Electrostatic induction is a redistribution of electrical charge in an object, caused by the influence of nearby charges.[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. Electrostatic induction should not be confused with electromagnetic induction; both are often referred to as 'induction'.
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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. This results in a region of negative charge on the object nearest to the external charge, and a region of virtual positive charge on the part away from it ("virtual" since there is no movement of positive charges because these are the atomic nuclei, that are stationary conforming the matrix of the metal. See next paragraph). 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 electrons, 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.
According to the band theory, instead of electrons moving towards a positive nearby charge, what happens is a displacement of the electronic density (surrounding the matrix of atomic nuclei) towards that external charge.
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 electroscope after grounding is always equal in sign to the external inducing charge. On the other hand, an opposite permanent charge on an object can be achieved if it is grounded from the opposite edge to that which is bearing the external induction charge.
A remaining question is how large the induced charges are. The movement of charge is caused by the force exerted by the electric field 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.[3] 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 carriers (electrons), enough to cancel out extremely large external electric fields.
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.[3] External electric fields induce surface charges on metal objects that exactly cancel the field within. Since the field is the gradient of the electrostatic potential, another way of saying this is that in electrostatics, the potential (voltage) throughout a conductive object is constant.
A similar induction effect occurs in nonconductive (dielectric) objects, and is responsible for the attraction of small light nonconductive objects, like scraps of paper or Styrofoam, to static electric charges.[4][5][6][7] 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 is called polarization, and the polarized molecules are called dipoles. 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 is the principle of operation of a pith-ball electroscope.[8]