1) short circuit means that the power supply is directly connected to the closed circuit with wires without passing through the load. (Usually, this is a serious circuit fault that should be avoided as much as possible, which will cause the circuit to burn and catch fire due to excessive current. )

2) In a series-parallel circuit, both ends of circuit elements or loads are directly connected by wires or switches. (This is a safe connection that will not burn out due to excessive current, and it is a partial or partial short circuit. For example, in order to prolong its service life, when a filament is broken or damaged, its internal special structure will automatically connect its two ends to make other small bulbs work normally. )

Explanation of short circuit in physics

In the operation of power system, when there is abnormal connection (short circuit) between phases or between phases to ground (or neutral line), a great current will flow. Its current value is much larger than the rated current, depending on the electrical distance between the short circuit point and the power supply. For example, when the generator is short-circuited, the maximum instantaneous value of short-circuit current flowing through the generator can reach 10 ~ 15 times of the rated current. In large-capacity power system, the short-circuit current can reach tens of thousands of amperes. This will have a serious impact and consequences on the normal operation of the power system.

There are four basic types of short circuit in three-phase system: three-phase short circuit, two-phase short circuit, single-phase short circuit and two-phase short circuit. In addition to the three-phase short circuit, the three-phase circuit is symmetrical, so it is also called symmetrical short circuit, and the other three are asymmetrical short circuits. In the power grid with neutral grounding, single-phase grounding has the most short-circuit faults, accounting for about 90% of all faults. In the power grid where the neutral point is not directly grounded, short-circuit faults are mainly various interphase short circuits.

When a short circuit occurs, it usually takes 3 ~ 5 seconds for the power system to transition from a normal stable state to a short-circuit stable state. In this transient process, the change of short-circuit current is very complicated. It has many components and its calculation needs an electronic computer. The maximum instantaneous value of short-circuit current will appear about half a cycle (0.0 1 sec) after short-circuit, which is called surge current. It will produce great electromotive force, which can be used to check the dynamic stability of mechanical stress when electrical equipment is short-circuited. The analysis and calculation of short-circuit current is one of the important contents of power system analysis. It provides an effective means for the selection of electrical equipment, the setting of relay protection and accident analysis in the planning, design and operation of power system.

In the electrical circuit, the phenomenon that the current suddenly increases due to various reasons is called short circuit. The collision between phase lines is called co-directional short circuit; The direct collision between phase line and ground wire, grounding conductor or earth is called short circuit to ground. When the short-circuit current suddenly increases, its instantaneous heat release is very large, which greatly exceeds the heat generated during the normal operation of the line. It can not only burn insulation, but also melt metal, causing combustible materials to burn and catch fire. The main reasons for short circuit are: 1, line aging, insulation damage and short circuit; 2. Overvoltage of power supply leads to insulation breakdown; 3. Small animals (such as snakes, rabbits, cats, etc. ) across bare wires; 4, all kinds of man-made confusion; 5. Outdoor overhead lines are slack and collide under strong wind; 6, line installation is too low and all kinds of transport goods or metal objects collision caused by short circuit.

Open circuit: The circuit is broken.

electric capacity

Capacitance is a physical quantity, which indicates the capacity of a capacitor to hold charge. We will increase the potential difference between the two plates of the capacitor by 1 volt, which is called the capacitance of the capacitor.

The symbol of the capacitor is C. In the international system of units, the unit of the capacitor is Farah, abbreviated as French, and the symbol is F. If the potential difference between the two stages is 1 volt when charging with 1, the capacitance of this capacitor is 1.

The formula of capacitance is: C=Q/U but the size of capacitance is not determined by q or u, that is, C=εS/4πkd. ε is a constant, which is related to the properties of dielectric. K is the electrostatic force constant.

The calculation formula of electric potential energy of capacitor is e = Cu 2/2.

In many electronic products, capacitor is an essential electronic component, which plays the role of smoothing filter of rectifier, power supply and decoupling, bypass of AC signal, AC coupling of AC /DC circuit and so on. Because there are many types and structures of capacitors, users need to know not only the performance indexes and general characteristics of various capacitors, but also the advantages and disadvantages, mechanical or environmental constraints of various components in a given use. This paper introduces the main parameters and uses of capacitors, which can be used as a reference for readers when choosing capacitor types.

