Electric charge

Type of interaction (attractive or repulsive) between loads of equal and different nature.

The electrical charge is an intrinsic physical property of some subatomic particles that is manifested by forces of attraction and repulsion between them through electromagnetic fields. Electrically charged matter is influenced by electromagnetic fields, being, in turn, a generator of them. The so-called electromagnetic interaction between charge and electric field is one of the four fundamental interactions of physics. From the point of view of the normal model, electric charge is a measure of the ability of a particle to exchange photons.

Electric charge is discrete in nature, a phenomenon experimentally demonstrated by Robert Millikan. For historical reasons, electrons were assigned a negative charge: –1, also expressed as –e. Protons have a positive charge: +1 or +e. Quarks are assigned fractional charge: ±1/3 or ±2/3, although they have not been observed free in nature.

Units of measure

In the International System of Units, the unit of electrical charge is called a coulomb or coulomb (symbol C). It is defined as the amount of charge that passes through the cross section of an electrical conductor in one second, when the electrical current is one ampere. Since the 26th General Conference on Weights and Measures in the International System of Units, the elementary charge is defined as 1.602 176 634 × 10^-19 C, without uncertainty. Since the charge of the electron is of the same magnitude as the proton, but negative, it takes 6.241 509 074 460 763 × 10¹⁸ electrons to gather one coulomb of negative charge.

In the Cegesimal System of Units (CGS) the electric charge of the electron is:

• e = 4.8×10^-10 Esu (electrostatic unit, that is, electrostatic unit of charge) = 4.8×10^-10 stat

History

Experiment of the comet of Benjamin Franklin.

Since Ancient Greece it has been known that when rubbing amber with skin, it acquires the property of attracting light bodies such as pieces of straw and small feathers. Its discovery is attributed to the Greek philosopher Thales of Miletus (ca. 639-547 BC), who lived about 2,500 years ago.

In 1600, the English physician William Gilbert observed that some materials behave like amber when rubbed and that the attraction they exert manifests itself on any body, even if it is not light. Since the Greek name for amber is ἤλεκτρον (ēlektron), Gilbert began to use the term electrical to refer to any material that behaved like that, which gave rise to the terms electricity and electrical charge. In addition, in Gilbert's studies the differentiation of electrical and magnetic phenomena can be found.

The discovery of the attraction and repulsion of elements by connecting them with electrical materials is attributed to Stephen Gray. The first to propose the existence of two types of charge is Charles du Fay, although it was Benjamin Franklin who, studying these phenomena, discovered how the electricity of bodies, after being rubbed, was distributed in certain places where there was more attraction; that's why he named them (+) and (-).

However, it was not until the middle of the 19th century when these observations were formally raised, thanks to the experiments on electrolysis carried out by Michael Faraday, around 1833, which allowed him to discover the relationship between electricity and matter; accompanied by the complete description of electromagnetic phenomena by James Clerk Maxwell.

Later, the work of Joseph John Thomson when discovering the electron and Robert Millikan when measuring its charge, were of great help to understand the discrete nature of the charge.

Nature of the cargo

Electric charge is an intrinsic property of matter that comes in two types. These are now named after what Benjamin Franklin called them: positive and negative charges. When charges of the same type meet, they repel and when they are different, they attract. With the advent of relativistic quantum theory, it was possible to formally demonstrate that particles, in addition to presenting an electric charge (whether null or not), present an intrinsic magnetic moment, called spin, which arises as a consequence of apply the theory of special relativity to quantum mechanics.

Elementary electric charge

Current research in physics suggests that electric charge is a quantized property. The most elementary unit of charge was found to be the charge that the electron has and is known as elementary charge, a charge to which the exact value of 1.602 176 634 × 10 has been assigned at the 26th General Conference on Weights and Measures^-19 C. The value of the electric charge of a body, represented as q or Q, is measured according to the number of electrons it possesses in excess or deficiency.

This property is known as Quantization of the load and the fundamental value corresponds to the electrical load value of the electron and to which it is represented as e. Any load q that exists physically, can be written as N× × e{displaystyle Ntimes e} being N an integer, positive or negative.

By convention, the charge of the electron is represented as -e, for the proton +e and for the neutron, 0. Particle physics postulates that the charge of quarks, particles that make up protons and neutrons, take fractional values of this elementary charge. However, free quarks have never been observed, and the value of their charge as a whole, in the case of the proton adds up to +e and in the case of the neutron adds up to 0.

Although we do not have a sufficiently complete explanation of why charge is a quantized quantity, which can only appear in multiples of the elementary charge, various ideas have been proposed:

• Paul Dirac showed that if there is a magnetic monopole, the electric charge must be quantified.
• In the context of Kaluza-Klein theory, Oskar Klein found that if the electromagnetic field was interpreted as a side effect of the curvature of a time of topology M× × S1{displaystyle {mathcal {M}}times S^{1}}, then the compassion of S1{displaystyle S^{1},} It would imply that the linear moment according to the fifth dimension would be quantified and hence the quantity of the load was deducted.

A coulomb corresponds to the charge of 6,241 509 074 × 10¹⁸ electrons.

