Electric charge
The electric charge or electricity quantity is one of the fundamental variables of the physics. It is expressed in the international system of units in the unit Coulomb and expressed with the Latin word quantum ' formula derived characters or. The electric charge is associated with the elementary building blocks of matter property. Electrically charged particles are subject to the electromagnetic interaction, one of the four fundamental forces of physics. The forces between electric charges cause the cohesion of atoms, molecules and solids, as well as the electricity and they produce phenomena such as thunderstorms and the crackling of the hair combs.
The total charge of a physical system is equal to the sum of the charges of the parts. There are positive and negative charges; in a system is the sum of positive charges is equal to the sum of the negative charges, the net charge is zero and the system is electrically neutral. In a closed system, the total charge is fixed because of the charge conservation. Physical systems can not wear any amounts of charge, but only integer multiples of the elementary charge. Moving electric charge is electrical current which is in turn connected with the phenomenon of magnetism.
The electric charge is a special case of the more general concept of the physical load. Is confusion excluded, is usually just use the term " charge ".
- 3.1 overall charge
- 3.2 charge conservation
- 3.3 charging
- 3.4 Relativistic invariant
- 3.5 quantum character
- 3.6 Electric charge in quantum field theory
- 4.1 Electric charge as the foundation of the theory of electricity
- 4.2 charge density and electric field
- 4.3 charge and electric current
Everyday observations
An impressive series of electric charges by friction electricity are flashes of lightning. Air is normally an insulator, but at too great boots there is a breakdown. In flashes, there is a sudden charge balance between differently charged areas in the thunderstorm cell or - less frequently - between a region in the thundercloud and the ground. Small sparks, which are accompanied by a crackling, can also occur when donning and doffing of garments or when combing.
Man has no specific sense organ for electrical charge. He can perceive them only indirectly, when an electric current flows through load balancing through the body. The perception of a mild electric shock when already mentioned above off of garments is the experience when you walk across the carpet and then touching a doorknob. Electric current can cause dragging pain in the tooth nerve, when the electrochemically dissimilar metals mouth ( for example, aluminum foil and amalgam) are in contact and forms a local element. Similarly, the tingling of the tongue caused by current flow when you contact both terminals of a suitable battery with wet tongue.
Charged objects can also be felt by forces. When packaging material, for example polystyrene particles small, seemingly executes self movements inserted, in physical terms, the repulsion or attraction of the same or not equally charged particles behind.
Almost all observable in everyday physical phenomena go back to either the gravity or the interaction of electric charges. To explain the chemical processes in general and experiential properties of matter electromagnetic forces between the electron shells of atoms are essential - even if you often have to take into account quantum mechanical properties such as the spin for a full understanding.
History
Naming
Probably already BC Thales of Miletus in ancient Greece carried out experiments to 550, in which the hazards of electric charges forces were observed. ( - Electron spoken Greek ηλεκτρόν ) found attractive force on bird feathers or hair, after the amber was rubbed with a dry fur example, it was one of a piece of amber.
The court physician of Queen Elizabeth I, William Gilbert, continued the work of Petrus Peregrinus from the 13th century and found out that other substances may also be electrified by friction. He introduced in his 1600 published book De magnets, Magnetisque Corporibus, et de Magno magnets Tellure ( German about: On the Magnet, Magnetic body and the large magnet earth) to the Neo-Latin borrowed term " electrica " for the phenomena, which he in connection with the Bernstein discovered. Later this term was used as electron becomes a synonym for the support of the negative elementary charge, the 1891 by George Johnstone Stoney as designated and documented in 1897 by Joseph John Thomson electron ( also the grated Bernstein takes a negative charge ).
One or two types of cargo
Gilbert applies for his work as the founder of the theory of electricity. He distinguished between the first electrical and magnetic attraction. His explanation for the attraction of a rubbed amber to other body was that he took a befindliches in all bodies influenced by friction fluid, which austräte by the heat of friction and the body umgäbe like a cloud of vapor. Other substances are tightened when entering into this mist, similar to the attraction of a stone through the earth. In Gilbert's description sounds from today's perspective on some of the modern concept of the field. However, the differences are considerable, particularly because of the haze is made up spilled fluid.
Otto von Guericke employed in his later works with static electricity of its results little is preserved. He invented a simple electrostatic generator in 1672, with which he could observe a number of phenomena, such as the induction, the line of electric charge, which luminous efficiency ( electroluminescence) and the fact that repel two homonymous electrified body. Until then, it was only known from the attraction effect of electricity, Gilbert's explanation of a fluid is no longer enough.