1. Nominal capacitance (CR): the capacitance value marked on the capacitor product.

The capacitance of mica and ceramic dielectric capacitors is low (about below 5000pF); The capacitance of paper, plastic and some ceramic media is in the middle (about 0.005 μ f10 μ f); Generally, the capacity of electrolytic capacitors is relatively large. This is a rough classification.

2. Category temperature range: The ambient temperature range that can work continuously as determined by the capacitor design depends on the temperature limit of its corresponding category, such as the upper category temperature, the lower category temperature and the rated temperature (the highest ambient temperature that can continuously apply rated voltage).

3. Rated voltage (UR): the effective value of the maximum DC voltage or the maximum AC voltage or the peak value of the pulse voltage that can be continuously applied to the capacitor at any temperature between the lower limit temperature and the rated temperature.

When capacitors are used in high voltage situations, we must pay attention to the influence of corona. Corona is caused by the gap between dielectric layer and electrode layer, which will not only produce parasitic signals that damage equipment, but also lead to dielectric breakdown of capacitors. Corona is particularly prone to occur under AC or pulsating conditions. For all capacitors, the sum of DC voltage and AC peak voltage should not exceed the DC voltage rating in use.

4. Tangent of loss angle (tgδ): Under the sinusoidal voltage with specified frequency, the loss power of capacitor is divided by the reactive power of capacitor.

What needs to be explained here is that in practical application, the capacitor is not a pure capacitor, and there is an equivalent resistance inside it. Its simplified equivalent circuit is shown in the following figure. In the figure, C is the actual capacitance of the capacitor, Rs is the series equivalent resistance of the capacitor, Rp is the insulation resistance of the medium, and Ro is the absorption equivalent resistance of the medium. For electronic equipment, the smaller the Rs, the better, that is to say, the smaller the power loss and the smaller the angle δ with the capacitor power.

This relationship is expressed by the following formula: tgδ=Rs/Xc=2πf×c×Rs. Therefore, this parameter should be carefully selected in application to avoid excessive self-heating and reduce equipment failure.

5. Temperature characteristics of capacitors: usually expressed as the percentage of the capacitance at the reference temperature of 20℃ to the capacitance at the relevant temperature.

Supplement:

1. Capacitance is generally represented by "c" plus a number in the circuit (for example, C 13 is represented by the number 13). Capacitor is an element with two metal films attached to each other and separated by insulating material. The characteristics of capacitors are mainly blocking DC and circulating AC.

The size of the capacitor means the amount of electric energy that can be stored. The blocking effect of capacitance on AC signal is called capacitive reactance, which is related to the frequency and capacitance of AC signal.

Capacitance XC= 1/2πf c (f stands for the frequency of AC signal, and c stands for capacitance) The commonly used capacitor types in telephones are electrolytic capacitor, ceramic capacitor, patch capacitor, monolithic capacitor, tantalum capacitor and polyester capacitor.

2. Identification method: The identification method of capacitance is basically the same as that of resistance, which is divided into three methods: direct standard method, color standard method and number standard method. The basic unit of capacitance is farad (f), and other units are millifarad (mF), microfarad (μF)/mju:/, nanofarad (nF) and picofarad (pF). Where: 1 Farah = 1000 millifarads (mF), 1 millifarads = 1000 microfarads (μF), 1 microfarads = 1000 nanofarads (nF).

The capacitance value of a large capacitor is directly indicated on the capacitor, such as 10 μF/ 16V.

The capacitance value of a capacitor with a small capacity is represented by letters or numbers on the capacitor.

Letter symbol:1m =1000μ f1p 2 =1.2pf1n =1000pf.

Digital representation: Generally, three digits are used to represent the capacity, the first two digits represent the effective digits, and the third digit is the ratio.

For example, 102 means10×102pf =1000pf224 means 22×104pf = 0.22μ f.

