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外文原文：1. Energy conversion and conservationThe conversion of mechanical energy to heat is by no means new to us. We are also familiar with other transformations of energy. Chemical energy is converted into heat when fuel burns. Electrical energy is transformed into heat and light in electrical lamps and electrical stoves. Radiant energy turns into heat when sunlight strikes an object which absorbs it. “All contradictory things are interconnected; not only do they coexist in a single entity in given conditions, but in other given conditions, they also transform themselves into each other.”In a word, all energies maybe converted from one form to another and what is more, they all can transform into heat by themselves. Heat is an energy of irregular motion of particles in a substance, at ordinary temperature it is less unable than any of the other energies.However, at high temperatures heat energy may be converted into energy of more usable forms. Some people have made different kinds of machines to convert heat into mechanical energy. Diesel and gasoline engines are designed to convert heat that is developed by the burning of fuel into mechanical energy for running tractors, trucks, and cars. The mechanical energy transformed from heat in a steam turbine is made to operate generators. And the generators, in turn, convert the mechanical energy. All these transformations are taking place every minute and everywhere in our daily life and production.In any energy transformation, there is some loss, but no energy is destroyed. The part that is lost is simply wasted. If all of the energies that are wasted were added to that used, the total would be found to be equal to the total supplied. The form may be changed, but the amount remains unchanged.The fact that energy can be changed from one form to another, but can neither be created nor destroyed, constitutes one of the most important laws in science, the law of conservation of energy. No one form of energy can be long conserved, but the total is conserved at any time. A machine may be designed to lift a much larger weight than the force that is applied, but it can never produce more work than was supplied to it. In other words, a machine cannot have an efficiency greater than one. Since man cannot create or destroyed energy, he must use the energy that is available to him.Some devices were designed for the purpose of doing work without the need of supplying energy. These are the so-called perpetual-motion machines. We say that such machines are impossible because they violate the law of conservation of energy. The attempt has never been successful. And it will never be successful.2.Generator and electricity(1) Faraday and his GeneratorThe electric current in our homes is produced in power stations which usually contain several generators. These are machines which generate electric current when they are turned. So there has to be some kind of engine to turn them.What kind of engine can we use? Steam engines are suitable, and so are oil engines. Sometimes the water of a great river can turn the generators, and so power stations are often built near dams.The water which is stored behind a dam flows out with great force when it is allowed to do so. We can use this force to turn machines which are called turbines. The water is led through big pipes to the turbines, and then they turn the generators. These supply the country with useful current.Michael Faraday (1791-1867) made the first generator. He was a great scientist. He studied gases and changed some of them into liquids. He made many discoveries in chemistry and electricity. Before his time scientists got their electric current from electric cells. Several cells together form a battery. An Italian, V olta, made the first battery and it produced a small current. Modern cells are boxes which contain acids and other materials such as metals or carbon rods. Faraday knew about V olta’s work, but he wanted to produce an electric current by using magnets.An electric current which flows through a coil of wire round an iron rod produces magnetism in it. Faraday wanted to do the opposite: he wanted to produce a current in a wire by using magnetism. He tried to do this for a long time, but he failed completely until he moved a wire near the magnets. Then his instruments showed that a small current was flowing in the wire. Either the magnet or the wire had to move. He made a small machine to turn a coil of wire near the magnets, and this generated a current. It was the first generator in the world.All modern generators depend on Faraday’s work. The magnets in them are usually electromagnets; even in an electromagnet a little magnetism remains in the iron after the current is switched off. As soon as the generator turns, a small current appears. This increases the magnetism, and so the current increases. This again increases the magnetism, and so on. In a few seconds there is quite a big current flowing in the wires. If a river turns the turbines, it does all the necessary work, and no fuel is needed. Those countries which have big and powerful rivers are lucky because they can geta lot of electric power from them.