Charles du Fay recognized in 1733 for tests of frictional electricity, that the two species could neutralize each other by electricity. He described the species as electricity glass electricity (French Electricité vitreuse ) and resin Electricity ( French Electricité résineuse ). The glass electricity equivalent in today's notation of the positive charge. Jean -Antoine Nollet developed from these experiments, the so-called " two- fluid theory." Thus surrounded, the two varieties electricity as a " two-fluid " ( the effluvium and the Affluvium ) the electrified body. This manner of speaking impressed thinking about the nature of electricity in the 18th century and is still alive today in the " two types of cargo " on.
In Benjamin Franklin - the topic of electrical phenomena - authored book Experiments and Observations on Electricity this coined the term charge ( engl. charge). Previously had to be spoken of " bodies that have been placed in an electric state ," Franklin led a view as in a loaded and unloaded account where entered by friction redistributions. So Franklin spoke of " a cargo " ( a fluid ), which changes only their residence and thus causes (positive or negative ) charge. William Watson came at the same time to a comparable assessment. Franklin could not explain why two equally emptied of charge bodies repel each other, only Franz Maria Aepinus corrected this deficiency with his point of view. In today's speech he saw the particulate matter while removing the cargo as it were in an ionized state.
The adoption of Franklin that the electricity of the glass existent and the resin Electricity is a shortage and that electricity always flows at the touch of charged and uncharged bodies in only one direction, it suggested that - in today's terminology - always the positive charges moving. Probably Franklin was led to this assumption, by the way the observable luminous phenomena in his experiments with charged metal tips.
This latest theory of electricity as " a fluid " of the idea of conservation of charge has been made to prevail. , The charges are not generated by the friction, but only separated from each other. Since the direction of the force between two charges with the help of the two- fluid model can be described by the signs of the charges involved simple, Charles Augustin de Coulomb took the dualistic model of " two fluids " and put the existence of two types of cargo basis. From today's point of view can be obtained with both models the same result.
In German-speaking notation of Franklin was probably spread primarily by Leonhard Euler and Georg Christoph Lichtenberg.
Quantitative experiments
Robert Boyle made in 1675 stated that electric attraction or repulsion takes place even through a vacuum, Francis Hauksbee deepened these studies using electrical luminous phenomena in a vacuum. Stephen Gray announced 1729 materials into electrically conductive and electrically insulating and demonstrated that the human body could conduct electricity.
In the last quarter of the 18th century, the focus of the now shifted (since by the Leyden jar, an impressive experimental agent had been found) very popular discussion of the theory of electricity towards quantitative studies on electrostatics. Specific research contributions were provided by Joseph Priestley and Charles Augustin de Coulomb. Coulomb published in 1785, the Coulomb law, which states that the amount of the force between two charged spheres is proportional to the product of the two quantities of charge and inversely proportional to the square of the distance of the ball centers. The force is attractive or repulsive depending on the sign of the charges in the direction of the line connecting the center points.
The 1832 formulated by Michael Faraday Faraday's laws provide a connection between electric charge flowed and metabolic rate ( amount of substance deposited on the electrodes ) ago during the electrolysis. In 1833 held at the Royal Society lecture Faraday demonstrated that the hitherto as " different electricity" be summed up "static" (or "ordinary" ), the "atmospheric ", the " physiological " (or " animal ") which " voltaic " (or " contact electricity" ), and in truth " thermoelectricity " only different aspects of the one - represented physical principle - of him "Magnet electricity" designated. Thus, it was also clear that the electric charge is the basic property of matter of all these phenomena. An important contribution of Michael Faraday to the theory of electricity was the systematic introduction of the field concept in the description of electric and magnetic phenomena.
In 1873, Frederick Guthrie discovered that a positively charged electroscope is discharged when they brought a grounded, glowing piece of metal in the vicinity. When a negatively charged electroscope nothing happens, resulting followed that glowing metal only leave negative charge and can this electric current to flow in only one direction. Thomas Edison has rediscovered this phenomenon in 1880 in experiments with incandescent and reported 1883 based thereon application for a patent. The " thermionic effect" is called by Edison and Richardson, who was given for the explanation of the Nobel Prize in 1928, Edison Richardson effect.
