What Is a Unit Load?

Load is a specialized word in human society, and pinyin is fù hè. Refers to the external resistance overcome by a machine or driver, and the requirements for the business capabilities of a system (such as circuit switching stations, postal, railways), and the weight carried by an object. Extended to the proportion of resources occupied.

Load power load

Load refers to the power and current passing through wires, cables, and electrical equipment (transformers, circuit breakers, etc.). This load is not a constant value but a fluctuation value that changes with time. Because the electrical equipment does not run at the same time, even if they are running at the same time, they may not all reach the rated capacity at the same time. In addition, the working system of each electrical equipment is also different, there are long-term, short-term, repeated short-time points. In the design, if the capacity of each electrical equipment is simply added up as the basis for selecting the wire, cable section and electrical equipment capacity, the result is not scientific. Either it is too large to make the equipment underloaded and uneconomical; or it is too small to cause overloaded operation, resulting in overheated insulation damage and increased wire loss, which affects the safe operation of wires, cables or electrical equipment. In severe cases, fire accidents can occur. To avoid this situation, a hypothetical load or calculated load is used to design the total load of the system. The use of calculated loads to select conductors, cable sections, and electrical equipment is closer to reality, because the thermal effects of calculated loads are equal to the thermal effects of varying loads.

Calculated load is also called required load or maximum load. The calculated load is an imaginary continuous load, and its thermal effect is equal to the maximum thermal effect produced by the actual variable load over a period of time. In the design of power distribution, the maximum average of 30 minutes is usually used as the basis for selecting electrical conductors based on civil thermal conditions.

The means for calculating the load is called load calculation. The methods commonly used in China to determine the calculation load include the need coefficient method and the binomial method. The advantage of the need coefficient method is that it is simple and suitable for the calculation of the load of the entire plant and the workshop substation. The binomial method is suitable for the calculation of the load of the main line and branch main line in the machining workshop with large capacity equipment. However, in determining the calculation load of branch trunks with a small number of equipments and widely differing equipment capacities, the use of the binomial method is more reasonable than the need coefficient method, and the calculation is simpler.

Load fire load

Fire load is a parameter to measure the amount of combustible materials contained in a building's interior, and it is a basic element for studying the characteristics of a fire during its comprehensive development. To put it simply, it is the total energy that all combustible materials in the building volume may release due to combustion. When a fire occurs in a building, the fire load directly determines the duration of the fire and the change in indoor temperature. Therefore, it is necessary to understand the concept of fire load and reasonably determine the value of fire load when designing fire protection of building structures.

Fire load value

Combustibles in buildings can be divided into fixed combustibles and contained combustibles. Fixed combustibles refer to combustibles used in structural materials and decoration materials such as walls, ceilings, floors, and combustible materials used in doors, windows, and fixed furniture. Contained combustibles refer to combustibles consisting of furniture, books, clothing, bedding, and furnishings. The fixed amount of combustibles is easily obtained from the design drawings of the building. It is difficult to accurately calculate the amount of contained combustibles. Generally determined by survey statistics.

Combustibles in buildings

There are many types of combustibles in buildings, and their calorific value varies with different material properties. In order to facilitate the research, in practice, a certain material is often converted into wood of equivalent calorific value according to the heat of combustion, and the weight of the equivalent wood is used to represent the quantity of combustibles, which is called the equivalent combustibles. Generally speaking, a large space contains more combustibles than a small space, so the amount of equivalent combustibles is related to the size of the building area or volume. In order to facilitate the study of fire behavior, the quantity of equivalent combustible wood per unit floor area in the fire area is generally defined as the fire load.

Determination of fire load

The relationship between the fire load and the severity of the fire is obvious. There is no fire without combustibles; the more fuel, the more severe the fire. Therefore, the calculation of fire load is very important. However, not only is the quantity of combustibles important, but also the type of combustible materials in a unit space. Because some materials release more energy per unit mass than others when burning. This is why fire loads are often expressed in MJ rather than kg. Sometimes, we use some data that we know well, such as by equivalently converting the thermal energy of all combustible materials in a space into the equivalent amount of wood to represent the fire load in the interval.

Fire load density

That is, Fire Load Density, the fire load per unit building area.

Load physiological load

The load or resistance that a muscle encounters before it contracts is called preload. It is the muscle that has a certain initial length before contracting. In a certain range, the tension generated by muscle contraction is proportional to the initial length of the muscle before contraction. If it exceeds a certain limit, it has a reverse relationship. That is, in the initial stage, as the initial length increases, the muscle tension also increases; when the muscle is at the optimal initial length (optimal front load), the contraction produces the maximum tension. When the initial length is increased, the muscle tone decreases. When the muscles are at the optimal initial length, if the isometric contraction is started, the shortening speed is the fastest, the shortening degree is the largest, and the work efficiency is the highest.