(2) Direct and Alternating CurrentsA direct current is, of course, useful. The electric system in a car uses the direct current. Besides, direct current is also used to meet some of he industrial requirements.However, at present, most cities make use of another kind of electric current going first in one direction and then in another, we give it the name of an alternating current.In spite of its being very useful a direct current system has one great disadvantage; namely, there is no easy, economical way in which one can increase or decrease its voltage. The alternating current does not have this disadvantage, its voltage may be increased or decreased with little energy loss by the use of a transformer. Using a transformer it is possible to transform power at low voltage into power at high voltage, and vice versa.In that manner, current can be generated at a voltage which is suitable for any given machine. In large power stations, the best suited voltage is often 6,300 or 10,500 V. Power being transmitted over long distances with less loss at high voltage than at low voltage, it is more economical to increase the voltage to 35,000 or 110,000 or even 220,000 V for transmission. Wherever the power is to be used, it is lowered to the voltage which satisfies that particular purpose, such as 220 V in homes, or 380 V in factories, etc. (3) Voltage and CurrentAll metals are good conductors because there is a great number of free electrons in them. These free electrons usually do not move in a regular way so that there is no current. However, when an electric field is set up, all the free electrons will be made to move in one direction. And an electric current is formed. Or to say, in order that an electric current can be produced in a conductor, an electric field must be built in it. An electric field is usually setup by applying a voltage between the two terminals of the conductor. Thus, the free electrons form an electric current in the conductor.There are two kinds of electric currents: direct current (D.C.) and alternating current (A.C.). Direct current is an electric current the charges of which move in one direction only. It is constant in value, unless the circuit conditions, such as the applied voltage or circuit resistance are changed. The changes of an alternating current change their direction regularly. First they flow one way, then the other. The difference between A.C. and D.C. depends upon the voltage applied. If the electric field applied is unchanged, the current produced is D.C. If the electric field applied is alternating, the current produced is A.C. Both A.C. and D.C. have their advantages and disadvantages and they are respectively used in different applications. Electric power is made at power stations, but it is usually needed far away. How is the current taken to far-off places?Thick wires usually carry it across the country, and steel pylons hold the wires above the ground. The pylons are so high that nobody can touch the wires at the top. The wires are not usually copper wires; they are made of aluminum, and thirty wires together form one thick cable. Aluminum is so light that the pylons can easily hold the cables up.It would not he cheap to drive very large currents through these cables. Large currents need very thick wires. If thin wires are used, they get hot or melt, and so the currents ought to be as small as possible. Can we send a lot of power if we use a small current? We can do so if the voltage is high. We need a small current and a high voltage; or a large current with a low voltage. The small current is cheaper because the wires need not be thick.The result is that the voltage has to be very high. The pressure in thealuminum cables may be 132,000 volts, and this is terribly high. The voltage of a small battery is usually between 1 and 9 volts. The is the kind of battery which we carry about in our pockets. A car battery has a voltage of 6 or 12 volts. In a house the pressure in the wires may be 230 volts, or something like that. Even 230 volts is high enough to kill a person, so what would happen if we touched one of the aluminum cables? The high voltage would drive a heavy current through our bodies to the earth.The wires are placed high up so that nobody can touch them. When they lead down to a house or a railway, the voltage is made lower. It can be changed easily; but if the voltage is lower, the current must be higher. If it is not, we will lose power. So the wires have to be thicker.The wires must never tough the steel pylons. If they did that, the current would escape to the earth through the steel. Steel is a good conductor of electricity, and so are most metals. We have to separate the wires from the pylons, and we do this with insulators.An insulator does not allow an electric current to flow through it; but a conductor lets it flow easily. Paper, air and glass are examples of good insulators. Another is porcelain. Porcelain is such a good insulator that it is widely used, and the aluminum cables hang down from the pylons on several separated porcelain insulators. Parts of these have to keep dry even when it rains, because water is a good conductor. So the insulators have a special shape and the rain cannot reach all parts of them.(4) ResistanceResistance is the opposition to the flow of electrons. The greater the resistance of a wire is, the less electric current will pass through it under the same voltage. The resistance of a wire depends mainly on the length, thecross-section, the material and the temperature of the wire.Copper is one of the best conductors that are used in electrical engineering.A long copper wire has a larger resistance than a short copper wire with the same cross-section. If two copper wires are equal in length, the wire with a larger cross-section will show smaller resistance.Now let’s study the effect of temperature on resistance. Measure the resistance of a conductor when a small current is passing though it, and then measure its resistance when a large current causes it red-hot. You will find the electrons meet more resistance when the conductor is hot than when it is cold. Accordingly a conductor which has a resistance of 100 ohms at 0℃will have a resistance of about 150 ohms at 100℃. The higher its temperature is, the more resistance it shows.3.Electric equipments(1) Electric wiresElectric wire is usually made of copper. Copper lets the electric current flow easily through it. We say that it has a low resistance. Some other metals also have a low resistance, but copper is the most useful. There are copper wires in millions of houses in the world.These wires carry the current to our lamps. There is a thin wire inside an electric lamp; you can see it if you look carefully. A thin wire has a higher resistance than a thick one. It