constant
The electrostatic potential or voltage between two points is defined as the energy (work) required to move a small positive charge through an electric field between the two points, divided by the size of the charge. If there is an electric field directed from point to point then it will exert a force on a charge moving from to . Work will have to be done on the charge by a force to make it move to against the opposing force of the electric field. Thus the electrostatic potential energy of the charge will increase. So the potential at point is higher than at point . The electric field at any point is the gradient (rate of change) of the electrostatic potential:
Since there can be no electric field inside a conductive object to exert force on charges , within a conductive object the gradient of the potential is zero[3]
Another way of saying this is that in electrostatics, electrostatic induction ensures that the potential (voltage) throughout a conductive object is constant.
In nonconductors, the electrons are bound to atoms or molecules and are not free to move about the object as in conductors; however they can move a little within the molecules. If a positive charge is brought near a nonconductive object, the electrons in each molecule are attracted toward it, and move to the side of the molecule facing the charge, while the positive nuclei are repelled and move slightly to the opposite side of the molecule. 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 of the molecule toward the charge. This effect is microscopic, but since there are so many molecules, it adds up to enough force to move a light object like Styrofoam.
This change in the distribution of charge in a molecule due to an external electric field is called dielectric polarization,[8] and the polarized molecules are called dipoles. This should not be confused with a polar molecule, which has a positive and negative end due to its structure, even in the absence of external charge. This is the principle of operation of a pith-ball electroscope.[11]
Related Research Articles
An electric current is a flow of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is defined as the net rate of flow of electric charge through a surface. The moving particles are called charge carriers, which may be one of several types of particles, depending on the conductor. In electric circuits the charge carriers are often electrons moving through a wire. In semiconductors they can be electrons or holes. In an electrolyte the charge carriers are ions, while in plasma, an ionized gas, they are ions and electrons.
In physics, a dipole is an electromagnetic phenomenon which occurs in two ways:
An electromagnetic field is a physical field, mathematical functions of position and time, representing the influences on and due to electric charges. The field at any point in space and time can be regarded as a combination of an electric field and a magnetic field. Because of the interrelationship between the fields, a disturbance in the electric field can create a disturbance in the magnetic field which in turn affects the electric field, leading to an oscillation that propagates through space, known as an electromagnetic wave.
Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge can be positive or negative. Like charges repel each other and unlike charges attract each other. An object with no net charge is referred to as electrically neutral. Early knowledge of how charged substances interact is now called classical electrodynamics, and is still accurate for problems that do not require consideration of quantum effects.
Voltage, also known as (electrical) potential difference, electric pressure, or electric tension is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to move a positive test charge from the first point to the second point. In the International System of Units (SI), the derived unit for voltage is the volt (V).
A Van de Graaff generator is an electrostatic generator which uses a moving belt to accumulate electric charge on a hollow metal globe on the top of an insulated column, creating very high electric potentials. It produces very high voltage direct current (DC) electricity at low current levels. It was invented by American physicist Robert J. Van de Graaff in 1929. The potential difference achieved by modern Van de Graaff generators can be as much as 5 megavolts. A tabletop version can produce on the order of 100 kV and can store enough energy to produce visible electric sparks. Small Van de Graaff machines are produced for entertainment, and for physics education to teach electrostatics; larger ones are displayed in some science museums.
In electromagnetism and electronics, electromotive force is an energy transfer to an electric circuit per unit of electric charge, measured in volts. Devices called electrical transducers provide an emf by converting other forms of energy into electrical energy. Other electrical equipment also produce an emf, such as batteries, which convert chemical energy, and generators, which convert mechanical energy. This energy conversion is achieved by physical forces applying physical work on electric charges. However, electromotive force itself is not a physical force, and ISO/IEC standards have deprecated the term in favor of source voltage or source tension instead.
Electrostatic discharge (ESD) is a sudden and momentary flow of electric current between two differently-charged objects when brought close together or when the dielectric between them breaks down, often creating a visible spark associated with the static electricity between the objects.
Lenz's law states that the direction of the electric current induced in a conductor by a changing magnetic field is such that the magnetic field created by the induced current opposes changes in the initial magnetic field. It is named after physicist Heinrich Lenz, who formulated it in 1834.
Static electricity is an imbalance of electric charges within or on the surface of a material. The charge remains until it can move away by an electric current or electrical discharge. The word "static" is used to differentiate it from current electricity, where an electric charge flows through an electrical conductor.
