I didn’t learn anything about electricity in junior high school physics.

Electricity

The word "electricity" in the West comes from the Greek word amber, and in China it comes from the phenomenon of thunder and lightning. Since the mid-18th century, research on electricity has gradually flourished. Each of its major discoveries has led to extensive practical research, thereby promoting the rapid development of science and technology.

Today, human life, scientific and technological activities, and material production activities are all inseparable from electricity. With the development of science and technology, certain research contents with specialized knowledge gradually become independent and form specialized disciplines, such as electronics, electrical engineering, etc. Electricity, also known as electromagnetism, is an important basic subject in physics.

A brief history of the development of electricity

Records of electricity can be traced back to the 6th century BC. As early as 585 BC, the Greek philosopher Thales had recorded that amber rubbed with a wooden block could attract small objects such as grass. Later, someone discovered that rubbed coal jade also had the ability to attract small objects. In the next 2000 years, these phenomena were regarded as properties of matter, like a magnet attracting iron, and no other major discoveries were made.

In China, at the end of the Western Han Dynasty, there were records of "tortoiseshell (tortoiseshell) absorbing small objects"; in the Jin Dynasty, there were further records of discharge phenomena caused by frictional electrification: "Modern people When you comb your hair or untie your clothes, there will be light when you comb and untie the knots, and there will also be a clicking sound."

In 1600, the British physicist Gibbert discovered that not only amber and coal jade can attract light and small objects after friction, but also a considerable number of substances have the property of attracting light and small objects after friction. He Note that these substances do not have the properties of a magnet to guide north and south after friction. To indicate the difference from magnetism, he called this property "electric" using the Greek phonetic pronunciation of amber. During the experiment, Gibbert made the first electroscope, which was a thin metal rod with a fixed center that could rotate. When it was close to rubbed amber, the thin metal rod could rotate and point toward the amber.

Around 1660, Gehrig of Magdeburg invented the first friction generator. He made a rotatable sphere shaped like a globe out of sulfur, and turned the sphere by rubbing it with dry palms to obtain electricity. Gallick's friction generator was continuously improved and played an important role in electrostatic experimental research. It was not replaced until the invention of the induction generator by Holtz and Tippler in the 19th century.

The research on electricity developed rapidly in the 18th century. In 1729, Gray in England discovered the difference between conductors and insulators when studying whether the electrical effect of amber could be transferred to other objects: metals can conduct electricity, silk does not, and he made the human body electrified for the first time. Gray's experiment attracted the attention of French Diffey. In 1733, Diffie discovered that insulated metals could also be triboelectrically charged, so he concluded that all objects could be triboelectrically charged. He called the electricity produced on glass "glassy", and the electricity produced on amber was the same as that produced on resin, called "resinic". He found that objects with the same electric charge repel each other; objects with different electric charges attract each other.

In 1745, Muschenbruck of Leiden, the Netherlands, invented the Leyden jar that could preserve electricity. The invention of the Leyden jar provided conditions for further research on electricity, and it played an important role in the dissemination of electrical knowledge.

At about the same time, Franklin in the United States did a lot of meaningful work, enriching people's understanding of electricity. In 1747, he proposed based on experiments: Under normal conditions, electricity is an element that exists in a certain amount in all substances; electricity, like fluids, can be transferred from one object to another through friction, but it cannot create electricity. ; The total amount of electricity in any isolated object is constant, which is commonly known as the law of conservation of charge. He called the excess part of the electricity gained by an object during friction as being positively charged, and the insufficient part of an object losing electricity as being negatively charged.

Strictly speaking, this unitary fluid theory of electricity does not seem correct today, but the terms he used for positive and negative electricity are still used today, and he also observed that the tips of conductors are more Easy to discharge etc. As early as 1749, he noticed that thunder and lightning had many similarities with discharges. In 1752, he conducted a lightning strike experiment by placing a kite into the clouds during a thunderstorm, proving that thunder and lightning were discharge phenomena. The most fortunate thing in this experiment was that Franklin was not electrocuted to death, because this was a dangerous experiment. Later, someone was electrocuted to death when repeating this experiment. Franklin also suggested using lightning rods to protect buildings from lightning strikes, first implemented by DeVos in 1745, which was probably the first practical application of electricity.