3, capacitance error table

Symbol F G J K L M

Allowable error1%2% 5%10%15% 20%

For example, the capacitance of a ceramic chip is 104J, which means the capacitance is 0. 1 μF with an error of 5%.

6 service life: the service life of capacitor decreases with the increase of temperature. The main reason is that the temperature accelerates the chemical reaction and degrades the medium with time.

7 Insulation resistance: As the electronic activity increases with the increase of temperature, the insulation resistance will decrease with the increase of temperature.

Capacitors include fixed capacitors and variable capacitors, in which fixed capacitors can be divided into mica capacitors, ceramic capacitors, paper/plastic film capacitors, electrolytic capacitors and glass glaze capacitors according to the dielectric materials used. Variable capacitors can also be glass, air or ceramic dielectric structures. The following table lists the letter symbols of common capacitors.

Introduction to capacitance classification

Name: Polyester (Polyester) Capacitor (CL)

Symbol:

Capacitance: 40p-4μ

Rated voltage: 63-630 volts

Main features: small volume, large capacity, resistance to damp heat and poor stability.

Purpose: Low-frequency circuit with low requirements on stability and loss.

Name: polystyrene capacitor (CB)

Symbol:

Capacitance: 10p- 1μ

Rated voltage:100V-30KV

Main features: stability, low loss and large volume.

Application: Circuits with high requirements for stability and loss.

Name: Polypropylene Capacitor (CBB)

Symbol:

Capacitance: 1000p- 10μ.

Rated voltage: 63-2000 volts

Main features: Its performance is similar to that of polystyrene, but its volume is small and its stability is slightly poor.

Purpose: to replace most of polyphenyl or mica capacitors, and to be used in circuits with high requirements.

Name: Mica Capacitor (CY)

Symbol:

Capacitance: 10p-0 0. 1μ

Rated voltage:100V-7KV

Main features: high stability, high reliability and low temperature coefficient.

Application: High-frequency oscillation, pulse and other demanding circuits.

Name: High Frequency Ceramic Capacitor (CC)

Symbol:

Capacitance: 1-6800p

Rated voltage: 63-500 volts

Main features: low high frequency loss and good stability.

Purpose: High frequency circuit.

Name: low frequency ceramic capacitor (CT)

Symbol:

Capacitance: 10p-4 4. 7μ

Rated voltage: 50V 50V-100V

Main features: small size, low price, large loss and poor stability.

Purpose: Low frequency circuit with low requirements.

Name: Glass Glaze Capacitor (CI)

Symbol:

Capacitance: 10p-0 0. 1μ

Rated voltage: 63-400 volts

Main features: good stability, low loss and high temperature resistance (200 degrees)

Application: Pulse, coupling, bypass and other circuits.

Name: aluminum electrolytic capacitor

Symbol:

Capacitance: 0. 47 - 10000μ

Rated voltage: 6. 3-450 volts

Main features: small volume, large capacity, large loss and large leakage.

Applications: power supply filtering, low frequency coupling, decoupling, bypass, etc.

Name: Tantalum electrolytic capacitor (CA) Niobium electrolytic capacitor (CN)

Symbol:

Capacitance: 0. 1 - 1000μ

Rated voltage: 6. 3-125v

Main features: loss and leakage are less than aluminum electrolytic capacitance.

Purpose: to replace aluminum electrolytic capacitors in demanding circuits.

Name: air dielectric variable capacitor

Symbol:

Variable capacitance: 100- 1500p

Main features: low loss and high efficiency; It can be made into linear type, linear wavelength type, linear frequency type and logarithmic type according to needs.

Applications: electronic instruments, radio and television equipment, etc.

Name: thin film dielectric variable capacitor

Symbol:

Variable capacitance: 15-550p

Main features: small size and light weight; The loss is greater than that of air medium.

Application: communication, broadcast receiver, etc.

Name: Thin Film Dielectric Trimming Capacitor

Symbol:

Variable capacitance: 1-29P

Main features: large loss and small volume.

Application: Radio recorders, electronic instruments and other circuits are used for circuit compensation.

Name: ceramic dielectric trimming capacitor

Symbol:

Variable capacitance: 0. 3 - 22p

Main features: less loss and small size.

