The physical essential chapters of the middle school entrance examination are summarized: "voice phenomenon" knowledge points and method skills summarization summary

The second grade students who have just started to study the eighth grade physics have also learned about two chapters. Many students will mistakenly think that physics is simple. Even now, I only have one formula. It is so difficult to say, and it is not like high school students saying physics is the most difficult subject.

Indeed, at the beginning of the eighth grade physics, the knowledge of simple, easy -to -understand, interesting and closely connected in life was put on the previous book. The scientific methods of physical problems, the standard steps of experimental inquiry, the good habit of reflection and induction ... instead of just to master the physical knowledge!

Knowing this, students in the second grade must not be gently affected by physics.

Even in the first few chapters that look simple and easy to understand and interesting, there are still many knowledge that is easy to, often wrong, and difficult to understand. If you only know how to memorize your death, or do not pay attention to the scientific inquiry process in the experiment, do not pay attention to the connotation of the physical quantity and extension, and not pay attention to the flexible use of two physical formulas, you still cannot achieve the ideal results.

Take the simplest and most interesting content: the chapter of "Sound Phenomenon", its content mainly includes: sound generation and communication, three characteristics of sound, and its decisive factors, the use of sound, the harm and control of noise.

Among them, the three characteristics of the sound are the most difficult to understand and almost belong to the knowledge points that must be tested every year!

Let's summarize today and summarize the must -have test and error -prone knowledge points in these knowledge points:

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1. The emergence and spread of sound

1. The sound is generated by the "vibration" of the object. The word "vibration" of the vibration must not be written as "shock"!

2. When the vibration stops, the sound stops, but the previous sound still spreads to the distance until the energy is exhausted.

3. All vocal objects are vibrating, and all vibration objects are sounding, but the sound can be heard but requires a lot of conditions: there must be sound source, there must be sound medium, the loudness must reach a certain degree, the frequency must be in the people to be human beings Within 20Hz-20000Hz).

4. Sound source can be solid, liquid, gas, and sound can also be spread in solids, liquids, and gas. Generally, the sound speed meets V solid greater than V liquid greater than V gas. The sound speed in the air. In the same medium, the higher the temperature, the larger the sound speed.

5. There are two most important and most common experiments in this chapter: one is "alarm clock or mobile phone ringtone in the vacuum cover"; the other is "the psychic bounce on the table tennis experiment". Explain here:

first:

The "alarm clock or mobile phone ringtone" experiment in the vacuum cover is "experimental facts+scientific reasoning"! Because we cannot achieve absolute vacuum, the conclusion of the last step "vacuum cannot pass" can only be obtained through scientific reasoning.

Follow: second:

The "alarm clock in the vacuum cover" experiment can only show that "vacuum cannot pass sound, and the dissemination of sound requires a medium";

The "mobile phone ringing in the vacuum cover" can explain two points:

(1) "Vacuum cannot be passed, and the sound of sound requires a medium; electromagnetic waves can be spread in real vacuum, and the transmission of electromagnetic waves does not require a medium"

(2) In the process of continuous breath, the sound of the sound becomes smaller, but the sound is unchanged!

Regarding the "Yin fork bounces on table tennis experiments", we must know that its role can be used to obtain two conclusions: "Verifying sounds are generated by the vibration of objects", "What factors are related to the sound of the sound!" Both use a very important and commonly used scientific method- "conversion method"! The former was to convert the tiny vibration of the sound fork into the vibration of the table tennis; the latter was to convert the loudness into the height of the table tennis!

6. When the echo heard the echo more than 0.1s later than the original sound, that is, the distance between the human and the obstacles is above 17m, to distinguish the echo and the original sound. Otherwise, the echo and the original sound will be enhanced by the original sound!

7. Be sure to pay attention to the "echo ranging" and its similar questions (laser ranging). Because the one -way distance is required, and the total time of two -way test is given in the test questions, the sound speed and time are multiplied by each other. At the time, the two -way distance is obtained, so one -way distance is required, and you must divide 2.

8. Generally, there are two ways to conduct sounds when people hear the sound: one is air conduction, and the other is bone conduction; related deafness is divided into "conduction deafness" and "neurotic deafness". To the sound.

Second, the three characteristics of the sound and its decisive factors

1. The three characteristics of sound include: tone, loudness, and sound!

2. Tone: That is the sound of sound! The height of the tone depends on the "frequency"! The frequency determines the level of tone! Here is the word "fast, slow" that often appears in the test questions. Any word when encountering these two words, without speaking, is directly discussing the level of "tone" instead of loudness! The frequency of fast vibration objects is large, and the sound is high! The frequency of slow vibration objects is small, and the tone is low!