Afterload refers to the load or resistance encountered after the muscle begins to contract. When muscles contract under post-load conditions, tension increases first, and then muscle contraction occurs. Within a certain range, the greater the post-load, the greater the tension generated, and the time it takes for the muscles to begin to shorten is delayed, the slower the rate of shortening. When the afterload increases to a certain value, the tension produced by the muscle reaches its maximum. At this time, the muscle does not shorten at all. The initial velocity is equal to zero. The tension produced by the muscle is inversely related to the initial velocity when it contracts. Therefore, muscles can only get the best results when they work with moderate afterload.

Cognitive load

Cognitive load refers to the total amount of psychological resources that a person needs in the process of information processing. Cognitive load theory was first proposed in 1988 by John Swdler, a cognitive psychologist at the University of New South Wales in Australia. It is based on early research by Miller et al., And its purpose is to teach In the design of materials and in the teaching process, the cognitive load that hinders learning is minimized, and the cognitive load that promotes learning is optimized, so that learners can reasonably use limited cognitive resources to achieve the maximum learning effect.

The theory of cognitive load believes that the level of "cognitive load" caused by a certain learning material is mainly determined by three basic factors: the complexity of the learning material, the organization and presentation of the learning material, and the knowledge experience of the learner. Therefore, three types of cognitive load are also formed: external cognitive load, internal cognitive load, and related cognitive load.

In 1988, J. Sweller, a psychologist at the University of New South Wales in Australia, formally proposed the Cognitive Load Theory (CLT) from the perspective of cognitive resource allocation based on the results of modern cognitive psychology research. Then the cognitive load theory began to be widely studied in the field of practical teaching, and some research results were obtained. Many studies have proved that the cognitive load theory provides a new theoretical framework for the study of cognitive processing in the teaching process, and has extremely realistic guiding significance for teaching practice. As Alexander (1997) demonstrated, cognitive load is regarded as an important factor in the design of teaching activities. Whether teaching activities are effective depends on (or in part depends on) whether it can reduce unnecessary cognitive loads.

First, the basic perspective of cognitive load theory

The cognitive load theory is based on the limited working memory of the human brain. In human information processing systems, short-term memory is a type of working memory that mainly processes information extracted from sensorymemory and long-term memory. Throughout the information processing system, It plays a role in controlling the information flow in the information processing system and plays a vital role in the smooth development of cognitive activities. Cognitive load theory is very concerned about the role of working memory in the learning process. Its basic views can be summarized as follows: (1) The capacity of working memory is extremely limited. The capacity of long-term memory is essentially unlimited. All information is Before entering long-term memory, information processing must be performed in working memory; (2) The learning process requires active use of working memory to understand (and process) materials and encode the information to be learned to be stored in long-term memory (3) If the amount of information the learner needs to process exceeds the amount of information the learner's working memory can process, then learning will become ineffective. On this basis, J. Sweller et al. Believe that various cognitive processing activities in the problem solving and learning process need to consume cognitive resources and generate a certain cognitive load. If the total amount of resources required for all activities exceeds The capacity of working memory will cause insufficient resource allocation, which will affect the efficiency of individual learning or problem solving. This situation is called cognitive overload (cognitive overload).

The concept and structure of cognitive load

Since the advent of cognitive load theory in the 1980s, people's understanding of the concept of cognitive load has been divided. For example, Cooper (1990) believes that cognitive load refers to the mental energy level required to process a certain amount of information; J. Sweller et al. (1998) believe that cognitive load is the load generated when a specific task is added to the learner's cognitive system; Xin Ziqiang et al. (2002) believed that cognitive load can be regarded as the mental labor required to process a certain amount of information. As the amount of processed information increases, cognitive load also increases; Lin Chongde et al. (2005) considered that cognitive load refers to Is the amount of intellectual activity imposed on working memory in one case; Lai Risheng et al. (2005) believed that cognitive load refers to the total amount of intellectual activity imposed on working memory in a certain situation; Yang Xinde et al. (2007) believed Cognitive load is the amount of mental capacity required to complete a certain task and perform mental activities on working memory. Combining the above different interpretations, the author believes that cognitive load is the sum of cognitive resources that people actually put into working memory in order to successfully complete specific work tasks, including necessary and unnecessary.