Quantitative research on charge interactions began in the late 18th century. In 1776, Priestley discovered that there was no charge on the inner surface of a charged metal container, and speculated that electricity and gravity have similar laws. In 1769, Robinson conducted an experiment to balance electric force and gravity on a small ball, and for the first time directly measured the interaction force between two charges being inversely proportional to the square of the distance. In 1773, Cavendish deduced that electric force is inversely proportional to the square of distance. His experiment was the prototype of the modern accurate verification of the electric law.

In 1785, Coulomb designed an ingenious torsion balance experiment and directly measured that the interaction force between two stationary point charges is inversely proportional to the square of the distance between them and proportional to the product of their electric charges. . Coulomb's experiment was recognized around the world, and the study of electricity began to enter the ranks of science.

In 1811, Poisson applied the potential theory developed by Laplace based on the law of universal gravitation in mechanics to electrostatics and developed the analytical theory of electrostatics.

Another important development in electricity in the late 18th century was the invention of the battery by the Italian physicist Volta. Before that, electrical experiments could only be carried out with Leyden jars of friction generators, and they could only Provides a brief current. In 1780, the Italian anatomist Galvani accidentally observed that a frog leg in contact with metal twitched. He further experimented and found that if two metals were used to contact the tendons and muscles of the frog's legs respectively, the frog's legs would also twitch when the two metals collided.

In 1792, Volta studied this carefully and believed that the twitching of the frog's legs was a sensitive response to electric current. Electric current is produced when two different metals are inserted into a certain solution and form a circuit, and the muscles provide this solution. Based on this idea, in 1799, he created the first chemical battery capable of producing continuous current. The device was a series of cylinders composed of silver sheets, zinc sheets and cardboard soaked in salt water stacked in the same order. , called a voltaic pile.

Since then, various chemical power sources have flourished. In 1822, Seebeck further discovered that by connecting a copper wire and a wire of another metal (bismuth) into a loop and maintaining different temperatures at the two joints, a weak and continuous current can also be obtained. This is the thermoelectric effect.

After the invention of chemical power source, it was soon discovered that many unusual things could be done with it. In 1800, Carlyle and Nicholson used low-voltage current to decompose water; in the same year, Ritter successfully collected two gases from the electrolysis of water and electrolyzed metallic copper from a copper sulfate solution; in 1807, Davy used a huge battery to The group successively electrolyzed metals such as potassium, sodium, calcium, and magnesium; in 1811, he used a battery pack composed of 2,000 cells to create a carbon electrode arc; from the 1850s, it became a powerful light source used in lighthouses, theaters and other places , it was not until the 1970s that it was gradually replaced by the incandescent lamp invented by Edison. In addition, voltaic batteries also promoted the development of electroplating, which was invented by Siemens and others in 1839.

Although Franklin had observed that Leyden jar discharge could magnetize steel needles as early as 1750, and even earlier in 1640, lightning had been observed to rotate the compass needle, but by the beginning of the 19th century, The scientific community still generally believes that electricity and magnetism are two independent effects. Contrary to this traditional concept, the Danish natural philosopher Oersted accepted the philosophical ideas of German philosophers Kant and Schelling on the unity of natural forces, and firmly believed that there was some connection between electricity and magnetism. After years of research, he finally discovered the magnetic effect of current in 1820: when the current passes through a wire, it causes the magnetic needle near the wire to deflect. The discovery of the magnetic effect of electric current opened up a new era in electrical research.

Oersted’s discovery first attracted the attention of French physicists, and in the same year he achieved some important results, such as Ampere’s experiments on the equivalence of current-carrying solenoids and magnets; Arago’s experiments on steel and The magnetization phenomenon of iron under the action of electric current; Biot and Savart's experiments on the force exerted by long direct current-carrying wires on magnetic poles; in addition, Ampere further conducted a series of exquisite experiments on current interaction. The rules of the interaction force between current elements obtained from these experimental analyzes are the basis for understanding the magnetic field generated by current and the effect of magnetic field on current.

The discovery of the magnetic effect of electric current opened a new field of electrical applications. Sturgeon invented the electromagnet in 1825, creating conditions for the widespread application of electricity. In 1833, Gauss and Weber built the first crude single-wire telegraph; in 1837, Wheatstone and Morse independently invented the telegraph machine. Morse also invented a set of telegraph codes. The telegraph machine he built could be used to Moving strips of paper are marked with dots and dashes to convey messages.

