This is a good question to connect with the realities of life, but the standard answers provided are incorrect and not a general typographical error. Because this problem also appears in some popular materials, and the same mistakes are given to the answers. The structure and measurement principle of the AC voltmeter and the understanding of the RMS value of the alternating voltage are a debatable question.
This brings the following thoughts: Thinking question I: The AC voltmeter in the question must have a reading. The structure of the AC voltmeter is different from the DC voltmeter. The DC voltmeter is composed of the galvanometer head G and the voltage dividing resistor A. The head G is a magnetoelectric meter and cannot be directly used to measure the AC voltage. Because the current is passed through the coil inside the ammeter, the generated magnetic moment is related to the direction of the current, and the thermal effect of the AC forward and reverse current is the same. , independent of current direction. Therefore, in order to measure AC with a DC meter, it is necessary to connect one in series with the DC voltmeter (pD. Crystal Diode D rectification, as shown by LT-JlTjN, because it is half-wave rectification, the positive ffi2 string alternating voltage is cut off. Half of the time, the meter measures only the voltage of a pulsating DC. In order to accurately measure the effective value of a complete sinusoidal alternating voltage, the voltage dividing resistor in the AC meter changes accordingly. In fact, it is measuring a pulsating DC voltage of a half wave, and the number read from the dial represents the effective value of the complete sinusoidal alternating voltage. * The conditions given by the above problem can be seen, the input to the electric blanket Is a sinusoidal alternating voltage, through the device P to make the input voltage into a city in June 2002, the test of the comprehensive test of science comprehensive ability test volume has such a question: (a) is the circuit diagram of the electric blanket, the electric heating wire is connected u =311Sin〗 00: r For the non-sinusoidal alternating voltage, the definition of the effective value is used, that is, the effective value of the alternating voltage is obtained according to the thermal effect of the current. Both AC and DC have their effective values. The effective value of DC can also be obtained by using the definition of AC rms. This should be said to be a generalization of the concept of AC RMS. Although the above problem is a pulsating direct current, we can use the rms definition of the alternating current to find the effective value of the voltage: for the first 772 in the image of Figure Ub, the same as the sinusoidal alternating voltage, the voltage RMS: (/=/*//5=220V, then we take a period of r to study the effective value of the pulsating DC, set the effective value of the voltage to f/', there is: this is the answer given in the title (B) The corresponding value, but considering the construction and measurement principle of the AC voltmeter, it is impossible to see this answer. Thinking question 3: Can the AC voltmeter be able to measure the effective value of the alternating voltage? As mentioned above, the AC voltmeter is set to measure the sinusoidal alternating voltage. In fact, half-wave rectification through the crystal diode D, the meter M is only a half cycle voltage condition, which is used to replace the RMS value of the whole cycle. Therefore, it is meaningless to use an AC voltmeter to measure the RMS value of a pulsating DC in the previous question, either no reading or reading The effective value of the original sinusoidal alternating voltage is 220V, so the exchange * teaching materials and teaching methods * talk about the effective value of the alternating current voltmeter and alternating voltage Wanliancheng (Luohe City High School ... Henan Luhe 462000) voltmeter measured values ​​and It is not the voltage rms value to be measured 156. On the other hand, when the AC voltage meter is used to measure the effective value of the AC voltage, if the AC voltage is an alternating voltage with a non-sinusoidal alternating voltage or a sinusoidal waveform distortion, The measured readings are also very different from the actual values. In fact, the measured number of turns is not the effective value of the alternating voltage. The alternating voltage shown (the forward and reverse parts of this alternating voltage are part of the sinusoidal alternating voltage, but the maximum value of the voltage is different), and the effective value of the alternating voltage can be obtained from the definition of the effective value. : In a period 71, a resistor / is added, and the heat generated by the alternating voltage as described is (1: there is a heat generated by adding a constant voltage, and there is (2 according to the effective value, the current heat effect is equal) As pointed out in the high school physics textbook: the values ​​measured by the general AC ammeter and the AC voltmeter are all valid values. It should be understood that it is limited to sinusoidal alternating current and is not applicable to other forms of alternating current. (Editor's note: There are many kinds of AC meters, and the measurement principle is not the same. This article only adds a half-wave rectification device to the magnetoelectric meter, which is correct for AC measurement. Other kinds of AC meters and their working principles, readers Please refer to the books on electrical measurement.) (Continued from page 4) * Self-domains such as: "Ideal self", "cooperation and competition, cherish life" and other topics with life meaning and life value are dusted or discarded; The choice of the subject is too social and adult, lacking research topics that are compatible with the students' thinking and knowledge level. Third, the method of inquiry only focuses on the procedures and methods that need to be followed to obtain scientific conclusions. The individualized inquiry method of students is blocked. In the scientific inquiry, the colorful exploration paths such as speculation, imagination, and sentiment are interrupted. The content of the ideal course starting from the students is replaced by the importance of the adult society, and the scientific inquiry learning is stuck in the quagmire of scientific rationalism. Can't extricate themselves, the pursuit of accepting its ideas is overhead, and the fact that value is weakened. To change this reality, Of course, scientific inquiry does not have to contradict scientific research, but the focus of our pursuit is the awareness of the problems formed by students, the ability to think independently, the scientific attitude of seeking truth from facts, and the personality of sound development. 2.2 Scientific inquiry should prevent the tendency to be excessively narrow in function. Scientific inquiry p learning is not only the way and the way to acquire knowledge - the more important baby is bred in the exploration of knowledge - the awareness of the problem, personally find and practice the way to solve the problem. Lead to changes in learning styles. However, in the practice of scientific inquiry learning, real schools often attach importance to their knowledge acquisition function, while ignoring the function of promoting the change of learning mode, so that the phenomenon of “teaching†learning methods and research methods indoctrination is made, which makes the function of scientific inquiry function. It has been greatly narrowed. Although guiding students to know how to explore is the proper goal of scientific inquiry, but from the actual exploration activities, only the learning methods and research methods are extracted for guidance, in fact, it is still the teaching of knowledge, but the method is taught. know how. The effectiveness of this is indeed doubtful, because the method of inquiry is personalized, prior teaching can not exhaust all the methods that may be used in student inquiry, and one-sided emphasis on operational skills is likely to hinder the growth of student problem awareness in the process of inquiry activities. . To avoid this phenomenon, it is necessary to make the acquisition and application learning methods and research methods themselves part of the scientific inquiry of students, let students face the research methods of the subject, and understand the meaning of the research methods based on the experience experienced. Of course, scientific inquiry that values ​​students' self-experience and inquiry does not mean giving up the guidance of teachers. If in the name of respecting the subjectivity, neglect the guidance, or even give up guiding the students to embark on a correct and reasonable way of exploration, the student's activities will be trapped in the misunderstanding of empiricism. In order to give full play to the function of scientific inquiry learning, and to avoid students' free inquiry activities and laissez-faire, teachers can guide students' inquiry process through appropriate means.
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