In 1897, Joseph John Thomson demonstrated that cathode rays consist of electrons. With a greatly improved vacuum he could determine for them the ratio of charge to mass. Thomson suggested that the electrons were already present in the atoms of the cathode, and presented in 1903 for the first time on an atomic model that the atoms ascribes an internal structure.
The discrete nature of electric charge, which was predicted by Faraday during his electrolysis experiments in the 19th century, could be confirmed in 1910 by Robert Andrews Millikan in the so-called Millikan experiment. In this trial, evidence was led that charged oil droplets are always loaded with an integer multiple of the elementary charge, he also provided a useful value for the size of the elementary charge.
Properties of the electric charge
Total charge
The electric charge can take positive or negative values. One often speaks of two kinds of electric charges. For example, an electron or a muon charge -1 e a positron or a proton charge 1 e
A particle and its antiparticle have exactly the equal and opposite amount of charge. For example, bears the Antiproton, the antiparticle of the proton, the charge -1 e
The absolute charge of a body or a material amount is the sum of all elementary charges contained. This is also the designations total charge, net charge or excess charge can be used. Since the summing sign of the charge is taken into account, the number of existing charges can be significantly larger than the total charge. For example, wear both the Δ particle Li2 ion and Fe 2 ion, the total charge " double positive ". The Δ particle has its charge, since it is made up of three up quarks composed with each charge. When Li2 ion, the total charge of the three positive protons in the nucleus and results in the negative electron in its electron shell, the Fe2 ion 50 charge carriers are involved, 26 protons and 24 electrons. The absolute charge and the total amount of charge present are thus not usually the same.
As a particle is electrically neutral on the one hand refers to carrying no load (for example, a neutrino ). On the other hand, a body is called a neutral carrying the same number of positive and negative elementary charges (such as an iron atom to 26 protons and 26 electrons). Since electric fields affect all existing charges, not only on excess charges, in the above example, the neutral iron atom is quite influenced by the electromagnetic interaction. Occur here as effects on the polarizability.
From a charge separation occurs when in certain regions of space charges of one sign predominate, there the absolute charge is therefore not zero. In charge separation within a body or component to specify the total charge is not sufficient. For example, can be both the charged as the uncharged capacitor zero total charge. During the uncharged capacitor is then electrically neutral on each plate, wear the charged capacitor plates both opposing (but equally numerous ) excess charges - in this case causes the charge separation electric field, which stores energy.
Conservation of charge
Under charge conservation is defined as the phenomenon that in any closed system, the existing amount of electric charge remains constant. This phenomenon has consequences: If matter formed from electro -magnetic radiation or photons, this must be done so that no charge is generated. The result is therefore in the pairing, for example, at the same time an electron and its antiparticle, the positron. This is the total charge of zero generated, the amount of charge is maintained. The same is true in reversing this process, the annihilation of a particle-antiparticle pair, in which the destroyed total charge is also zero.
As with any basic physical conservation law is based the principle of conservation of electric charge on observations and experiments. So far, all this respect relevant experiments have confirmed the electric charge conservation - some with very high accuracy. In formal theoretical description of electrodynamics the charge conservation is expressed by the continuity equation, which is a consequence of the Maxwell's equations ( see section charge and electric current ). A more abstract property of electrodynamics is its invariance (often also called symmetry) under gauge transformations of the quantum electrodynamics is obtained as the gauge theory. After Noetherian theorem, the electric charge is linked as a conserved quantity with the invariance of electrodynamics under gauge transformations as well.
In apparent contradiction to the charge conservation is the speech of a charge generating or charging. But that is a local accumulation of charges of one sign meant, so actually a charge separation ( and no production).
Charging
To charge (in terms of excess charge ) of a previously neutral body it must absorb or emit charge carriers. But even with an uneven charge distribution in a total neutral body is called " Charging." This is about due to an applied electric field or by motions on a molecular scale. A polarized material, the charge is bound before, " floating " charge carriers are in the induction moved in a conductor.
A well-known from everyday life mechanism for the separation of charges is the friction. Example, if you rub a balloon on a sweater, then electrons are transferred from one material to the other, so that the electron and the residual ion core are separated. Such static electricity is a special case of contact electricity. The Van de Graaff generator uses both touch electricity and induction.