In 1855, Thomson (Kelvin) solved the problem of slow signal transmission speed in underwater cables. In 1866, the Atlantic cable based on Thomson's design was successfully laid. In 1854, the French telegraphist Bourser proposed the idea of ??using electricity to transmit sound, but it did not become a reality. Later, Rice successfully experimented in 1861, but did not attract attention. Bell invented the telephone in 1861. As a receiver, it is still used in modern times, and its transmitter was improved by the carbon transmitter invented by Edison and the microphone invented by Hughes.

Not long after the magnetic effect of current was discovered, several different types of galvanometers were designed and made, providing conditions for Ohm to discover the circuit laws. In 1826, inspired by Fourier's theory of heat conduction in solids, Ohm believed that the conduction of electricity was very similar to the conduction of heat, and that the power supply acted like the temperature difference in heat conduction. In order to determine the circuit laws, he first used a voltaic pile as a power source to conduct experiments. Because the performance of the voltaic pile at that time was very unstable, the experiment was unsuccessful. Later, he switched to using a thermoelectromotive force with a constant temperature at two contact points and thus a highly stable temperature. Through experiments, he found that the current intensity in the circuit is directly proportional to what he called the "test power" of the power supply, and the proportional coefficient is the resistance of the circuit.

Since the law of conservation of energy had not yet been established at that time, the concept of electrochemical force was vague. It was not until 1848 that Kirchhoff investigated the concepts of potential difference, electromotive force, and electric field intensity from the perspective of energy. , harmonizing Ohmic theory with electrostatic concepts. On this basis, Kirchhoff solved the branch circuit problem.

The outstanding British physicist Faraday was engaged in experimental research on electromagnetic phenomena and made extremely important contributions to the development of electromagnetism. The most important contribution was the discovery of electromagnetic induction in 1831. Then he did many experiments to determine the laws of electromagnetic induction. He found that when the magnetic flux in a closed coil changes, an induced electromotive force is generated in the coil. The magnitude of the induced electromotive force depends on the rate of change of the magnetic flux with time. Later, Lenz gave a description of the direction of induced current in 1834, and Neumann generalized their results to give a mathematical formula for induced electromotive force.

Faraday built the first generator based on electromagnetic induction. In addition, he conducted extensive research on the connection between electrical phenomena and other phenomena. In 1833, he successfully proved that triboelectricity and the electricity generated by voltaic cells were the same. In 1834, he discovered the law of electrolysis. In 1845, he discovered the magneto-optical effect and explained it. He also studied in detail the paramagnetism and diamagnetism of matter, polarization phenomena and electrostatic induction phenomena, and experimentally proved the law of conservation of charge for the first time.

The discovery of electromagnetic induction has opened up new prospects for the development and widespread utilization of energy. In 1866, Siemens invented a practical self-exciting motor; at the end of the 19th century, long-distance transmission of electric energy was realized; electric motors were widely used in production and transportation, thus greatly changing the face of industrial production.

Extensive research on electromagnetic phenomena enabled Faraday to gradually form his unique concept of "field". He believed that: lines of force are material, they permeate the entire space, and connect charges with different signs and different magnetic plates respectively; electricity and magnetism are not transmitted through distance action in empty space, but through electric power lines and magnetic lines of force. , they are an indispensable component for understanding electromagnetic phenomena, and they are even more valuable for research than the "sources" that generate or "collect" lines of force.

Faraday's fruitful experimental research results and his novel field concepts prepared the conditions for a unified theory of electromagnetic phenomena. Physicists such as Neumann and Weber have made many important contributions to the understanding of electromagnetic phenomena. However, they have summarized all existing electrical knowledge since Coulomb from the perspective of action at a distance and have not succeeded in establishing a unified theory. . This work was completed in the 1860s by the outstanding British physicist Maxwell.

Maxwell believed that changing magnetic fields excite vortex electric fields in the surrounding space; changing electric fields cause changes in the electric displacement of the medium, and changes in electric displacement, like currents, excite vortex magnetic fields in the surrounding space. Maxwell expressed them explicitly with mathematical formulas, thus obtaining the universal equations of the electromagnetic field - Maxwell's equations. Faraday's idea of ??lines of force and the idea of ??electromagnetic action transmission are fully reflected in it.

Maxwell further derived from his system of equations that electromagnetic action propagates in the form of waves. The propagation speed of electromagnetic waves in vacuum is equal to the ratio of the electromagnetic unit of electricity to the electrostatic unit, and its value is the same as that of light in vacuum. The speed of propagation is the same, so Maxwell predicted that light is also an electromagnetic wave.