tries to stop the flow of current. Then it gets very hot.The thin wire is not made of copper; it is made of tungsten. All metals melt when they get hot. (Mercury melts at a lower temperature than our usual ones.) Tungsten does not melt easily. It has to be very hot indeed before it melts.When the tungsten gets hot, it also gets bright. It shines and gives a good light. It also lasts a long time without breaking.An American, Edison, invented the first small electric lamp. He wanted a thin wire for his lamp, and tried to make one; but he had a lot of trouble. Thin wires easily melt if they are made of copper. He decided to use carbon because it does not melt. He tried cotton and hundreds of other materials to make his thin piece of carbon. But at first all of them broke. They were too thin and weak. They had to be thin because they had to shine brightly. Thick pieces do not have a high resistance. So they did not get hot enough, and they gave no light. Edison did not stop trying; and after a lot of trouble he made his first lamp.Our tungsten lamps are better than the old carbon lamps. They are brighter and they last longer. The tungsten does not easily melt or break. There is not much air inside an electric lamp; we have to take it out. Air contains oxygen, and the hot tungsten could burn in it. Usually we put some gas in the place of the air.Electric fires also have wires which get hot. These wires are thick, but they are not made of copper. They have a high resistance. A large current flows though them and makes them hot. So we can use electric fires in winter to keep us warm.In some houses an electric current also makes the water hot. This is useful when we want a bath. The wires get hot like the wires of electric fires; but we must keep them away from the water. We have to separate the wires from the water with some special material. It is not safe to let an electric wire tough water. Water has a low resistance to an electric current. Sometimes a person touches an electric wire with a wet hand; he ought not to do this. He mightkill himself. The water lets the current flow easily to his body. Then it can escape to the ground through his legs. The current can easily flow through his body; and it can go through his heart. Then his heart will stop beating. (2) Switches and fusesAn electric switch is often on a wall near the door of a room. Two wires lead to the lamp in the room. The switch is fixed in one of them. The switch can cause a break in this wire, and then the light goes out. The switch can also join the two parts of the wire again; then we get a light.Switches can control many different things. Small switches control lamps and radio sets because these do not take a large current. Larger switches control electric fires. Other switches can control electric motors.Good switches move quickly. They have to stop the current suddenly. If they move slowly, an electric spark appears. It jumps across the space between the two ends of the wire. This is unsafe and it heats the switches are sometimes placed in oil. Sparks do not easily jump though oil, and so the oil makes the switch safer.A large current makes a wire hot. If the wire is very thin, even a small current makes it hot. This happens in an electric lamp.The electric wires in a house are covered with some kind of insulation. No current can flow through the insulation; so the current can never flow straight from one wire to the other. But the insulation on old wires can tough. A large current may flow; and if this happens, the wires will get very hot. Then the house may catch fire.Fuses can stop this trouble. A fuse is only a thin wire which easily melts. It is fixed in a fuse-holder. The fuse-holder is made of some material which cannot burn. A large current makes the fuse hot and then it melts away. Wesay that the fuse “blows”. The wire is broken, and no current can flow. So the house does not catch fires go out because there is no current.When a fuse blows, something is wrong. We must find the fault first. Perhaps two wires are touching. We must cover them with new insulation of some kind. Then we must find the blown fuse and repair it. We put a new piece of fuse-wire in the holder. (Sometimes we can find the others are cold.) If we do not repair the fault first, the new fuse will blow immediately.Some people get angry when a fuse blows. So they put a thick copper wire in the fuse-holder! Of course this does not easily melt; if the current rises suddenly, nothing stops it. The thick wire easily carries it. Then the wires of the house may get very hot, and the house may catch fire. Some of the people in it may not be able to escape. They may lose their lives. So it is always best to use proper fuse-wire. This will keep everyone and everything in the house safe.(3) AutotransformersA transformer in which the primary and secondary windings are connected electrically as well as magnetically is called an autotransformer. Figure shows a connection diagram of an autotransformer. If this transformer is to be used as a step-down transformer, the entire winding ac forms the primary winding and the section ab forms the secondary winding. In other words, the section ab is common to both primary and secondary. As in the standard two-winding transformer, the ratio of voltage transformation is equal to the ratio of primary to secondary turns if the losses and exciting currents are neglected and Figure 11 represents an autotransformer winding with a total of 220 turns, with the sections ab and bc having 150 and 70 turns respectively. If a voltage of 440 V is applied to the winding ac, the voltage across each turnwill be 2 V. The voltage from a to b will then be 150×2,or 300 V.When a non inductive load of 30 ohms is connected to winding ab, a current, I X, of 300/30 or 10A flows and the power output of the transformer is 300×10 or 3,00 W. Neglecting the transformer losses, the power input must be 3,000 W and the primary current 3,000/440 or 6.82 A.An application of Kirchhoff’s current