In batteries and accumulators chemical reactions are utilized to redistribute a large amount of charge carriers ( electrons or ions). As with the capacitor, the total charge is zero. Unlike this, however, the voltage rises not nearly linearly, but remains approximately constant. Therefore, the capacitance is given as the energy storage capacitor in Farads ( Coulomb per volt ), while the capacity of a battery is characterized as a charge amount - in ampere-hours, where 1 Ah equal to 3600 coulomb applies.
Charge separation can also be caused by electromagnetic waves, eg light, caused: Leaving light of sufficiently high frequency to a metal surface meet and placed in a vacuum a second metal plate in the area, creates a charge difference between them, because by the light electrons from the first plate are released, the move part of the second plate (outer photoelectric effect).
Relativistic invariant
The charge Q of the body is not only conserved, but also irrespective of its speed. That is, the electric charge is a relativistic invariant, the overall charge of an article is not changed by the length of contraction. This property is the load with the mass of a system together invariant, but differs, for example, of the energy. From this example it can be seen that relativistic invariance is not self-evident for conserved quantities, but is itself an additional property.
On computational level can the relativistic invariance of the charge Q understand by it perceives as a volume integral over the charge density
Under a Lorentz transform of the charge density transforms as the time component of a four-vector, that undergoes a change similar to the time dilation; however, the volume element undergoes a Lorentz contraction. These two effects cancel each other exactly, so that the charge itself remains unchanged.
Interference experiments ( for example, by Claus Jönsson ) with electrons of various velocities show directly that their charge is independent of the speed. Also otherwise, the charge of a solid takes a change in temperature change, because the increased speed of its components due to the higher thermal energy of the electrons but on average receive a much greater rate than the more massive positive nuclei.
Quantum character
Electrically charged matter can not wear any amounts of charge. The charges of all known elementary particles have been measured experimentally with the result that all leptons and their antiparticles carry always integer multiples of the elementary charge e. From the building blocks of atoms carry proton and electron charge respectively, the neutron no (electrical) charge. The currently most accurate value of the constants of nature is e = 1.602 176 565 (35 ) · 10-19 C. Although the quarks carry charge or not, but quarks occur never free (see confinement), but only in bound states, the hadrons, which turn carry always integer multiples of the elementary charge. Thus all the particles freely occurring carry integer multiples of the elementary charge.
This is theoretically grounded in the electroweak model by the electric charge on the weak hypercharge and weak isospin is returned. Why, however, the weak hypercharge and weak isospin of only assume certain values can not be explained by the model. Therefore, so far is the "cause" of the observed quantization of charge unclear; it belongs, in the opinion of well-known scientists of the biggest mysteries of physics. According to Paul Dirac's consideration to a magnetic monopole, the existence of such a particle would casually return the charge quantization to the quantization of angular momentum.
Outside of atomic structures, it is usually permissible to look at the charge as a continuous variable. Even a minute current from a nanoamperes is a directed charge transport of about six billion electrons per second, passing a cross-sectional area of the conduit. This means that individual elementary charges not recognizable in most aspects of electrical engineering. Beyond the countability the cargo can be viewed as a continuum, and a differential quotient dQ / dt is a valid mathematical construction.
Electric charge in quantum field theory
In the framework of quantum field theory, the elementary charge is the coupling constant of the electromagnetic interaction. From the point of view of the renormalization group, however, the coupling constant of quantum field theories not constants, but depend on the energy scale. The elementary charge is dependent on the energy scale, being larger with increasing energy. This means that at very high energies the charged particles stronger interaction. As a result, are more likely in high-energy particle reactions by the electromagnetic interaction. The probability that, for example, the collision of two electrons, an electron -positron pair is formed increases with the energy of the collision.
The electroweak model states that the electromagnetism is only an effective interaction at low energies, which remains after a spontaneous symmetry breaking via the Higgs mechanism. At higher energies, thus pits two interactions at the point of electromagnetism and the electric charge is replaced by the weak hypercharge and weak isospin. Accordingly, the electrical charge can be considered composed in a sense, as these two types of charge.
The symmetry of positive and negative charge is for the quantum field theory of meaning. The transformation which inverts the sign of all the electric charges in a system of particles, C is mentioned. Other important transformations in the following are P, the point of reflection of the room at the zero point, and T is the reversal of the direction of time. The CPT theorem, a fundamental statement of all quantum field theories, stating that scattering processes are exactly alike when applying all three of these transformations on the system. This does not apply to the individual transformations. There is parity-violating processes which occur differently if only P is applied, and of CP violation is when a process runs differently than its space - charge and mirrored counterpart.