In 1888, Hertz designed and produced an electromagnetic wave source and an electromagnetic wave detector based on the oscillation properties of capacitor discharge. He detected electromagnetic waves through experiments, measured the wave speed of electromagnetic waves, and observed that electromagnetic waves, like light waves, have polarization. Properties, able to reflect, refract and focus. Since then, Maxwell's theory has gradually been accepted by people.

Maxwell’s electromagnetic theory was confirmed through Hertzian electromagnetic wave experiments, opening up a new field - the application and research of electromagnetic waves. In 1895, Russia's Popov and Italy's Marconi respectively realized the transmission of radio signals. Later, Marconi improved the Hertz oscillator into a vertical antenna; Braun of Germany further divided the transmitter into two oscillator lines, creating conditions for expanding the signal transmission range. In 1901 Marconi established the first transatlantic radio link. The invention of the electron tube and its application in circuits made it easy to transmit and receive electromagnetic waves, promoted the development of radio technology, and greatly changed human life.

The electron theory proposed by Lorenz in 1896 applied Maxwell's equations to the microscopic field and attributed the electromagnetic properties of matter to the effects of electrons in atoms. This can not only explain the polarization, magnetization, conductivity and other phenomena of matter as well as the absorption, scattering and dispersion of light by matter; but also successfully explain the normal Zeeman effect about the splitting of the spectrum in the magnetic field; in addition, Lorentz also Based on the electron theory, the formula for the speed of light in moving media was derived, which pushed Maxwell's theory one step forward.

In the theoretical systems of Faraday, Maxwell and Lorentz, it is assumed that there is a special medium "ether", which is the carrier of electromagnetic waves. Only in the ether reference system, the speed of light in vacuum is strictly The ground has nothing to do with direction, and Maxwell's equations and Lorentz force formula are strictly true only in the ether reference system. This means that the laws of electromagnetism do not comply with the principle of relativity.

Further research on this issue led Einstein to establish the special theory of relativity in 1905. It changed the original view and determined that special relativity is a basic principle of physics, which denies the ether The existence of reference frames modifies the space-time transformation relationship between inertial reference frames, making it possible for Maxwell's equations and the Lorentz force formula to hold in all inertial reference frames.

The establishment of the special theory of relativity not only developed the electromagnetic theory, but also played a huge role in the future development of theoretical physics.

Basic content of electricity

The content of electrical research mainly includes electrostatics, electrostatic magnetism, electromagnetic fields, circuits, electromagnetic effects and electromagnetic measurements.

Electrostatics is a discipline that studies the electric field generated by stationary charges and the effects of electric fields on charges. There are only two types of electric charges, called positive and negative. Like charges repel each other, and dissimilar charges attract each other. Charge obeys the law of conservation of charge. Charge can be transferred from one object to another, and the algebraic sum of the charges remains unchanged during any physical process. The so-called charging is nothing but the separation or transfer of positive and negative charges; the so-called disappearance of charges is nothing but the neutralization of positive and negative charges.

The interaction force between stationary charges conforms to Coulomb's law: the magnitude of the interaction between two stationary point charges in a vacuum is proportional to their product and inversely proportional to the square of the distance between them; The direction of the force is along the line between them. Charges with the same sign repel and charges with different signs attract.

The interaction between charges is through the electric field generated by the charges. The electric field generated by charges is described by electric field strength (referred to as field strength). The electric field strength at a certain point in space is defined by the electric field force experienced by the test charge at that point in positive units. The electric field strength follows the field strength superposition principle.

Common substances can be divided into two types according to their conductive properties: conductors and insulators. There are movable free charges in a conductor; an insulator, also called a dielectric, has only bound charges in its body.

Under the action of the electric field, the free charges in the conductor will move. When the composition and temperature of the conductor are uniform, the condition for achieving electrostatic equilibrium is that the electric field intensity inside the conductor is equal to zero everywhere. According to this condition, several properties of the electrostatic balance of conductors can be derived.

Magnetostatics is the study of the magnetic field generated when the current is steady and the force of the magnetic field on the current.

The directional flow of charge forms an electric current. There is a magnetic interaction between currents, and this magnetic interaction is transmitted through a magnetic field, that is, the current generates a magnetic field in the space around it, and the magnetic field exerts a force on the current placed in it. The magnetic field produced by an electric current is described by the magnetic induction intensity.

Electromagnetic field is a subject that studies electromagnetic phenomena and laws that change over time.