law to point a shows that when IX is 10 A and IH is 6.82 A then the current form b to a must be 3.18 A. Similarly, the current from b to c must be 6.82 A.Thus the section of the winding that is common to both primary and secondary circuits carries only the difference in primary and secondary currents. In effect, the transformer in the example transforms only 3.18×300=954W rather than the full circuit power of 3000W. The percentage of power transformed is 100×954/3,000 or 31.8 percent. This is the same as the percent voltage difference between the primary and secondary voltage or (440－300)/440=0.318 or 31.8 percent. Since only a part of the circuit power or KV A is transformed by an autotransformer, it is smaller and more efficient than a two-winding transformer of a similar rating.For some applications that require a multivoltage supply, an autotransformer in which the winding is tapped at several points is used. The connections from the various taps are brought out of the tank to terminals or to a suitable switching device so that any one of several voltages may be selected.Autotransformers are used when voltage transformations of near unity are required. Such an application of an autotransformer is in “boosting” a distribution voltage common application is in the starting of ac motors, in which case the voltage applied to the motor is reduced during the startingperiod.Autotransformers are not safe, however, for supplying a low voltage from a high-voltage source; for, if the winding that is common to both primary and secondary should accidentally become open, the full primary voltage will appear across the secondary terminals. The requirements of safety codes should always be followed whenever autotransformers are applied.(4) High-voltage fusesHigh-voltage fuses are used both indoors and outdoors for the protection of circuits and equipment with voltage ratings above 600 V. These are many types of fuses and they are mounted in many different ways. Some of the more commonly used fuses and mountings are mentioned briefly in the following paragraphs.Expulsion fuses consist of a fusible element mounted in a fuse tube and depend upon the vaporization of the fuse element and the fuse-tube liner to expel conducting vapors and metals from the fuse tube, thereby extinguishing the arc formed when current is interrupted. Another type of fuse, called the liquid fuse, depends on a spring mechanism to separate quickly the ends of the melted fuse element in a nonflammable liquid to extinguish the arc. Still another type of fuse is the solid-material fuse, in which the arc is extinguished in a hole in a solid material. In one type of solid-material fuse a spring mechanism similar to that of the liquid fuse is used to separate the arcing terminals when the fuse blows. In this fuse, overload and low fault currents are interrupted in a small cylindrical chamber in the solid arc-extinguishing material, and large fault currents are interrupted in a lager chamber in the same fuse holder.High-voltage fuses are often mounted in the same enclosure withdisconnect switches to provide short-circuit protection and switching facilities for circuits and equipment. Typical equipment of this type removed from its enclosure is shown in circuits and consists of a three-pole load-interrupter switch above and three solid-material fuses below.Outdoor high-voltage fuses for low-capacity overhead lines are mounted in distribution fuse cutouts. Cutouts consist of a fuse support and fuse holder in which the fuse link is installed. One commonly used type of fuse cutout is the drop-out enclosed cutout. In this type of cutout, the fuse holder is enclosed within a porcelain housing. The fuse holder which contains the fuse link is mounted on the inside of the hinged enclosure door and is so arranged that it is connected into the circuit when the cutout door is closed. When the fuse link melts in clearing a short circuit, the cutout door drops open, thereby providing an indication of the blown fuse. The cutout is placed back in service by inserting a new fuse link in the fuse holder and closing the cutout door.Fuse mountings for high interrupting-capacity high-voltage outdoor fuses are mounted on insulators, as shown in Fig.5-11. The size of the insulators and the spacing between phases is dependent upon the protection of circuits, transformers and other equipment where the system short-circuit currents are high.(5) High-voltage Circuit BreakersThe term high-voltage circuit breaker as used here applies to circuit breakers intended for service on circuits with voltage ratings higher than 600 V. High-voltage circuit breakers have standard voltage ratings of from 4,160 to 765,000 V and three-phase interrupting ratings of from 50,000,000 kV A. Breakers with even higher ratings are being developed.During the early development of electrical systems, the vast majority of high-voltage breakers used were oil circuit breakers. However, air circuit breakers of the magnetic and compressed-air types have been developed and are now in common use.The magnetic air circuit breaker is available in ratings up to and including 750,000 kV A at 13,800 V. In this type of breaker the current is interrupted between separable contacts in air with the aid of magnetic blowout coils. As the main current-carrying contacts part during the interruption of a fault, the arc is drawn out in a horizontal direction and transferred to arcing contacts. At the same time, the blowout coil is connected into the circuit to provide a magnetic field to draw the arc chutes. The arc accelerates upward, aided by the magnetic field and natural thermal effects, into the arc chutes where it is elongated and divided into small segments. The arc resistance increases until, as the current passes through zero, the arc is broken; after this it does not reestablish itself.The general construction of the magnetic power circuit breaker is somewhat similar to the large air circuit breaker used on low-voltage circuits except