Electric field

The electric field is defined as the electric force per unit charge; it surrounds an electric charge, and exerts a force on other charges in the field, attracting or repelling them. The direction of the field is taken to be the direction of the force it would exert on a positive test charge. The electric field is radially outward from a positive charge and radially in toward a negative point charge. Electric fields are created by electric charges, or by time-varying magnetic fields. On an atomic scale, the electric field is responsible for the attractive force between the atomic nucleus and electrons that hold atoms together, and the forces between atoms that cause chemical bonding.

Introduced by Michael Faraday, the electric field propagates at the speed of light and exerts a force on every electrically charged object. In the international system of units, it is measured in newtons on coulombs (N/C), or in volts on meters (V/m). If it is generated only by the stationary space charge distribution, the electric field is called electrostatic and is conservative.

Experimental evidence

Experimentally it is verified the attraction or repulsion between bodies with electric charge, corresponding to two states of electrization of matter. Charge is defined positive when there is a lack of electrons in the object, negative in the presence of an excess. Bodies electrified both positively or both negatively repel each other, while oppositely electrified bodies attract each other.

To measure the electrization of a body is used an instrument called leaf electroscope, consisting of a glass ampoule in which is inserted a metal rod which, inside the ampoule, has two very thin metal tabs, called leaves, while outside it can be put in contact with a charged body. Putting in contact with the rod a charged body, the tongues move away from each other in proportion to the electrization of the body that has been put in contact.

From these experimental evidences, in the second half of the eighteenth century Charles Augustin de Coulomb formulated Coulomb’s law, which quantifies the attractive or repulsive electric force that two electrically charged point bodies exchange at a distance. Starting from this law it can be said that an electrically charged body produces in the surrounding space a field such that, if an electric charge is introduced, it is affected by a force, called Coulomb force, directly proportional to the product of the two charges and inversely proportional to the square of their distance.

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