The summary of physics knowledge in the eighth grade volume II of people's Education Press It must be the second volume of the eighth grade

The summary of physics knowledge in the eighth grade volume II of people's Education Press It must be the second volume of the eighth grade

PEP 8th grade physics
Chapter one sound phenomenon
Acoustic phenomenon
1. The occurrence of sound
All objects that are making sound are vibrating. When the vibration stops, the sound stops
Sound is produced by the vibration of an object, but not all the sound produced by vibration can be heard
2. Sound transmission
The transmission of sound needs medium, but vacuum cannot transmit sound
(1) Sound depends on all gases, liquids and solids as media. These media are called media. Even if astronauts on the moon talk face-to-face, they also need to rely on radio. That is because there is no air on the moon and vacuum can't transmit sound
(2) Sound in different media has different propagation speed, generally speaking, solid > liquid > air
The speed of sound in the air is about 340 m / s
3. Echo
In the process of sound transmission, when encountering obstacles and being reflected back, the sound that people hear again is called echo
The condition of distinguishing echo from original sound: the echo reaches the ear of the person who reaches the target more than 0.1 second later than the original sound. Therefore, the sound must be reflected back by an obstacle with a distance of more than 17 meters before the echo can be heard
When it is less than 0.1 second, the reflected sound can only strengthen the original sound
The echo can be used to measure the depth of the sea or the distance between the speaker and the obstacle
4. Musical sound
The sound of regular vibration is called musical sound
Three elements of musical sound: pitch, loudness and timbre
The higher the frequency, the higher the tone
The loudness is related to the amplitude of the vibration of the speaker and the distance from the sound source to the human ear
The quality of sound produced by different speakers is called timbre. It is used to distinguish different sounds
5. Noise and source
From the physical point of view, noise refers to the sound produced by the speaker when it vibrates irregularly. From the perspective of environmental protection, all the sounds that hinder people's normal rest, study and work, as well as the sounds that interfere with people's listening, belong to noise
6. The division of sound level
People use decibels to divide the sound level. 30db-40db is an ideal quiet environment. If it is more than 50dB, sleep will be affected. If it is more than 70dB, conversation will be interfered and work efficiency will be affected. If you live in a noise environment of more than 90dB for a long time, hearing will be affected
7. Ways to reduce noise
It can be weakened at the sound source (noise elimination), in the process of transmission (sound absorption) and at the human ear (sound insulation)
Chapter 2 light phenomenon
1. Light source: the object that can emit light by itself is called light source
2. Light propagates along a straight line in a homogeneous medium
The atmosphere is uneven. When light comes from outside the atmosphere to the ground, it bends (mirage, the sun is still below the horizon in the morning, the twinkling of stars, etc.)
3. Speed of light
The speed of light propagation in different substances is generally different, and the fastest in vacuum
The propagation velocity of light in vacuum: v = 3 × 108 M / s, which is close to that in air, 3 / 4V in water and 2 / 3V in glass
4. Application of light linear propagation
It can explain many optical phenomena: laser collimation, shadow formation, lunar eclipse, solar eclipse formation, pinhole imaging and so on
5. Light
Light: a straight line indicating the propagation direction of light, that is, draw a straight line along the propagation route of light, and draw an arrow on the straight line to indicate the propagation direction of light (light is imaginary and does not actually exist)
6. Reflection of light
When light shoots from one medium to the interface of another medium, part of the light returns to the original medium, which changes the direction of light propagation. This phenomenon is called light reflection
7. The law of light reflection
The reflection light is on the same plane as the incident light and the normal; the reflected light and the incident light live separately on both sides of the normal; the reflection angle is equal to the incident angle
It can be summarized as: "three lines are coplanar, two lines are separated, and two corners are equal."
reason
The reflected light is determined by the incident light, and the word "reverse" should be used in the narration
Conditions of reflection: the junction of two media; occurrence: the point of incidence; result: return to the original medium
The reflection angle increases and decreases with the increase of the incident angle. When the incident angle is zero, the reflection angle also becomes zero
8. Two reflection phenomena
Specular reflection: the parallel light is reflected by the interface and is emitted parallel along a certain direction. The reflected light can only be received in a certain direction (the reflecting surface is a smooth plane)
Diffuse reflection: the parallel light is reflected to different directions after being reflected by the interface, that is, the reflected light can be received in different directions (the reflecting surface is a rough plane or curved surface)
Note: both specular reflection and diffuse reflection follow the law of light reflection
9. In the reflection of light, the light path is reversible
10. The effect of plane mirror on light
(1) Imaging (2) changing the direction of light propagation
11. Characteristics of planar mirror imaging
(1) (2) the line between the image and the object is perpendicular to the mirror, and the distance between the image and the object and the mirror is equal
The image and object formed by the plane mirror are symmetrical figures with the mirror as the axis
12. The difference between real image and virtual image
The real image is formed by the convergence of the real light, which can be connected with the screen and, of course, can be seen with the eyes
The virtual image is not formed by the convergence of the actual light, but by the intersection of the reverse extension lines of the actual light
13. Application of plane mirror
(1) Reflection in water (2) plane mirror imaging (3) periscope
Chapter 3 lens and its application
1. Refraction of light
When light slants from one medium into another, its propagation direction will generally change. This phenomenon is called refraction of light
The refraction of Ricoh and the reflection of light both occur at the junction of two kinds of media, only the reflected light returns to the original medium, and the refracted light enters into another medium. Because the propagation speed of light in two different materials is different, the propagation direction changes at the junction of two kinds of media, which is the refraction of light
Note: at the junction of the two media, refraction must occur at the same time as reflection,