Purpose: Fine tuning high frequency oscillation circuit.

Name: Monolithic Capacitor

The biggest disadvantage is that the temperature coefficient is very high, so it is unbearable to make the oscillator drift steadily. The capacitor of a 555 oscillator we made is next to 7805. After starting, the frequency of oscilloscope changes slowly, and it is much better to change it to polyester capacitor later.

Characteristics of monolithic capacitors:

Large capacitance, small volume, high reliability, stable capacitance, high temperature resistance, good moisture resistance and so on.

Scope of application:

Widely used in electronic precision instruments. Resonance, coupling, filtering and bypass of various small electronic devices.

Capacity range:

0.5 pf- 1μF

Withstand voltage: twice the rated voltage.

It is said that monolithic capacitors are also called multilayer ceramic capacitors, which are divided into two types. 1 model has good performance, but its capacity is small, generally less than 0. 2U, and the other is called Type II, which has large capacity but average performance.

In terms of temperature drift:

The positive temperature of monolith is about+130, and the negative temperature coefficient of CBB is -230.

In terms of price:

Tantalum-niobium capacitors are the most expensive, monolith and CBB are cheaper, and tiles are the lowest, but there is a kind of high-frequency zero-temperature drift black-spot tiles that are slightly more expensive. Mica capacitor has a higher Q value and a slightly higher price.

conservation of energy

The change of energy in quantity follows the most common and basic law of nature, that is, the law of energy conservation.

The law of conservation of energy was independently discovered by more than 10 scientists of different majors in five countries. Meyer, Joule and Helmholtz are the main contributors. Meyer is a German doctor. Meyer published a paper entitled "On the Force of Inorganic Boundary" in 1842 from the study of metabolism, which further expressed the idea of energy conservation in physical and chemical processes. Joule was a British physicist in 1843. He studied and measured the equivalent relationship between thermal energy and mechanical work. 1847, he did the best experiment to determine the mechanical equivalent of heat. Since then, the experimental method has been continuously improved, and the measurement results have been reported to 1878. The accurate experimental results provide unquestionable experimental evidence for the establishment of the law of conservation of energy. Helmholtz is a German physicist and physiologist. 1847, he published the book On the Conservation of Force, gave the mathematical expressions of different forms of energy, and studied their mutual transformation, thus this book became a historical document with great influence on the demonstration of the law of conservation of energy. In the process of discovering this law, in addition to the above three, there are Kano in France, Moore in Germany, Segen in France, Hess in Switzerland, Holzman in Germany, Grove in Britain, Keldin in Denmark, and Elon in France, all of whom have independently published papers on the conservation of energy and contributed to the discovery of the law of conservation of energy.

The law of conservation of energy points out that "all substances in nature have energy, which can neither be created nor destroyed, but can only be transformed from one form to another, and from one object to another. In the process of energy transformation and transmission, the total amount of energy is constant".

Energy can be converted into various forms of energy that people need under certain conditions. For example, coal gives off heat after burning, which can be used for heating; It can be used to generate steam, promote the conversion of steam engines into mechanical energy, and promote the conversion of turbogenerators into electrical energy. Electric energy can be converted into mechanical energy, light energy or heat energy by motors, lights or other electrical appliances. Another example is solar energy, which can heat water by collecting hot gas or generate steam to generate electricity; Solar energy can also be directly converted into electric energy through solar cells. Of course, these transformations all follow the law of conservation of energy.

In English, conservation of energy is called conservation of energy.

The concrete manifestation of energy conservation

Conservative mechanical system: under the condition that only conservative force does work, the energy of the system is mechanical energy (kinetic energy and potential energy), and the conservation of energy is embodied in the law of conservation of mechanical energy.

Thermodynamic system: energy is expressed as internal energy, heat and work, and the expression of energy conservation is the first law of thermodynamics.

Relativistic mechanics: in relativity, mass and energy can be transformed into each other. Considering the energy change caused by mass change, the law of conservation of energy still holds. Historically, the law of conservation of energy in this case is also called the law of conservation of mass and energy.