that they are all electrically operated. These breakers are used extensively in metal-clad switchgear assemblies in industrial plants, steel mills, and power plants.Compressed-air breakers (sometimes called air-blast breakers) depend upon a stream of compressed air directed toward the interrupting contacts of the breaker to interrupt the arc formed when current is interrupted. This type of breaker was introduced to the American market in 1940 and since that time it has become universally accepted for use in heavy-duty indoor applications. More recently, air-blast breakers have been developed for use inextra-high-voltage outdoor stations with standard ratings up to 765,000 V.Oil circuit-breaker contacts are immersed in oil so that the current interruption takes place under oil which by its cooling effect helps quench the arc. Since oil is an insulator, the live parts of oil circuit breakers may be placed closer together than they could be in air. The poles of small oil circuit breakers are all placed in one oil tank, but in the large high-voltage breaks each pole is in a separate oil tank. Tanks of small breakers are suspended from a framework so that the tanks may be lowered for inspection of the contacts. The tanks of very large oil circuit breakers rest directly on a foundation and have hand holes for access to the contact assembly.The oil tanks of oil circuit breakers are usually sealed, the electrical connections between the external circuit and the contacts in the tank being made through porcelain bushings. The breaker contacts are opened and closed by means of insulated lift rods on which the movable contacts are mounted. The lift rods are connected to the operating mechanism by means of a mechanical linkage, so that the contacts of all poles of the breaker are opened and closed together.Only the very small oil circuit breakers are manually operated. The larger oil circuit-breaker mechanisms are either pneumatically operated or spring operated. Pneumatic operators obtain the closing and tripping energy from compressed air provided by a small, automatically controlled air compressor that maintains enough compressed air for several operations of the breaker in an air receiver. Spring operators derive their energy from a spring that is compressed by a small electric motor.Indoor oil circuit breakers are generally assembled into metal-clad switchgear units, although oil breakers are being replaced in many cases byair circuit breakers. Oil circuit breakers are used under adverse atmospheric conditions such as in oil refineries where there is danger of explosion from any open arc. Outdoor oil circuit breakers are usually frame-mounted and are set individually on concrete footings, with open overhead connections being made to the breaker bushings. A typical frame-mounted outdoor oil circuit breaker is shown in Fig. 5-12.(6) Protective relaysLow-voltage air circuit breakers ordinarily have self-contained series trip coils of either the instantaneous or time-delay types. The tripping energy is supplied by the flow of the short-circuit current through the trip coil. Power circuit breakers seldom use series trip coils but are equipped with trip coils designed to operate from a storage battery or a reliable source of alternating current. Auxiliary devices called protective relays, designed to detect the presence of short circuits on a system, are used to connect the breaker trip coils to the source of tripping power and thereby trip the breaker. Protective relays are said to be selective when they trip only the circuit breakers directly supplying the defective part of the system and no other circuit breakers. When relays and circuit breakers are selective, short circuits are removed from a system with a minimum of service interruption. Of course, it is also desirable to isolate the defective system element as quickly as possible. To this end, relays and circuit breakers have been developed that will clear a shot circuit in less than 0.1 s. however, selectivity being more important than speed, the tripping of some circuit breakers on a system is delayed intentionally to gain selectivity in clearing faults at certain locations on a system.Protective-relay operating elements are connected to high-voltage circuits。
常微分方程的英文Ordinary Differential EquationsIntroductionOrdinary Differential Equations (ODEs) are mathematical equations that involve derivatives of unknown functions with respect to a single independent variable. They find application in various scientific disciplines, including physics, engineering, economics, and biology. In this article, we will explore the basics of ODEs and their importance in understanding dynamic systems.ODEs and Their TypesAn ordinary differential equation is typically represented in the form:dy/dx = f(x, y)where y represents the unknown function, x is the independent variable, and f(x, y) is a given function. Depending on the nature of f(x, y), ODEs can be classified into different types.1. Linear ODEs:Linear ODEs have the form:a_n(x) * d^n(y)/dx^n + a_(n-1)(x) * d^(n-1)(y)/dx^(n-1) + ... + a_1(x) * dy/dx + a_0(x) * y = g(x)where a_i(x) and g(x) are known functions. These equations can be solved analytically using various techniques, such as integrating factors and characteristic equations.2. Nonlinear ODEs:Nonlinear ODEs do not satisfy the linearity condition. They are generally more challenging to solve analytically and often require the use of numerical methods. Examples of nonlinear ODEs include the famous Lotka-Volterra equations used to model predator-prey interactions in ecology.3. First-order ODEs:First-order ODEs involve only the first derivative of the unknown function. They can be either linear or nonlinear. Many physical phenomena, such as exponential decay or growth, can be described by first-order ODEs.4. Second-order ODEs:Second-order ODEs involve the second derivative of the unknown function. They often arise in mechanical systems, such as oscillators or pendulums. Solving second-order