When the magnetic flux passing through the closed conductor coil changes, an induced current is generated in the coil. The direction of the induced current can be determined by Lenz's law. The induced current in the closed coil is the result of the induced electromotive force, which obeys Faraday's law: the magnitude of the induced electromotive force on the closed coil is always proportional to the time rate of change of the magnetic flux passing through the coil.

Maxwell’s equations describe the laws that electromagnetic fields generally follow. Combined with the medium equation of matter, the Lorentz force formula and the law of conservation of charge, it can in principle solve various macroscopic electrodynamic problems.

An important result derived from Maxwell's equations is the existence of electromagnetic waves. The changing electromagnetic field propagates in the form of electromagnetic waves. The propagation speed of electromagnetic waves in vacuum is equal to the speed of light. This also shows that light is also a type of electromagnetic wave, so the wave theory of light is included in the scope of electromagnetic theory.

Circuit includes the study of DC circuit and AC circuit, which is an integral part of electricity. DC circuits study the circuit laws and properties under the condition of stable current; AC circuits study the circuit laws and properties under the condition of periodic changes in current.

A DC circuit is connected by conductors (or wires), and the conductors have a certain resistance. Under steady conditions, the current does not change with time, and the electric field does not change with time.

According to the properties of the steady electric field, the basic laws of conduction and the concept of electromotive force, various practical laws of DC circuits can be derived: Ohm's law, Kirchhoff's circuit law, and some effective and simple methods for solving complex circuits Theorems: equivalent power supply theorem, superposition theorem, reciprocity theorem, duality theorem, etc. These practical laws and theorems form the theoretical basis of circuit calculation.

AC circuits are much more complex than DC circuits. Changes in current over time cause changes in the electric and magnetic fields in space, so there are electromagnetic induction and displacement currents, and there are electromagnetic waves.

Electromagnetic effect The electrical effect in matter is the link between electricity and other physical subjects (even non-physical subjects). There are many types of electrical effects in matter, many of which have become or are developing into specialized fields of study. For example:

Electrostriction, piezoelectric effect (electricity and polarity produced by mechanical pressure on dielectric crystals) and inverse piezoelectric effect, Seebeck effect, Peltier effect (two different metals) or at a semiconductor joint, heat is released when current passes in a certain direction, and heat is absorbed when the current is reversed), Thomson effect (a temperature gradient is maintained in a metal conductor or semiconductor, and heat is released when current passes in a certain direction, When the current reverses, it absorbs heat), thermistor (resistance in semiconductor materials changes sensitively with temperature), photoresistor (resistance in semiconductor materials changes sensitively with light), photovoltaic effect (semiconductor materials generate potential differences due to light) ,etc.

The study of various electrical effects helps to understand the structure of matter and the basic processes that occur in matter. In addition, technically, they are also the basis for realizing energy conversion and non-electrical measurement methods.

Electromagnetic measurement is also an integral part of electricity.

The development of measurement technology is closely related to the theoretical development of the discipline. The development of theory promotes the improvement of measurement technology; the improvement of measurement technology verifies theories on a new basis and promotes the discovery of new theories.

Electromagnetic measurement includes the measurement of all electromagnetic quantities, as well as the measurement of other related quantities (frequency, phase angle, etc. of alternating current). Various special instruments (ammeters, voltmeters, ohmmeters, magnetic field meters, etc.) and measurement circuits have been designed and produced using the principles of electromagnetism, which can meet the measurement needs of various electromagnetic quantities.

Another important aspect of electromagnetic measurement is the electrical measurement of non-electric quantities (length, speed, deformation, force, temperature, light intensity, composition, etc.). Its main principle is to convert the measurement of non-electricity into the measurement of electromagnetic quantity by utilizing a certain effect of the interconnection between electromagnetic quantity and non-electricity. Since electrical measurement has a series of advantages: high accuracy, wide range, small inertia, easy operation, long-distance remote measurement and automation of measurement technology, non-electrical electrical measurement is constantly developing.

Electricity and other subjects

Electricity, as a branch of classical physics, has been quite well developed in terms of its basic principles. It can be used to explain various phenomena in the macroscopic field. electromagnetic phenomena.

In the 20th century, with the development of atomic physics, nuclear physics and particle physics, human understanding has deepened into the microscopic field. On the interaction between charged particles and electromagnetic fields, classical electromagnetic theory has encountered difficulty. Although classical theory has given some useful results, many phenomena cannot be explained by classical theory. The limitation of classical theory is that the description of charged particles ignores their wave nature, while the description of electromagnetic waves ignores their particle nature.