The equivalent substitution method refers to the method that in the study of a physical phenomenon and law, due to the special limitations of the experiment itself or the limitations of the experimental equipment, it is impossible or difficult to directly reveal the physical essence, and the equivalent phenomenon with similar or the same characteristics is used for substitution. If this method is used properly, it can not only draw a conclusion smoothly, but also be easily accepted and understood by students.
For example, in the experiment of exploring the imaging law of flat mirror, it is easy for students to accept the glass plate instead of the flat mirror because of its similar imaging characteristics. Moreover, the glass plate is transparent, and the candle behind the glass plate can be observed through it, which is convenient for studying the characteristics of the image and revealing the law. In our teaching, teachers pay attention to guiding students to summarize methods and inspire them from the way of thinking on the basis of students' experience of the experimental process. They will consciously use relevant experimental designs in the future. For example, after learning to measure resistance by voltammetry, students are required to design an experiment. In the above experiment, a voltmeter or ammeter is missing, and other equipment remains unchanged. There is also a fixed resistance with known resistance to choose from, so it is necessary to measure the unknown resistance. What should I do? Students can design with the idea of equivalent replacement.
Conversion method
Some physical quantities are not convenient for direct measurement, and some physical phenomena are not convenient for direct observation. By transforming them into physical phenomena equal to or related to them, we can draw a conclusion. For example, in the experiment of studying the relationship between electric heating power and resistance, the heat generated by current passing through two resistance wires with different resistance values cannot be directly observed and compared, but we can observe the temperature change of kerosene by converting it into the heat absorption of kerosene, thus deducing that the resistance emits more heat. In teaching, we might as well ask: why do we use kerosene, which seems to have nothing to do with the experiment, when studying the relationship between electric heating power and resistance? Arouse students' thinking and discussion. On the basis of summarizing the function of kerosene in this experiment, we put forward: can we use other methods instead of kerosene to observe the heating of resistor after electrifying? This can disperse students' thinking, train students' transformed thinking methods and improve students' ability to design experiments.
In the physics experiment of junior middle school, the length of railway line, the diameter of coin and the height of cone on the map are measured by soft wire with the idea of transformation method and the cooperation of ruler and triangle.
similar
Analogy is a method of reasoning. In order to clarify the physical problems to be expressed, concrete, tangible and familiar things are often used to compare the abstract, intangible and unfamiliar things to be explained, and some features of a familiar object are used to understand and master some features of another similar object. For example, when studying the function of voltage, with the help of the visible and familiar experiment of "water pressure forms water flow", it is revealed that voltage is the cause of current formation. For example, in the experiment of studying the magnetic field of electrified solenoid, in order to accurately remember the relationship between the north pole of electrified solenoid and the current direction, the clenched right fist is compared to solenoid, and the four fingers are coils and point to the current direction, so the thumb refers to the north pole. In this way, the image is intuitive and easy for students to understand and remember. Of course, there are other analogy methods to give full play to students' subjective initiative and find an analogy method that is more in line with students' reality. Figure 9.3-6 of the experimental textbook Physics (the second volume of the eighth grade) published by People's Education Press gives people a good enlightenment.
model method
The method of revealing the shape, characteristics and essence of prototype through model is called model method. Model method indirectly studies the properties and laws of object prototype with the help of material model similar to prototype or abstract thinking model reflecting the essence of prototype. Generally speaking, physical problems are realized by introducing models (symbols, formulas, tables, objects, etc.). It is convenient for us to explain the internal structure, changes and relationships of things that are difficult to observe directly.
For example, the magnetic field is described by magnetic induction lines, and the schematic diagram of light and force is studied.