ODEs requires two initial conditions.Applications of ODEsODEs have wide-ranging applications in different scientific and engineering fields. Here are a few notable examples:1. Physics:ODEs are used to describe the motion of particles, fluid flow, and the behavior of physical systems. For instance, Newton's second law of motion can be formulated as a second-order ODE.2. Engineering:ODEs are crucial in engineering disciplines, such as electrical circuits, control systems, and mechanical vibrations. They allow engineers to model and analyze complex systems and predict their behavior.3. Biology:ODEs play a crucial role in the study of biological dynamics, such as population growth, biochemical reactions, and neural networks. They help understand the behavior and interaction of different components in biological systems.4. Economics:ODEs are utilized in economic models to study issues like market equilibrium, economic growth, and resource allocation. They provide valuable insights into the dynamics of economic systems.Numerical Methods for Solving ODEsAnalytical solutions to ODEs are not always possible or practical. In such cases, numerical methods come to the rescue. Some popular numerical techniques for solving ODEs include:1. Euler's method:Euler's method is a simple numerical algorithm that approximates the solution of an ODE by using forward differencing. Although it may not provide highly accurate results, it gives a reasonable approximation when the step size is sufficiently small.2. Runge-Kutta methods:Runge-Kutta methods are higher-order numerical schemes for solving ODEs. They give more accurate results by taking into account multiple intermediate steps. The most commonly used method is the fourth-order Runge-Kutta (RK4) algorithm.ConclusionOrdinary Differential Equations are a fundamental tool for modeling and analyzing dynamic systems in various scientific and engineering disciplines. They allow us to understand the behavior and predict the evolution of complex systems based on mathematical principles. With the help of analytical and numerical techniques, we can solve and interpret different types of ODEs, contributing to advancements in science and technology.。
物理IG先行课程大纲课程目标：1.掌握IGCSE运动学和力学部分和热物理学的内容，解决初中遗留的物理问题。
2.对理科内容进行中英文衔接，方便理解。
3.为秋季开学做准备，培养对物理的学习兴趣。
第一节课: physical quantities and SI base units, measurement, density-Understand what is a physical quantity-Understand the SI base units of fundamental physical quantities-The relationship between SI base units and derived units-Use and describe the use of rules and measuring cylinders to find a length or a volume-Understand that a Vernier caliper and micrometer screw gauge are used to measure very small distances-Use and describe the use of clocks and devices, both analogue and digital, for measuring an interval of time-Obtain an average value for a small distance and for a short interval of time by measuring multiples (including the period of a pendulum)-Know the prefixes: T, G, M, k, d, c, m, μ, n, p-Describe an experiment to determine the density of a liquid and of a regularly shaped solid and make the necessary calculation-Describe the determination of the density of an irregularly shaped solid by the method of displacement-Recall and use the equation ρ =m/V第二节课: Motion-Understand that vectors have a magnitude and direction-Demonstrate an understanding of the difference between scalars and vectors and give common examples-Distinguish between speed and velocity，displacement and distance,-Determine graphically the resultant of two vectors- Define speed and calculate average speed from total distance/ total time- Define and calculate acceleration using change of velocity/ time taken- Calculate speed from the gradient of a distance-time graph- Calculate acceleration from the gradient of a speed-time graph- Calculate the area under a speed-time graph to work out the distance travelled- Recognise motion for which the acceleration is constant and not constant第三节课：Newton’s laws of motion, force, mass and weight-Distinguish mass and weight-Describe the ways in which a force may change the motion of a body-Recognise that if there is no resultant force on a body it either remains at rest or continues at constant speed in a straight line-Find the resultant of two or more forces acting along the same line-Recall and use the relationship between force, mass and acceleration (including the direction), F = ma-Describe qualitatively motion in a circular path due to a perpendicular force-State that the acceleration of free fall for a body near to the Earth is constant-Describe qualitatively the motion of bodies falling in a uniform gravitational field with and without air resistance (including reference to terminal velocity)-Newton’s third law of motion第四节课：Turning effect, equilibrium, centre of mass-Describe the moment of a force as a measure of its turning effect and give everyday examples-Calculate moment using the product force perpendicular distance from the pivot -Apply the principle of moments to the balancing of a beam about a pivot-Recognise that, when there is no resultant force and no resultant turning effect,a system is in equilibrium-Perform and describe an experiment to determine the position of the centre of mass of a plane lamina-Describe qualitatively the effect of the position of the centre of mass on the stability of simple objects第五节课：Energy, work and Power-Identify changes in kinetic, gravitational potential, chemical, elastic (strain), nuclear and internal energy that have occurred as a result of an event or process -Apply the principle of conservation of energy to examples involving multiple stages-Recall and use the expressions kinetic energy = 1/2mv2 and change in gravitational potential energy = mgΔh-Demonstrate understanding that work done = energy transferred-Relate (without calculation) work done to the magnitude of a force and the distance moved in the direction of the force-Relate (without calculation) power to work done and time taken,第六节课：Momentum-Understand