Hello everyone uu from CSDN, today Xiaoyalan’s content is my professional course for this semester, first of all, I learned circuit models and circuit laws, including circuits and circuit models, reference directions of current and voltage, electric power and energy, Circuit elements, resistive elements, voltage and current sources, Kirchhoff's laws. So now, let's enter the world of circuit analysis
circuit theory
focus
Circuits and Circuit Models
Reference direction for current and voltage
electrical power and energy
circuit components
Resistive element
circuit theory
Circuit theory is one of the important basic theories of contemporary electrical engineering, electronic science and technology, information and communication engineering, control science and engineering, computer science and technology. Circuit theory and these disciplines promote and influence each other. After a long journey of more than a century, circuit theory has developed into a discipline with complete system, rigorous logic and strong vitality.
Human beings' understanding of electromagnetic phenomena begins with the observation of static electricity and magnetostatic phenomena.
In 1729, the British Gray divided materials into two categories: conductors and insulators.
American scientist Franklin made a lot of experiments in the study of electricity, and in 1749 he proposed the concept of positive and negative electricity.
From 1785 to 1789, French physicist Coulomb quantitatively studied the interaction between two charged bodies, and obtained the earliest law of electrostatics in history - Coulomb's law. This is a leap in human understanding of electromagnetic phenomena.
Before the 19th century, the application of electricity and magnetism was still rare.
In 1800, the Italian physicist Volta invented the voltaic battery, which can continuously convert chemical energy into electrical energy and maintain a continuous current in one direction. This invention has epoch-making significance, and it has laid a material foundation for people to study electrochemistry, electromagnetism and their applications in depth. Soon afterwards, the chemical effect of electric current, thermal effect and the use of electricity for lighting were discovered.
In 1820, the Danish physicist Oersted discovered the magnetic effect of current through experiments, built a bridge between electricity and magnetism, and opened a breakthrough in modern electromagnetism.
In 1825, the French scientist Ampere proposed the famous Ampere's loop law. He began to measure the magnetic effect of the circuit in 1820, and found that two current-carrying wires can attract each other and repel each other. This discovery became the basic law of mechanics and made theoretical preparations for the invention of electric motors.
In 1826, German physicist Ohm proposed the famous Ohm's law based on years of experiments: at a constant temperature, the current in the wire loop is equal to the ratio of the electromotive force to the resistance in the loop. Ohm extended this law to any section of wire, and concluded that the current in the wire is equal to the ratio of the voltage to the resistance of this section of wire.
In 1831, British physicist Faraday discovered the phenomenon of electromagnetic induction. As he continued Oersted's experiments, he was convinced that since electricity produces magnetism, magnetism can also produce electricity. He finally discovered that a conductor moving in a magnetic field would generate an induced electromotive force, which would generate a current in a closed conductor loop. This discovery became the basic principle of generators and transformers, making it possible to convert mechanical energy into electrical energy.
In 1832, American physicist Henry proposed the self-inductance coefficient L that characterizes the self-induction effect in the coil.
In 1834, Russian physicist Lenz proposed the law of the direction of induced current, namely the famous Lenz's law.
In 1838, American painter-turned-American Morse invented the telegraph. In 1844, he sent a telegraph message from Washington to Baltimore, 40 miles away.
The advent of the telegraph increased the need for circuit analysis and calculations.
In 1845, after in-depth research on Ohm's work, the German scientist Kirchhoff proposed two basic laws of circuits - Kirchhoff's current law (KCL for short) and Kirchhoff's voltage law (KVL for short). . It is the law that the voltage and current must obey in the lumped parameter circuit.
In 1853, British physicist Thomson used the circuit model of resistance, inductance and capacitance to analyze the discharge process of the Leiden bottle and obtained the frequency of electrical oscillation. In the same year, the German physicist Helmholtz proposed the equivalent generator theorem in the circuit. Due to the increase in international communications, submarine telegraph cables between Britain, France, Italy, and Turkey were built in Europe from 1850 to 1855. Telegraph signals are transmitted through long-distance cables, resulting in signal attenuation, delay, and distortion.