The speed of light must change in refraction, but not in reflection
2. Refraction law of light
When light slants into water or other medium from air, the refracted light is in the same plane with the incident light and normal, and the refracted light and incident light occupy two sides of the normal; the refracted angle is less than the incident angle; when the incident angle increases, the refracted angle also increases; when the light is perpendicular to the surface of the medium, the propagation direction does not change, and the light path is reversible in refraction
The principle of refraction can be divided into three points: (1) three lines are coplanar; (2) two lines are separated; (3) the relationship between two angles can be divided into three cases: ① when the incident light is perpendicular to the interface, the refraction angle is equal to the incidence angle, which is equal to 0 °; ② when the light slants from air into water and other media, the refraction angle is less than the incidence angle; ③ when the light slants from water and other media into air, the refraction angle is greater than the incidence angle
3. In the refraction of light, the light path is also reversible
4. Lens and classification
Lens: made of transparent material (usually glass), at least one surface is part of a sphere, and the thickness of the lens is much smaller than the radius of the sphere
Classification: convex lens: thin edge, thick center
Concave lens: thick edge, thin center
5. Main optical axis, optical center, focus and focal length
Main optical axis: a straight line passing through two ball centers
Optical center: there is a special point on the main optical axis, through which the direction of light propagation remains unchanged. Focus: the convex lens can make the light parallel to the main axis converge at a point on the main optical axis, which is called the focus of the lens, and is represented by "F"
Virtual focus: the light parallel to the main optical axis becomes divergent after passing through the concave lens. The reverse extension line of divergent light intersects at a point on the main optical axis, which is not the convergence point of actual light, so it is called virtual focus
Focal length: the distance from the focal point to the optical center is called focal length, expressed by "F"
Each lens has two focal points, a focal length and an optical center
6. The effect of lens on light
Convex lens: converges light
Concave lens: divergent to light
7. Imaging law of convex lens
Object distance (U) imaging size virtual real object position image distance (V) application
U > 2F zoom out the f < V U magnifying glass on both sides of the real image lens
[convex lens imaging law oral decision memory method]
"One focus is divided into virtual and real, two focus is divided into size; the virtual image is on the same side, and the far image becomes larger; the real image is on the opposite side, and the far image becomes smaller."
8. In order to make the screen like "upright" (up), slide should be inserted upside down
9. The lens of the camera is equivalent to a convex lens, and the film in the darkbox is equivalent to a light screen. We adjust the focusing ring, not the focus, but the distance from the lens to the film. The farther the object is from the lens, the closer the film should be to the lens
The fourth chapter is the change of state
1. Temperature: the degree of heat and cold of an object is called temperature
2. Centigrade temperature (symbol: T, unit: centigrade)
Sweden's schersius regulation: ① the temperature of pure ice water mixture is set at 0 ℃; ② the temperature of pure water boiling at 1 standard atmospheric pressure is set at 100 ℃; ③ the temperature between 0 and 100 ℃ is divided into 100 equal parts, each equal part is one ℃
3. Thermometers
Principle: the liquid is made of the nature of heat expansion and cold contraction
Construction: glass shell, capillary, glass bubble, scale and liquid
Use: before using the thermometer, pay attention to observe the range and recognize the division value
When using a thermometer to measure the temperature of a liquid, do the following three things:
① All the glass bubbles of the thermometer should be immersed in the object to be measured; ② read after the indication is stable; ③ when reading, do not take the thermometer out of the liquid, and the line of sight should be level with the upper surface of the liquid level,
4. Main differences between thermometer, experimental thermometer and thermometer
How to construct the scale value
There is a necking on the top of the glass bulb of the thermometer, which is 35-42 ℃ and 0.1 ℃ away from the human body to read
The experimental thermometer has no - 20 - 100 ℃, 1 ℃ can't leave the reading of the object to be measured, and can't swing
There is no - 30 - 50 ℃ for the thermometer, and 1 ℃ is the same as above
5. Melting and solidification
When a substance changes from a solid state to a liquid state, it is called melting
The change of a substance from a liquid to a solid is called solidification, which is exothermic
6. Melting point and freezing point
Solids can be divided into crystalline and amorphous
Melting point: crystal has a certain melting temperature, called melting point; non crystal has no melting point
Freezing point: crystal has a certain freezing temperature, called freezing point; non crystal has no freezing point
The freezing point of a substance is the same as its melting point
The conditions of crystal melting are as follows: 1) reaching the melting point temperature; 2) continuing to absorb heat from the outside
The conditions for the liquid to solidify into crystal are as follows: (1) reaching the freezing point temperature; (2) continuing to release heat to the outside world
[memory] some common crystals and non crystals
7. Vaporization and liquefaction
The change of substance from liquid state to gas state is called vaporization. There are two different ways of vaporization: evaporation and boiling, both of which are endothermic
The change of a substance from a gaseous state to a liquid state is called liquefaction. There are two different ways of liquefaction: lowering the temperature and compressing the volume
8. Evaporation phenomenon
Definition: evaporation is the phenomenon that liquid can vaporize at any temperature and only on the surface of liquid
Factors affecting evaporation speed: liquid temperature, liquid surface area, liquid surface air flow speed
9. Boiling phenomenon
Definition: boiling is a violent vaporization phenomenon that occurs at a certain temperature inside and on the surface of the liquid at the same time
The conditions of liquid boiling are as follows: (1) temperature reaches boiling point; (2) continue to absorb heat
10. Ascending and condensing
The direct change of substance from solid state to gas state is called sublimation, and the direct change of substance from gas state to solid state is called sublimation
Sublimation and sublimation in daily life
Sublimation endothermic, sublimation exothermic
[memory method]
Evaporation boiling
difference
Severity and temperature of occurrence site