A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor carrying a high voltage. It represents a local region where the air has undergone electrical breakdown and become conductive, allowing charge to continuously leak off the conductor into the air. A corona discharge occurs at locations where the strength of the electric field around a conductor exceeds the dielectric strength of the air. It is often seen as a bluish glow in the air adjacent to pointed metal conductors carrying high voltages, and emits light by the same mechanism as a gas discharge lamp. Corona discharges can also happen in weather, such as thunderstorms, where objects like ship masts or airplane wings have a charge significantly different from the air around them.
In electromagnetism, an eddy current is a loop of electric current induced within conductors by a changing magnetic field in the conductor according to Faraday's law of induction or by the relative motion of a conductor in a magnetic field. Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field. They can be induced within nearby stationary conductors by a time-varying magnetic field created by an AC electromagnet or transformer, for example, or by relative motion between a magnet and a nearby conductor. The magnitude of the current in a given loop is proportional to the strength of the magnetic field, the area of the loop, and the rate of change of flux, and inversely proportional to the resistivity of the material. When graphed, these circular currents within a piece of metal look vaguely like eddies or whirlpools in a liquid.
In electromagnetism, the magnetic moment or magnetic dipole moment is the combination of strength and orientation of a magnet or other object or system that exerts a magnetic field. The magnetic dipole moment of an object determines the magnitude of torque the object experiences in a given magnetic field. When the same magnetic field is applied, objects with larger magnetic moments experience larger torques. The strength of this torque depends not only on the magnitude of the magnetic moment but also on its orientation relative to the direction of the magnetic field. Its direction points from the south pole to north pole of the magnet.
The electroscope is an early scientific instrument used to detect the presence of electric charge on a body. It detects charge by the movement of a test object due to the Coulomb electrostatic force on it. The amount of charge on an object is proportional to its voltage. The accumulation of enough charge to detect with an electroscope requires hundreds or thousands of volts, so electroscopes are used with high voltage sources such as static electricity and electrostatic machines. An electroscope can only give a rough indication of the quantity of charge; an instrument that measures electric charge quantitatively is called an electrometer.
In physics, the electric displacement field or electric induction is a vector field that appears in Maxwell's equations. It accounts for the electromagnetic effects of polarization and that of an electric field, combining the two in an auxiliary field. It plays a major role in topics such as the capacitance of a material, as well the response of dielectrics to electric field, and how shapes can change due to electric fields in piezoelectricity or flexoelectricity as well as the creation of voltages and charge transfer due to elastic strains.
Ion wind, ionic wind, corona wind or electric wind is the airflow of charged particles induced by electrostatic forces linked to corona discharge arising at the tips of some sharp conductors subjected to high voltage relative to ground. Ion wind is an electrohydrodynamic phenomenon. Ion wind generators can also be considered electrohydrodynamic thrusters.
The versorium was the first electroscope, the first instrument that could detect the presence of static electric charge. It was invented in 1600 by William Gilbert, physician to Queen Elizabeth I.
Faraday's ice pail experiment is a simple electrostatics experiment performed in 1843 by British scientist Michael Faraday that demonstrates the effect of electrostatic induction on a conducting container. For a container, Faraday used a metal pail made to hold ice, which gave the experiment its name. The experiment shows that an electric charge enclosed inside a conducting shell induces an equal charge on the shell, and that in an electrically conducting body, the charge resides entirely on the surface. It also demonstrates the principles behind electromagnetic shielding such as employed in the Faraday cage. The ice pail experiment was the first precise quantitative experiment on electrostatic charge. It is still used today in lecture demonstrations and physics laboratory courses to teach the principles of electrostatics.
This article provides information on the following six methods of producing electric power.
Friction: Energy produced by rubbing two material together.
Heat: Energy produced by heating the junction where two unlike metals are joined.
Light: Energy produced by light being absorbed by photoelectric cells, or solar power.
Chemical: Energy produced by chemical reaction in a voltaic cell, such as an electric battery.
Pressure: Energy produced by compressing or decompressing specific crystals.
Magnetism: Energy produced in a conductor that cuts or is cut by magnetic lines of force.
↑ Fleming, John Ambrose (1911). "Electricity". In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol.9 (11thed.). Cambridge University Press. pp.179–193, see page 181, second para, three lines from end. ... the Swede, Johann Karl Wilcke (1732–1796), then resident in Germany, who in 1762 published an account of experiments in which....
This page is based on this Wikipedia article Text is available under the CC BY-SA 4.0 license; additional terms may apply. Images, videos and audio are available under their respective licenses.