In 1854, Thomson published the cable transmission theory, which analyzed these phenomena.
In 1857, considering that the overhead transmission line is different from the cable, Kirchhoff obtained a complete equation of voltage and current on the transmission line including the self-inductance coefficient, which is called the telegraph equation or Kirchhoff's equation. At this point, the circuit theory including the transmission line is basically established.
In 1866, the German engineer Siemens discovered the principle of the electric motor and used it to improve the generator. Due to the increasingly wide application of electricity in various aspects, such as lighting, point solution, electroplating, electric drag, etc., there is an urgent need to obtain electric energy more conveniently to improve efficiency and reduce costs. In 1881, the DC high-voltage transmission experiment was successful. However, since high voltage DC is not convenient for users to use directly, long-distance AC high voltage transmission was realized on the basis of the invention of the transformer in the same year. Since then, the age of electrification has begun.
In 1873, the British physicist Maxwell summarized the laws of various electromagnetic phenomena discovered at that time, expressed it as Maxwell's equations, predicted the existence of electromagnetic waves, and laid a solid foundation for circuit theory.
In 1888, the German physicist Hertz proved that the electromagnetic waves predicted by Maxwell did exist through painstaking repeated experiments.
In 1876, American inventor Bell invented the telephone. Bell was only a teacher in a school for the deaf at that time, but with his sensitive understanding of the effect of electric current and unremitting efforts, he achieved the purpose of communicating with each other through wires. After continuous improvement, by 1878, he achieved the first long-distance call between Boston and New York, 200 miles away.
In 1879, the American inventor Edison invented the carbon filament light bulb. In 1912, American Coolidge invented the tungsten filament light bulb, which became the most popular lighting appliance. The widespread use of electric lights is a great popularization of electric energy applications and has changed people's lives.
In 1880, the British Hopkinson proposed a law for calculating magnetic circuits that is similar in form to Ohm's law. At the end of the 19th century, the rapid development of alternating current technology promoted the establishment of alternating current circuit theory. In 1893, German-American scientist Steinmetz proposed the complex number symbol method (phasor method) for analyzing AC circuits, using complex numbers to represent sinusoidal AC, which simplified the calculation of AC circuits. The vector diagram proposed by the Swiss mathematician Argend has also become a powerful tool for analyzing AC circuits. These theories and methods laid the foundation for the development of circuit theory in the future.
In 1894, the Italian Marconi and the Russian physicist Popov invented the radio respectively. The 20-year-old Marconi, who has no formal university education, uses a Hertz spark oscillator as a transmitter to generate intermittent electromagnetic wave signals through the opening and closing of electric keys. In 1895, the transmission distance of the signal he transmitted was more than 1Km; in 1897, the signal transmitted could be received beyond 20Km, and the era of radio communication began.
The invention of electric vacuum devices has advanced the development of electronic engineering a big step.
British scientist Thomson repeatedly tested between 1895 and 1897, proving that electrons do exist. Subsequently, British scientist Fleming invented a practical vacuum diode based on the thermal diode invented by Edison. It has unidirectional conductivity and can be used for rectification or detection. In 1907, American Forrest invented the vacuum triode, which can amplify weak electrical signals. In 1914, Forrester used a vacuum triode to form an oscillating circuit, which made the radio communication system more advanced.
The needs of applications have led to the emergence and development of large-scale power generation and transmission and distribution.
At the end of the 19th century, there was also a "controversy between AC and DC".
The party represented by Edison advocated the application of direct current; the other party represented by Tesla and Westinghouse advocated the application of alternating current. It was not until the invention of alternators, induction motors, transformers, etc. that the advantages of the AC system were fully demonstrated, and the AC system was widely used. By the 1930s, the voltage of the power transmission line had reached 22×10^4V, and the power supply range reached hundreds of kilometers, forming a relatively complex power network.