The first determinations of the charge of the electron were carried out between 1910 and 1917 by Robert Andrews Millikan.

e=1C6,241509074× × 1018=1,602176634× × 10− − 19C{displaystyle e={frac {1 mathrm {C}{6,2415074times 10^{18}}}}}=1,602176634times 10^{-19}mathrm {C} }

Since the coulomb may not be manageable in some applications, being too large, its submultiples are also used:

1 milliculombie = 1C1,000=1mC{displaystyle {frac {1mathrm {C}{1}}=1 mathrm {mC} }

1 microculombie = 1C1,000,000=1μ μ C{displaystyle {frac {1mathrm {C}{1,000.000}}}=1mu mathrm {C} }

Sometimes the CGS system is also used, whose unit of electrical charge is the Franklin (Fr). The value of the elementary charge is then approximately 4.803×10^–10 Fr.

Load properties

Principle of conservation of charge

In agreement with the experimental results, the principle of conservation of charge establishes that there is neither destruction nor net creation of electric charge, and affirms that in any electromagnetic process the total charge of an isolated system is preserved.

In an electrification process, the total number of protons and electrons is not altered, there is only a separation of the electrical charges. Therefore, there is no destruction or creation of electric charge, that is, the total charge is conserved. Electric charges may appear where there were none before, but they will always do so in such a way that the total charge of the system remains constant. In addition, this conservation is local, it occurs in any region of space, however small it may be.

Like other conservation laws, the maintenance of the electric charge is associated with a symmetry of the lagrangian, called in quantum invariance gauge physics. Thus by Noether's theorem to every symmetry of the lagrangian associated with a uniparametric group of transformations that leave the invariant lagrangian corresponds to a preserved magnitude. The maintenance of the load implies, like the conservation of the mass, that at each point of the space is satisfied an equation of continuity that relates the derivative of the density of electrical load to the divergence of the vector electric current density, this equation states that the net change in the density of load ρ ρ {displaystyle rho } within a prefixed volume V{displaystyle V} is equal to the integral of electric current density J{displaystyle J} on the surface S{displaystyle S} that locks the volume, which in turn is equal to the intensity of electric current I{displaystyle I}:

− − ▪ ▪ ▪ ▪ t∫ ∫ Vρ ρ dV=∫ ∫ SJ⋅ ⋅ dS=I=− − ▪ ▪ Q▪ ▪ t{displaystyle}{frac {partial }{partial t}}int _{Vrho ,dV=int _{S}mathbf {J} cdot mathbf {dS} = l=-{frac {partial Q}{partial t}}}}

Relativistic invariant

Another property of electric charge is that it is a relativistic invariant. This means that all observers, regardless of their state of motion and speed, will always be able to measure the same amount of charge. Thus, unlike space, time, energy or linear momentum, when a body or particle moves at speeds comparable to the speed of light, the value of its charge will not change.

Electric charge density

The amount of electric charge per unit length, area, or volume found on a line, surface, or region of space, respectively, is called electric charge density. Therefore it is distinguished in these three types of charge density. It would be represented with the Greek letters lambda (λ), for linear charge density, sigma (σ), for surface charge density and ro (ρ), for density volumetric load.

There can be both positive and negative charge densities. It should not be confused with the density of charge carriers.

Although electric charges are quantized with q and, therefore, multiples of an elementary charge, sometimes the electric charges in a body are so close to each other that they can be assumed to be uniformly distributed over the body of which they are a part. The main characteristic of these bodies is that they can be studied as if they were continuous, which makes their treatment easier, without losing generality. Three types of electric charge density are distinguished: linear, superficial and volumetric.

Linear charge density

Used on linear bodies such as threads.

λ λ =QL{displaystyle lambda ={frac {Q}{L}}}}

Where Q{displaystyle Q} it's the load locked in the body and L{displaystyle L} It's the length. The International Unit System (IS) is measured in C/m (several per metre).

Surface Charge Density

Used for surfaces, for example a thin metal sheet such as aluminum foil.

σ σ =QS{displaystyle sigma ={frac {Q}{S}}}

Where Q{displaystyle Q} it's the load locked in the body and S{displaystyle S} It's the surface. In the SI is measured in C/m² (keys per square meter).

Volumetric charge density

It is used for bodies that have volume.

ρ ρ =QV{displaystyle rho ={frac {Q}{V}}}}

Where Q{displaystyle Q} it's the load locked in the body and V{displaystyle V} volume. In the SI is measured in C/m³ (keys per cubic meter).

Ways to change the electric charge of bodies

Electrification is the effect of gaining or losing electrical charges, normally electrons, produced by an electrically neutral body. The types of electrification are the following:

1. Contact e-mail: When we put a body loaded in contact with a driver, a transfer of charge from one body to the other can be given and the driver is loaded, positively if he "gave electrons" or negatively if he "winned them".
2. Friction e-electrization: When we rub an insulation with some kind of materials, some electrons are transferred from the insulation to the other material or vice versa, so when both bodies are separated they are left with opposite loads.
3. Induction charge: If we approach a body charged negatively to an isolated driver, the repulsion force between the charged body and the electrons of valence on the surface of the conductor makes these move to the farthest part of the driver to the charged body, remaining the nearest region with a positive load, which is noticed by having an attraction between the charged body and this part of the driver. However, the driver's net load remains zero (neutral).
4. Charge for the photoelectric effect: It happens when electrons are released on the surface of a conductor when irradiated by light or other electromagnetic radiation.
5. Charge by electrolysis: Chemical decomposition of a substance, produced by the passage of a continuous electric current.
6. Charge for thermoelectric effect: It means producing electricity by heat action.