From a structural point of view, cognitive load is composed of intrinsic cognitive load (IntrinsicCognitiveLoad), external cognitive load (ExtraneousCognitiveLoad), and related cognitive load (GermaneCognitiveLoad): (1) the intrinsic cognitive load consists of the inherent characteristics of the material itself (such as Difficulty and complexity) and the learner's original knowledge level, as well as the interaction between the two, determine that the internal cognitive load is relatively fixed and cannot be changed by InstructionDesign, but recently some scholars believe that the internal Cognitive load can be changed (Pollocketal, 2002; Peteretal, 2004); (2) External cognitive load comes from the presentation and teaching design of teaching materials, which is generally related to the irrational organization and design of teaching content, which can be adopted The reorganization and design of teaching content are adjusted; (3) Relevant cognitive load is information related to the subjective field of the learner, and refers to the amount of cognitive resources that the learner intends to invest in the process of schema construction and automation [ 2], it is related to students 'cognitive efforts, improves the students' related cognitive load, and can guide students to use the surplus I cognitive resources carry out deep-level schema construction. Therefore, Bannert (2002) regards it as a tool for schema construction and automation [3].

Enlightenment of cognitive load theory to teaching

In order to prevent the total cognitive load of students from exceeding their working memory capacity in teaching, the teaching principle of the cognitive load theory is to reduce the students' internal cognitive load and external cognitive load as much as possible; and to ensure that there is a surplus of working memory resources Under the premise, students should be appropriately guided to invest more mental effort, increase their cognitive load, and achieve schema acquisition and rule automation. As mentioned earlier, students' internal cognitive load in teaching activities is related to the characteristics of teaching materials and students' cognitive level. External cognitive load is related to the presentation of teaching materials and the level of teaching design. The related cognitive load is related to students' cognitive Knowledge effort (MentalEffort). Therefore, we can optimize the structure of students' cognitive load in teaching activities from the following aspects to improve teaching effectiveness.

(I) Controlling Intrinsic Cognitive Load: Fully consider the characteristics of teaching materials and students' cognitive level and their interaction

It is generally believed that the intrinsic cognitive load is determined by the nature of the learning materials and cannot be changed by changes in instructional design, but experiments by Pollocketal (2002) and Peteretal (2004) have raised objections. [4] The two experiments have in common the use of the information ordering principle of materials to reduce the intrinsic cognitive load. Pollocketal first displays the learning materials in batches to students, and then displays them all at once. This sorting method of learning materials, especially for beginners, can better promote deep understanding of learning materials (quoted from Bannert, 2002 ). Peteretal changed the traditional method of Example Instruction, decomposing the example into several components that can be understood separately. The final results of his five experiments consistently show that the new example presentation method reduces the material brought to students. Intrinsic cognitive load.

TracyClarke, Paul Ayres, and John Sweller also studied a progressive approach to changing inherent cognitive load in their book "The Impact of Sequencing and Prior KnowledgeKnowledgeon Learning Mathematics: Through Spreadsheet Applications". They use Spreadsheet software to develop students' mathematical skills. Students are divided into two categories. One class of students is not very familiar with Spreadsheet, the other group of students have a better understanding of Spreadsheet. Each class of students is divided into two, and they receive two kinds of first training: one is to use Spreadsheet software first, and then to use this software to develop mathematical skills; the other is to practice Spreadsheet software skills and practice mathematical skills at the same time get on. The results show that for students who are not familiar with Spreadsheet in advance, sequential presentation is better than concurrent presentation, and test results are higher; but for students who have already learned Spreadsheet in advance, the situation is the opposite. This shows that for students with low levels of knowledge and skills, the strategy of "learning skills first and then learning the concepts of specific content areas" can improve their learning; for them, technical content is likely to have high element interaction, If technical skills and concepts in specific subject areas are studied simultaneously, the inherent cognitive load may increase [5]. For students with low skill levels, the technical content does not have high-element interaction, while learning technical skills and concepts in specific subject areas, the inherent cognitive load will not increase. When deciding on a sequential strategy, the level of student's technical skills and the level of interactivity of the elements of the content are crucial variables (Kalyuga et al., 2003; Van Merrienboer, Kirschner & Kester, 2003).

(2) Reduce external cognitive load: optimize the presentation of teaching materials and improve the level of teaching design

Reducing the external cognitive load is the focus of "theoretical research of teaching cognitive load". To reduce the students' external cognitive load in teaching, many researchers have put forward their views from different angles. On the basis of reading many documents, the author believes that it can be summed up in one sentence, that is, to optimize the presentation of teaching materials and improve the level of teaching design. Here are a few examples to illustrate.