Take two most commonly examined examples:

The shorter the string in the string instrument (the finger holds different positions), the shorter the air column in the tube instrument (the fingers hold different holes), the faster The larger the tone, the higher the tone. The lower the lower.

When knocking on the bottle and blowing bottle, the changes in the tone depends on the length of the main sound. During the knock, the water column is the main sound. The shorter the water column, the higher the tone. The shorter, the higher the tone! When it comes to the frequency, you must pay attention to human "sounding sound" and "unable to smell"!

The frequency range that human ear can hear is between 20Hz-20000Hz. The sound below 20Hz is a secondary sound wave, and the sound of 20000Hz is an ultrasonic wave. Remember that the second sounds and ultrasonic waves are all sounds! Different animals are different from people. Sometimes, in an environment where people think that in the environment, the dogs hear secondary sounds and cats hear ultrasonic waves, and they all become alert. What is even more amazing is that the elephant can use the second sound wave communication information that cannot be heard by people. The secondary sound waves in nature are often caused by large natural disasters such as earthquakes, volcanoes, typhoons, tsunami, etc. These secondary sound waves often cause the internal organs of some animals or fish to be ruptured and the frequency of the second sound waves is the same as that of its internal organs. Causes internal organs).

3. Like: The strength of the sound (or size)! The size of the loudness mainly depends on the "amplitude"! The larger the amplitude, the greater the loudness! Of course, the loudness is also related to the distance from the distance. The farther the distance, the smaller the loudness. The longest "force" in the test question, as long as it encounters strong force, indicates a large amplitude, the loudness is strong! Small force and small shadow, the loudness is small!

4. Tone: The quality of sound (sound quality, sound)! The sound depends on the "material, structure" of the sound!

5. Based on the above, many students do not know that there is no relationship between the three special nature of the sound. Remember: the sound of the sound of high sound is not necessarily loud, and the sound tone with loud loudness is not necessarily high! For example, mosquito sounds small, high sound, high sound sound, low sound sound, low sound tone! Therefore, there is nothing to do between tone, loudness, and sound!

6. About the waveform of music:

We can compare the tone height of the tone by observing the degree of density of the waveform (the highest point or the minimum point of the number or the lowest point), and the thinner the waveforms, the slower the vibration vibration, the smaller the frequency, and the lower the tone! The higher the higher!

By observing the amplitude (the highest point of the waveform or the lowest point to the balance position), the comparison is loud! The larger the amplitude, the greater the loudness, and the smaller the other!

By observing the shape of the waves, the sound is the same! Different shapes, the sound is different!

Related exercises are as follows:

7. For example, the "variable tone whistle" often appeared in textbooks and test questions, some use the piston up and down to promote the changes in the air column, and some use scissors to cut short! When the piston is pushed up or cuts with scissors, the air column becomes shorter and the tone becomes higher!

Third, the use of sound

The types of regular exam questions in this section are as follows:

1. Example of ultrasonic transmission information: bats of bats, ultrasonic blind guidance, reversing radar (this radar uses ultrasonic waves, most of the other radar uses electromagnetic waves), sons, B -ultrasound, detection cracks.

Example of ordinary sound wave transmission information: stethoscope.

2. Example of ultrasonic transmission of energy: cleaning clocks, cleaning glasses, and removing stones.

Examples of ordinary sound waves: sound waves extinguish the candle.

3. Echo wall: The principle of multiple reflection of the wall in the wall is used.

4. Harm and control of noise

1. From the perspective of physics: noise is the sound made when the sound is not regular vibration, and the waveform of the noise is messy. Le Yin is the sound made by the sound of the sound to make a regular vibration, and the waveform of Le Yin is ruled.

2. From the perspective of environmental protection: Everyone who hinders rest, study, work, and interference is noise!

3. Dating is "unit of strong and weak sound", not level!

4.0 decibel is the weakest voice that people can just hear, rather than no sound, no sound!

5. Control noise can start from three aspects:

"Sound source" to prevent noise generation; "during the process of communication" blocking the spread of noise; "at the ears" to prevent noise from entering the ear!

The above content is all the knowledge points and errors of the full knowledge of the "Sound Phenomenon" chapter of the junior high school! It can be used as an important reference for preview and review.

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