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. 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. 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.
History
The phenomenon of induction was well-known before it was understood. British scientist Isaac Newton gave a report on it to the Royal Society in 1675. Detailed study by British scientist John Canton in 1753 and Swedish professor Johan Carl Wilcke in 1762 lead Canton, Benjamin Franklin and Franz Aepinus to develop explanations in terms of the then prevailing theories of electricity.
Explanation
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. 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. The two rules of induction are:
- 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
thumb|upright=1.5|Surface charges induced in metal objects by a nearby charge. The [[electrostatic field (lines with arrows) of a nearby positive charge <span style="color:red;">(+)</span> causes the mobile charges in metal objects to separate. Negative charges <span style="color:blue;">(blue)</span> are attracted and move to the surface of the object facing the external charge. Positive charges <span style="color:red;">(red)</span> 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. 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. (see picture of cat, above), as well as static cling in clothes.
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,
See also
Stephen Gray