1. Present teaching materials according to the principle of proximity to reduce attention dispersal and characterization preservation. That is, the presentation of teaching materials in teaching should be as close as possible in space and time. Information involving multiple sources should be physically integrated to reduce the pressure on limited working memory and release cognitive capacity for other information processing. [6] Take the presentation of images and explanatory text as examples. In space, interpretive text should be integrated into the image as much as possible; in time, it should be presented at the same time instead of continuously to form a whole. In this way, learners can find the corresponding visual and word information with the shortest visual search, and it is easy to establish a connection between them to promote information integration. Otherwise, images and explanatory texts are presented separately. "Learners tend to look at the text first and then the corresponding diagrams" (Hegart & Judt, 1989). After reading the text, on the one hand, some cognitive resources are needed to save the text representation Formation of representation preservation; on the other hand, it takes a certain amount of cognitive resources to search for corresponding image information, forming a distraction of attention, which brings additional cognitive load. Therefore, "the complete presentation of text and images is more conducive to the learning of learners than the separate presentation of text and images (Moreno & Mayer, 1999; Mayer, 2001)".

2. Eliminate redundant information to avoid redundant effects (RedundancyEffects). Redundant information is information that is not necessary for the achievement of teaching goals. It is also called redundant information. PaulChandler and JohnSwe11er use experiments to prove that redundant information is not neutral, they will have a negative effect on learning and understanding. [7] The existence of redundant information often forces students to pay attention to some information that could not be paid attention, thereby wasting valuable working memory resources. Therefore, it will bring an additional burden on working memory, which will increase students' external cognitive load. Moreno and Mayer also believe that "when the text is presented in narrative mode alone, the learner's understanding of the presented material is better than the presentation in both narrative and screen text mode" [8]. The textual information presented in screen mode is redundant here. Should be removed.

3. Use the Advance Organizer strategy to change students' cognitive readiness. People's understanding of new knowledge is always based on their original cognitive structure (Cognitive Construction). When the original cognitive structure supports the new knowledge, it is relatively easy for people to understand the new knowledge; on the contrary, if The lack of content that supports new knowledge in the original cognitive structure or related knowledge in the original cognitive structure is not activated, and people's understanding of new knowledge will become relatively difficult. Teachers should use the advance organizer strategy to present students with a higher level of abstraction and inclusive guidance material than the current learning task before the new learning task begins, so as to activate the new knowledge in their cognitive structure. The content of knowledge is well prepared for learning new knowledge. This avoids the cognitive burden of searching for relevant knowledge in long-term memory while learning new knowledge, thereby reducing the external cognitive load.

In addition, by separating audiovisual information, expanding working memory capacity; organizing information presentation to reduce students' cognitive search; abstracting information visualization to reduce students' cognitive difficulty; simplifying complex information (such as using charts) to simplify students' cognitive processing; Control the speed of presentation of teaching materials and leave enough time for cognitive processing; teach students cognitive strategies to improve students' cognitive efficiency; differentiate complex tasks to prevent students from overloading cognitive load; use working-example ) Teaching, simplifying the process of acquiring cognitive skills, etc. can alleviate students' external cognitive load from different perspectives.

(3) Improve related cognitive load: Stimulate students' learning motivation and guide them to increase their cognitive efforts

Relevant cognitive load refers to the amount of cognitive resources that learners intend to invest in the process of schema construction and automation. It is conducive to the acquisition of schemas and the automation of rules, to optimize the cognitive structure of students, and to remove a large amount of complexity and disorder. The information is combined into a simple and orderly knowledge system, which effectively reduces the cognitive load of working memory, thereby saving limited working memory resources. The automation of rules also allows us to process information with the minimum working memory capacity and reduce the burden of working memory. Not only that, the increase in related cognitive load is also conducive to the construction of meaning, helping students to better understand and master what they have learned. It can be seen that under the condition of ensuring that the total cognitive load is not overloaded, appropriately increasing the relevant cognitive load of the students is beneficial to teaching.

The key to improving the relevant cognitive load of students is to stimulate students' learning motivation (especially internal motivation), so that students can increase their own cognitive efforts. To this end, educators need to transform the learning requirements of society and educators to students into the learners' internal learning needs through a variety of ways and means. For example: cultivating and motivating students' learning interest; carrying out purpose-oriented education; using heuristic teaching; establishing a reasonable reward mechanism and so on.