In the 20th century, in 1911, the British engineer Heaviside proposed the concept of impedance, and also proposed an algorithm to solve the transient process of the circuit. In 1918, the symmetrical component method proposed by Futek simplified the analysis of asymmetrical three-phase circuits. This method is still a common method for analyzing the asymmetrical operation of three-phase AC motors and power systems. In 1920, Campbell and Wagner studied the filter circuit of the ladder structure. In 1924, Foster proposed the reactance theorem of the inductor-capacitor two-terminal network. Since then, the network synthesis theory for designing circuits with given frequency characteristics has been established.
After the invention of the electron tube, electronic circuit technology developed rapidly.
In 1932, the Swedish Nyquist proposed a criterion for judging the stability of the closed-loop system from the frequency characteristics of the open-loop transfer function of the feedback circuit. In 1945, American Porter published the book "Network Analysis and Feedback Amplifier", summarizing the principle of negative feedback amplifier, thus forming a frequency domain analysis method for analyzing linear circuits and control systems, and has been widely used.
In World War II, the emergence of radar and modern control technology played a role in promoting the development of circuit theory.
On December 24, 1947, Bratton, Bardeen and Shockley of Bell Laboratories invented a point-contact transistor. This is a brand new semiconductor device with small size, stable electrical performance and low power consumption. Since this invention was announced to the world in 1948, it has been applied to communication, television, computer and other fields soon, and electronic technology has entered the semiconductor era.
In 1958, the integrated circuit was invented, which manufactured the resistors, capacitors, diodes, transistors and wires that constitute the electronic circuit on a semiconductor chip of a few square millimeters, thus greatly reducing the volume. Now integrated circuits have developed from small-scale integrated circuits containing dozens of transistors to very large-scale integrated circuits containing millions of transistors, thus electronic technology has entered the era of integrated circuits.
At the same time, electronic computers and various microprocessors have also undergone several generations of changes. The electronic computer ENIAC applied in 1947 contained 18,000 electronic tubes, 30t, and consumed 50kW. At present, an electronic computer with the same function made of integrated circuits weighs less than 300g and consumes only 1/2W. At present, computers have been widely used in fields such as production, national defense, scientific research, management, education, and medical care.
focus:
- Reference direction of voltage and current
- Characteristics of Resistive Elements and Power Elements
- Kirchhoff's laws (KCL, KVL)
Circuits and Circuit Models
The generator of electrical energy or electrical signal is called the power supply, and the electrical equipment is called the load.
Since the voltage and current generated in the circuit are generated under the action of the power supply, the power supply is also called the excitation source or excitation; the voltage and current generated in the circuit by the excitation are called the response.
Call the stimulus the input and the response the output.
expand:
The relationship between frequency and inductance and capacitance
Reference direction for current and voltage
Reference direction of current
When the current, voltage, charge and other variables in the circuit change with time, they are generally represented by lowercase letters i, u, q, etc., and capital letters I, U, Q indicate that the corresponding variable is a constant value.
Reference direction of voltage
Then look at a few simple questions:
High school knowledge can be solved
associative reference direction
electrical power and energy
Determination of whether the circuit absorbs or emits power
Now, let's look at an example:
circuit components
The lumped parameter element means that the physical phenomena related to electricity and magnetic field are "lumped" by the element. There are no electric and magnetic fields outside the element. If there is an electric field outside the component, the current entering and leaving the terminal may be different; if there is a magnetic field outside the component, the voltage between the two terminals may not be a single value.
Resistive element
The unit of conductance is Siemens
Alright, this is the end of today's content for Xiao Yalan. The circuit analysis has just started, and I find it very difficult, but I will try my best to learn it. Let's learn the rest of Kirchhoff's laws. A blog will talk about it carefully
