In electrostatics, one of the fundamental is Coulomb's law. It is used in physics to determine the force of interaction between two stationary point charges or the distance between them. This is a fundamental law of nature that does not depend on any other laws. Then the shape of the real body does not affect the magnitude of the forces. In this article, we will explain in simple terms Coulomb's law and its application in practice.
Content:
- Discovery history
- The wording
- Coulomb's formula for a dielectric medium
- How the forces are directed
- Application in practice
Discovery history
Sh.O. Pendant in 1785 for the first time experimentally proved the interactions described by the law. In his experiments, he used a special torsion balance. However, back in 1773 it was proved by Cavendish, using the example of a spherical capacitor, that there is no electric field inside the sphere. This indicated that the electrostatic forces change depending on the distance between the bodies. More precisely, the square of the distance. Then his research was not published. Historically, this discovery was named after Coulomb, the same name is also given to the value in which the charge is measured.
The wording
The definition of Coulomb's law reads: In a vacuum F interaction of two charged bodies is directly proportional to the product of their modules and inversely proportional to the square of the distance between them.
It sounds short, but it may not be clear to everyone. In simple words: The more charge the bodies have and the closer they are to each other, the greater the force.
And vice versa: If you increase the distance between the charges, the force will become less.
The formula for the Coulomb rule looks like this:
Designation of letters: q is the amount of charge, r is the distance between them, k is the coefficient, depends on the selected system of units.
The value of the charge q can be conditionally positive or conditionally negative. This division is very arbitrary. When bodies come into contact, it can be transmitted from one to another. Hence it follows that one and the same body can have a charge different in magnitude and sign. A point charge is a charge or a body whose dimensions are much smaller than the distance of possible interaction.
It should be borne in mind that the environment in which the charges are located affects the F interactions. Since it is almost equal in air and in vacuum, Coulomb's discovery is applicable only for these media, this is one of the conditions for the application of this type of formula. As already mentioned, in the SI system, the unit of measure for charge is Coulomb, abbreviated Cl. It characterizes the amount of electricity per unit of time. Derived from base SI units.
1 Cl = 1 A * 1 s
It should be noted that the dimension of 1 C is redundant. Due to the fact that the carriers repel each other, it is difficult to keep them in a small body, although the current of 1A itself is small if it flows in the conductor. For example, a current of 0.5 A flows in the same 100 W incandescent lamp, and more than 10 A in an electric heater. Such a force (1 C) is approximately equal to the one acting on a body with a mass of 1 ton from the side of the globe.
You might have noticed that the formula is practically the same as in the gravitational interaction, only if masses appear in Newtonian mechanics, then charges appear in electrostatics.
Coulomb's formula for a dielectric medium
The coefficient taking into account the values of the SI system is determined in N2* m2/Кл2. It is equal to:
In many textbooks, this coefficient can be found in the form of a fraction:
Here E0= 8.85 * 10-12 Cl2 / N * m2 is an electrical constant. For a dielectric, E is added - the dielectric constant of the medium, then Coulomb's law can be used to calculate the forces of interaction of charges for a vacuum and a medium.
Taking into account the influence of the dielectric, it has the form:
From here we see that the introduction of a dielectric between the bodies reduces the force F.
How the forces are directed
Charges interact with each other depending on their polarity - the same ones repel, and the opposite (opposite) ones attract.
By the way, this is the main difference from a similar law of gravitational interaction, where bodies are always attracted. The forces are directed along the line drawn between them, called the radius vector. In physics, denoted as r12 and as a radius vector from the first to the second charge and vice versa. The forces are directed from the center of the charge to the opposite charge along this line, if the charges are opposite, and in the opposite direction, if they are of the same name (two positive or two negative). In vector form:
The force applied to the first charge from the side of the second is denoted as F12. Then, in vector form, Coulomb's law looks like this:
To determine the force applied to the second charge, the notation F21 and R21.
If the body has a complex shape and it is large enough that at a given distance it cannot be considered a point charge, then it is divided into small sections and each section is considered as a point charge. After geometric addition of all the resulting vectors, the resulting force is obtained. Atoms and molecules interact with each other according to the same law.
Application in practice
Coulomb's works are very important in electrostatics; in practice, they are used in a number of inventions and devices. A striking example is a lightning rod. With its help, buildings and electrical installations are protected from thunderstorms, thereby preventing fire and equipment failure. When it rains with a thunderstorm, an induced charge of large magnitude appears on the ground, they are attracted towards the cloud. It turns out that a large electric field appears on the surface of the earth. Near the tip of the lightning rod, it has a large value, as a result of which a corona discharge is ignited from the tip (from the ground, through the lightning rod to the cloud). The charge from the earth is attracted to the opposite charge of the cloud, according to Coulomb's law. The air is ionized, and the electric field strength decreases near the end of the lightning rod. Thus, the charges do not accumulate on the building, in which case the probability of a lightning strike is small. If a blow to the building occurs, then through the lightning rod all the energy will go into the ground.
In serious scientific research, the greatest structure of the 21st century is used - the particle accelerator. In it, the electric field does the work to increase the energy of the particle. Considering these processes from the point of view of the effect on a point charge by a group of charges, then all the relations of the law turn out to be true.
Finally, we recommend watching the video, which provides a detailed explanation of the Coulomb's Law:
Useful on the topic:
- Joule-Lenz law
- The dependence of the resistance of the conductor on temperature
- Gimp rules
- Ohm's law in simple words