the concepts of momentum and impulse-Recall and use the equation, momentum = mass*velocity, p = mv-Recall and use the equation for impulse Ft = mv – mu-Apply the principle of the conservation of momentum to solve simple problems in one dimension第七节课：Energy resources and pressure-Describe how electricity or other useful forms of energy may be obtained from: –– chemical energy stored in fuel–– water, including the energy stored in waves, in tides, and in water behind hydroelectric dams–– geothermal resources–– nuclear fission–– heat and light from the Sun (solar cells and panels)–– wind-Understand that the nuclear fusion in the Sun is the source of energy for all our energy resources except geothermal, nuclear and tidal-Give advantages and disadvantages of each method in terms of renewability, cost, reliability, scale and environmental impact-Recall and use the equation p = F / A-Relate (without calculation) the pressure beneath a liquid surface to depth and to density, using appropriate examples, and use the equation p = hρg-Describe the simple mercury barometer and its use in measuring atmospheric pressure-第八节课：Simple kinetic molecular model of matter, evaporation and origin of pressure- State the distinguishing properties of solids, liquids and gases- Describe qualitatively the molecular structure of solids, liquids and gases in terms of the arrangement, separation and motion of the molecules- Show an understanding of the random motion of particles in a suspension as evidence for the kinetic molecular model of matter (Brownian motion)- Describe evaporation in terms of the escape of more-energetic molecules from the surface of a liquid- Relate evaporation to the consequent cooling of the liquid- differences between evaporation and boiling- Demonstrate an understanding of how temperature, surface area and draught over a surface influence evaporation- Explain pressure in terms of the change of momentum of the particles striking the walls creating a force第九节课：heat capacity and latent heat-Explain, in terms of the motion and arrangement of molecules, the relative order of the magnitude of the thermal expansion of solids, liquids and gases-Understand differences between internal energy, thermal energy and temperature Show an understanding of what is meant by the thermal capacity of a body and use the equation thermal capacity = mc-Define specific heat capacity, and use the equation change in energy = mcΔT-Describe an experiment to measure the specific heat capacity of a substance-Describe melting and boiling in terms of energy input without a change in temperature-Use the terms latent heat of vaporisation and latent heat of fusion and give a molecular interpretation of latent heat-Define specific latent heat, and use the equation energy = ml-Describe an experiment to measure specific latent heats for steam and for ice。
分子动力学英文Molecular Dynamics (MD) refers to a computer simulation technique that is widely used in various fields of physical, chemical, and biological sciences to study the behavior of molecules and materials at the molecular level. It is a mathematical and computational approach that models the motion of a system of molecules by solving the equations of motion of the individual particles.Step 1: Theoretical backgroundThe basic principle of MD is the Newton's laws of motion, which describe the motion of a particle as a function of its position, velocity, and acceleration. MD simulates the motion of a system of particles by calculating the forces between them and then integrating the equations of motion to obtain their trajectories in time.Step 2: Simulation ProcessMD simulates the motion of a system of molecules in a box, where the size and shape of the box, the number of molecules and their positions, and the temperature and pressure of the system can be controlled. The simulation process involves the following steps:- Initialization: The simulation box is set up, and the initial positions and velocities of the molecules are assigned based on a chosen distribution.- Force calculations: The forces acting on the molecules are calculated using interatomic or intermolecular potential energy functions that describe the interactions between the particles.- Time integration: The equations of motion are solved numerically to obtain the positions and velocities of the molecules at each time step. The time step is typically inthe femtosecond range to capture the fast vibrations and rotations of the molecules.- Analysis: The trajectories of the molecules are analyzed to obtain various properties of the system, such as thediffusion coefficient, the radial distribution function, and the energy profile.Step 3: ApplicationsMD has a wide range of applications in materials science, chemistry, biology, and even finance. It can be used to study the behavior of materials under different conditions, such as melting, solidification, and deformation. It can also be used to investigate the interaction between molecules andbiological systems, such as protein-ligand binding and drug discovery. In finance, MD has been applied to study the behavior of financial markets and the dynamics of price movements.In conclusion, molecular dynamics is a powerful simulation technique that allows researchers to study the behavior of molecules and materials at the molecular level.It provides insights into the physical and chemicalproperties of the system that cannot be obtained from experiments alone. With the development of more sophisticated algorithms and computer hardware, MD is expected to play an increasingly important role in scientific research and technological innovation in the future.。
