Electric charges in physics, the unit of measurement and their properties, what are conductors and insulators and what are they used for. Coulomb’s law.
The discovery that if you rub amber with a woolen cloth, the amber attracts elder shavings dates back to the 5th century. When a body acquires this property, it is said to become electrified , and the physical quantity that occurs is called electric charge .
By rubbing ebonite rods and glass rods with woolen cloths, we note that rods of the same material repel each other while rods of different materials attract each other: this observation implies that two different electrical states exist : that of ebonite, called negative , and that of glass, called positive .
We can deduce that electric charges of different signs attract each other, while electric charges of the same sign repel each other.
ELECTRIC CHARGE: UNIT OF MEASUREMENT
The unit of measurement of electric charge is the Coulomb (C). A Coulomb is the electric charge that, placed in a vacuum at a distance of one meter from an identical charge, repels it with a force of:
COULOMB’S LAW
There are other factors, besides the electric charge , that influence the attraction or repulsion between pairs of electrified objects: the mutual distance (the further two objects are, the less force is exerted between them), the medium placed between them (there are media, called insulators, that attenuate the interactions). Towards the end of the 19th century, the French physicist Coulomb, using an instrument called a torsion balance , carried out measurements on the force acting between electric charges . This instrument is made up of a wire f with a certain torsional elasticity (that is, if twisted it manifests a restoring force towards the rest configuration) to which a small metal sphere charged S is attached: by bringing a small sphere R, charged in the same way, close to this, the two repel each other and, as a result of this repulsion, the sphere S twists the wire until the elastic torsional reaction balances the force with which the two charges repel each other.
By appropriately calibrating the instrument, it is possible to establish Coulomb’s law : two point-like electric charges (i.e. of negligible size compared to their distance) attract or repel each other with a force directed along their connecting line; the magnitude of this force is directly proportional to the product of the charges and inversely proportional to the square of their distance, the constant of proportionality being dependent on the medium interposed between them.
PROPERTIES OF ELECTRIC CHARGES
Conductors and insulators . Why is a rubbed body able to exert repulsive or attractive actions? What happens at the microscopic level? In matter , at the subatomic level, there are particles with an electric charge (protons and electrons): the simplest model of an atom , the so-called planetary one, foresees that at the centre of the atom there is a nucleus made up of protons , with a positive charge, and neutrons , that is, electrically neutral particles.
These “nucleons” are held together by nuclear attractive forces that are much stronger than the electrical forces that prevent the protons from separating. Around the nucleus “rotate”, like planets around the sun , the electrons, electrical particles that are arranged in different layers.Each chemical element is electrically neutral and has a different atomic number, that is, a different number of electrons (equal to the number of protons).
This causes different elements to bond to others differently according to their chemical properties . Materials are divided into conductors , substances in which there are free electrons, that is, electrons with considerable mobility, and electrical insulators (or dielectrics), substances in which there are no free electrons.
HOW TO REVEAL THE CHARGE
Two simple devices for detecting charge are the electric pendulum and the electroscope .
- The electric pendulum consists of a ball of light material, covered with tin foil and suspended from a silk or cotton thread. If we bring a charged body close to the ball, we first observe an attraction and then, after the ball has come into contact with the body, a repulsion. In the experiment, a part of the charge of the body has passed to the ball. To know the sign of the charge of a given body, you need to know the charge of the ball, then bring the body close to it and see if the force exerted is repulsive or attractive. In this way, you know the sign of the unknown force.
- The electroscope consists of a metal rod ending at one end with a small sphere and at the other with two very light metal leaves; the whole is placed in a glass case.
By bringing an electrified body into contact with the sphere, the electric charge, through the conducting rod , is transmitted to the leaves which, due to the repulsive force between their charges, diverge. Even when the electrified body is removed from the end of the electroscope: the leaves remain open, which shows that a part of the charge of the electrified body remains on them. If we bring a positively charged electroscope close to a negatively charged body we will notice a decrease in the divergence of the leaves.