Conjunction with other sizes
Electric charge as the foundation of the theory of electricity
Electrically charged bodies generate electric fields and are themselves affected by such fields. Compared to the gravitational force between the charge carriers - - is very large between the charges, the Coulomb force whose intensity works. It attracts between a positive and a negative charge between two like charges repelling. In this case, the Coulomb's law, the distance of the charge is important. With static electric charges, charge distributions and the electric fields of charged bodies dealt electrostatics.
When charging of bodies have to expend energy to separate opposite charges attract each other. This energy is after the charge separation in front of electric field energy. The voltage indicates how much work or energy is needed to move an object with a certain electric charge in an electric field.
When moving electric charges is called electric current. The movement of electric charges leads to magnetic forces and the electromagnetic fields; this is described by Maxwell's equations and special relativity. By moving charges in general shape and in busy electrodynamics. The interaction of charged particles, which is done with photons, in turn, is the subject of quantum electrodynamics.
Charge density, electric field
This description of electrical interactions between elementary particles is practically feasible only for systems with few particles. For many considerations, however, it is quite sufficient to work with spatially and temporally averaged suitable sizes, because no notice details for this macroscopic point of view are negligible. In this sense, the equations of electrodynamics were established, without needing to know the submicroscopic structure of matter. Through the process of averaging the basic equations of electrodynamics are not formally changed. Whether or exact averaged equations are meant arises from the context.
The description of the charge distribution is done with the space charge density or the surface charge density. Based on the Coulomb field of a point charge is obtained for the field produced by the space charge electric field in the vacuum, the Gaussian law:
Here, the electric field constant. This clearly means the Gaussian law that electric field lines start from positive charges ( sources) and end at negative charges (sinks).
In relativity theory the electric field is combined with the magnetic field in the field strength tensor. The charge density ( the speed of light multiplied ) together with the electric current density a four-vector.
Charge and electric current
When an electric current flows, then by a surface ( for example, the cross-sectional area of an electrical conductor ) through flowing charge amount - based on the amount of time required for this - referred to as electric current.
Simply stated corresponds to the connection of electrical power and charge the statement:
The amount of charge that was moved in the time between and follows from the integration of both sides:
For a temporally constant current, the relationship between charge and current simplified to:
Based on this equation is also particularly easy to realize that the unit can be used as Coulomb represent. Through this relationship the base units ampere and second, the Coulomb in the International System of Units is set.
Because of conservation of charge, the charge quantity changes in a particular region of space only exactly to the extent that such charges into or flow out in this region of space. The charge conservation thus corresponds to the continuity equation. The observed charge is equal to the volume integral of the charge density within the spatial area. The change over time of this charge is equal to the surface integral of the current density over the surface of the closed volume (, read " the edge of" ), and is equal to the electric current. The current direction out of the volume is defined as positive:
In other notation, the continuity equation of the statement is correct:
Here, the charge density and the current density.
Measurement of the electric charge
The amount of charge of 1 coulomb corresponds to about 6.24 · 1018 elementary charges. For the determination of total charges, the charge carriers can be counted therefore usually not easy.
Indirectly, the from or accrued amount of charge can be determined by measuring the current intensity: When a current flows constant strength during the time he transported the cargo. Generally, the charge which has passed into or through a body, the integral of the electric current over time. If the discharge time short compared with the period of oscillation of a ballistic galvanometer, so can the charge be read directly as the amplitude of the oscillation initiated.
Basically, you can also determine the value of a charge by the fact that the amount of force is measured in an electric field of known field strength on a charged test body. The definition of the field strength provides the relationship
This method is subject to serious limitations: The test piece must be small, portable and highly electrically isolated. Its cargo must not significantly affect the electric field, but this is difficult to verify. Therefore, the charge should be low - but then the force is difficult to measure.
The disadvantages listed has another method does not, it succeeds even very large loads. It is based on the relationship between the capacitance of a capacitor and the electric voltage:
With the measured charge, a capacitor of known capacitance is charged and the measured voltage. This measurement must be made, however, high impedance, that is, so that they only negligible takes the capacitor little bit of stored charge. This is done with an electroscope or better with an impedance converter. However, in this method, the capacity of the charge source must be known, since a portion of the charge remains there. The non-voltage measurement with an integrator (without input resistance also referred to as charge amplifiers ) avoids this problem.