electrostatic induction

electrostatic induction

Modification in the distribution of electric charge on one material under the influence of an electric charge on a nearby object. It occurs whenever any object is placed in an electric field. When a negatively charged object is brought near a neutral object, it induces a positive charge on the near side of the object and a negative charge on the far side. If the negative side of the original object is momentarily grounded, the negative charge may escape, so that the object becomes positively charged by induction.

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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 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'.

Explanation

A normal 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 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. If the external charge is negative, the polarity of the charged regions will be reversed. Since this is just a redistribution of the charges, 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 picture is that only the negative charges in matter, 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.

Charging an object by induction

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. This 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.

Induction in dielectric objects

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 plastic foam, to static electric charges. In nonconductors, the electrons are bound to atoms 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 foam. This can be demonstrated with a pith-ball electroscope.

Footnotes

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