mof 分子动力学Molecular Dynamics Simulation (MDS) is a powerful computational technique used to study the motion and behavior of atoms and molecules over time. It has become an essential tool in various fields of science, including chemistry, physics, and materials science. This article will provide an overview of MDS and discuss its applications and advantages.In MDS, the motion of atoms and molecules is simulated by solving classical equations of motion. By numerically integrating these equations, the positions and velocities of the particles can be calculated at each time step. The simulation starts from an initial configuration and proceeds by iteratively updating the positions and velocities of the particles based on the forces acting on them.One of the key advantages of MDS is its ability to provide atomic-level insights into the behavior of complex systems. By simulating the motion of individual particles, scientists can study phenomena that are difficult or impossible to observe experimentally. For example, MDS can be used to investigate the folding of proteins, the diffusion of molecules in liquids, or the behavior of materials under extreme conditions.MDS also allows for the exploration of different physical and chemical properties of materials. By simulating the behavior of atoms and molecules under different conditions, scientists can study the thermodynamics, kinetics, and transport properties of materials. This information is crucial for the design and development of new materials with specific properties.In addition to its scientific applications, MDS is also used in industrial settings for the optimization of processes and the design of new products. For example, MDS can be used to study the flow of fluids in pipes, the behavior of nanoparticles in suspensions, or the interactions between drugs and biological targets. By simulating these processes, scientists and engineers can gain valuable insights that can lead to improved efficiency and performance.Despite its many advantages, MDS does have some limitations. One major limitation is the computational cost associated with simulating large systems or long time scales. As the number of particles or the simulation time increases, the computational requirements can become prohibitivelyhigh. However, advances in computer hardware and simulation algorithms have greatly improved the efficiency of MDS, allowing for the simulation of increasingly complex systems.Another limitation of MDS is the accuracy of the interatomic potential, which describes the forces between atoms and molecules. The choice of potential can greatly influence the results of the simulation, and it is important to select an appropriate potential for the system under study. Developing accurate and reliable potentials is an active area of research in the field of MDS.In conclusion, MDS is a powerful computational technique that allows scientists to study the motion and behavior of atoms and molecules. It has numerous applications in various scientific and industrial fields and provides atomic-level insights into complex systems. Despite its limitations, MDS continues to play a crucial role in advancing our understanding of the microscopic world.。
分子动力学模拟流程Molecular dynamics simulation is a powerful tool in the field of computational chemistry and physics. It allows researchers to study the movement of atoms and molecules over time, providing valuable insights into the behavior of materials at the molecular level. By simulating the interactions between particles based on classical mechanics, scientists can explore various physical and chemical processes in great detail.分子动力学模拟是计算化学和物理领域中一种强大的工具。
它允许研究人员随着时间的推移研究原子和分子的运动，为在分子水平上材料的行为提供有价值的见解。
通过基于经典力学对粒子之间的相互作用进行模拟，科学家可以详细地探索各种物理和化学过程。
One of the key advantages of molecular dynamics simulation is its ability to capture the dynamics of complex systems that are difficult to study experimentally. By monitoring the trajectories of individual particles in a simulated environment, researchers can observe how macroscopic properties emerge from the interactions of atoms and molecules. This information is crucial for understanding the behaviorof materials under different conditions and for designing new materials with desired properties.分子动力学模拟的一个关键优势是它能够捕获实验难以研究的复杂系统的动态。
第11卷第1期 2021年1月农业工程Agricultural EngineeringVol. 11 No. 1J a n.2021基于E D E M_F lu e n t耦合的钙果风筛式清选装置仿真与试验吴楠，贺俊林，刘少华，何永强(山西农业大学农业工程学院，山西晋中030801 )摘要：针对钙果收获装置作业后钙果含杂率高的问题，设计了一种钙果风筛式清选装置。
运用E D E M-F l u e n t耦合方法，以风速、振动筛振幅和振动频率为试验因素，钙果的清洁率和损失率为评价指标，对清选过程进行了仿真分析，并依据仿真结果进行台架试验。
仿真分析与台架试验表明，随着风速、振动筛振动频率与振幅的增加，钙果清洁率先增大后减小，损失率一直增加。
最优工作参数组合为风速10 m/s、振幅10 m m和振动频率9 H z，最优工作参数组合条件下的钙果清洁率96.3%，损失率3. 4%。
研究表明，EDEM-F l u e n t耦合仿真的运用有助于钙果清选研究，研究结果可为钙果清选装置的设计与优化提供理论依据。
关键词：钙果；清选装置；E D E M-F lu e n t;台架试验中图分类号：S225 文献标识码：A文章编号：2095-1795(2021)01-0082-06Simulation and Experiment of Air Screen Cleaning Device forC e r a s u s h u m ilis Based on EDEM-Fluent CouplingWU Nan,HE Junlin,LIU Shaohua,HE Yongqiang(College o f Agricultural Engineering, Shanxi Agricultural University，Jinzhong Shanxi 03080J , China)A b stract：In order to solve problem of high impurity rate of Cerasus humilis after operation of Cerasus humilis harvesting unit, a kind of air screen cleaning device for Cerasus humilis was designed. With coupling method of EDEM-Fluent, cleaning process was simulated and analyzed by taking vibration am plitude, frequency and wind speed as experimental factors, cleaning rate and loss rate of Cerasus humilis as evaluation indexes, and cleaning device were tested according to simulation results. Simulation a- nalysis and bench test showed that with increase of wind speed, vibration frequency and am plitude, cleaning rate of Cerasus humilis firstly increased but then d ecreased, and loss rate always increased. Optimal operating param eter combination was wind speed 10 m/s, amplitude 10 mm and vibration frequency 9 Hz. Under optimal operating param eter combination, cleaning rate of Cerasus humilis was 96. 3%, and loss rate was 3. 4% . Study indicated that application of EDEM-Fluent coupling simulation was helpful to research of Cerasus humilis cleaning. Study results could provide a theoretical basis for design and optimization of device for Cerasus humilis cleaning.K eyw ords：Cerasus humilis, cleaning device, EDEM-Fluent, bench test〇引言钙果，学名欧李，富含钙元素，营养含量高[^。
