Induction of physics knowledge in the last semester of the second grade of junior high school

Notes on physical knowledge in the first volume of the second day of junior high school

1, the appearance of sound

Everything that makes a sound vibrates. When the vibration stops, the sound will also stop.

Sound is produced by the vibration of objects, but not all the sounds produced by vibration can be heard by human ears.

2. Transmission between voices

The transmission of sound needs medium, and vacuum cannot transmit sound.

(1) All gases, liquids and solids can transmit sound. These substances as media are called media. Even if astronauts on the moon talk face to face, they still need to rely on radio. That's because there is no air on the moon, and vacuum can't spread sound.

(2) Sound travels at different speeds in different media. Generally speaking, solid >: liquid > air.

The speed of sound in the air is about 340 meters per second.

Step 3 echo

In the process of sound propagation, the sound that people hear again when they encounter obstacles is called echo.

The distinguishing condition between echo and original sound: the echo reaches people's ears more than 0. 1 second later than the original sound. Therefore, the sound must be reflected by obstacles with a distance exceeding 17m, so that people can hear the echo.

Below 0. 1 sec, the reflected sound can only strengthen the original sound.

Use echo to measure the depth of the sea or to detect how far an object is from an obstacle.

4. Music

The sound made by an object when it vibrates regularly is called music.

Three elements of musical sound: tone, loudness and timbre.

The sound level is called tone, which is determined by the vibration frequency of the speaker. The higher the frequency, the higher the pitch.

The size of sound is called loudness, which is related to the amplitude of speaker vibration and the distance from the sound source to the human ear.

The quality of sound produced by different vocal bodies is called timbre. Used to distinguish different sounds.

5. Noise and its sources

From the physical point of view, noise refers to the sound made by the vocal body when it vibrates irregularly. From the perspective of environmental protection, all sounds that hinder people's normal rest, study and work, as well as those that interfere with people's desire to hear, belong to noise.

6, the division of sound level

People use decibels to divide sound levels. 30 dB -40 dB is an ideal quiet environment. If it exceeds 50dB, it will affect sleep; If it exceeds 70dB, it will interfere with the call and affect the work efficiency. If you live in a noise environment above 90dB for a long time, it will affect your hearing.

7. Ways to reduce noise

It can be weakened at the sound source (noise elimination), during propagation (sound absorption) and at the human ear (sound insulation).

Chapter II Light Phenomenon

1, light source: An object that can emit light by itself is called a light source.

2. Light propagates in a straight line in a uniform medium.

The atmosphere is unbalanced. When light hits the ground from outside the atmosphere, it will bend (a mirage, when you see the sun in the morning, the sun is still below the horizon, and the stars are flashing, etc. ).

3. the speed of light

The propagation speed of light in different substances is generally different, and it is the fastest in a vacuum.

The propagation speed of light in vacuum is V = 3× 108 m/s, which is close to this speed in air, 3/4V in water and 2/3V in glass.

4. Application of linear propagation of light.

Many optical phenomena can be explained: laser collimation, shadow formation, lunar eclipse, solar eclipse, pinhole imaging and so on.

5. light

Light: a straight line indicating the direction of light propagation, that is, draw a straight line along the direction of light propagation, and draw an arrow on the straight line to indicate the direction of light propagation (light is imaginary, but actually does not exist).

6. Light reflection

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 reflected light is on the same plane as the incident light and the normal line; Reflected light and incident light are separated on both sides of the normal; The reflection angle is equal to the incident angle.

It can be summarized as: "Three lines * * * plane, two lines are separated, and the two angles are equal."

Understand:

The reflected light is determined by the incident light, and the word "anti" should be used as the head in the narrative.

Conditions of reflection: the junction of two media; Place of occurrence: incident point; Result: Return to the original media.

The reflection angle increases with the increase of incident angle and decreases with the decrease of incident angle. When the incident angle is zero, the reflection angle also becomes zero.

8, two kinds of reflection phenomenon

Specular reflection: parallel rays are reflected by the interface and emitted in parallel in a certain direction, and the reflected rays can only be received in a certain direction (the reflecting surface is a smooth plane).

Diffuse reflection: the parallel light is reflected in 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 influence of plane mirror on light.

(1) imaging (2) changing the propagation direction of light.

1 1, the characteristics of plane mirror imaging

(1) is an isometric virtual image. (2) The connecting 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.

Understanding: The image and object formed by the plane mirror are symmetrical figures with the mirror as the axis, that is, the plane mirror is the middle vertical line connecting the object and the image.

12, the difference between real image and virtual image

The real image is the gathering of actual light, which can be received by the screen and of course seen by the eyes.

The virtual image is not formed by the convergence of actual light, but by the intersection of opposite extension lines of actual light, which can only be seen by the eyes and not received by the screen.

13, the application of plane mirror

(1) reflection in water (2) flat mirror imaging (3) periscope

Chapter III Lenses and Their Applications

1, light refraction

When light inclines from one medium to another, the propagation direction generally changes, which is called light refraction.

Understanding: the refraction of light occurs at the junction of two media just like the reflection of light, except that the reflected light returns to the original medium and the refracted light enters the other medium. Because the propagation speed of light in two different substances is different, the propagation direction changes at the junction of two media, which is the refraction of light.

Note: At the junction of two media, refraction must occur simultaneously.

In refraction, the speed of light must change, while in reflection, the speed of light remains the same.

2, the law of refraction of light

When light is obliquely incident from the air into water or other media, the refracted light is on the same plane as the incident light and the normal line, and the refracted light and the incident light are separated on both sides of the normal line; The refraction angle is smaller than the incident angle; When the incident angle increases, the refraction angle also increases; When the light is perpendicular to the surface of the medium, the propagation direction is unchanged and the refraction of the light path is reversible.

Understanding: The law of refraction is divided into three points: (1) Three lines * * * Plane (2) Two lines are separated (3) The relationship between the two angles is divided into three situations: ① When the incident light is incident on the vertical interface, the refraction angle is equal to the incident angle is equal to 0; (2) When light is obliquely incident from air to water and other media, the refraction angle is smaller than the incident angle; (3) When light enters the air obliquely from a medium such as water, the refraction angle is greater than the incident angle.

3. The optical path is also reversible in terms of light refraction.

4, lens and classification

Lens: Made of transparent material (usually glass), at least one surface is a part of a spherical surface, and the thickness of the lens is much smaller than its spherical radius.

Classification: convex lens: thin edge and thick center.

Concave lens: thick edge and thin center.

5. Main optical axis, optical center, focal point and focal length

Principal optical axis: a straight line passing through two spherical centers.

Optical center: there is a special point on the main optical axis, through which the propagation direction of light remains unchanged. Focus: The point at which the convex lens can converge the light parallel to the main axis 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, and the backward extension line of the divergent light intersects with a point on the main optical axis, which is not the convergence point of the actual light, so it is called virtual focus.

Focal length: The distance from the focal point to the optical center is called focal length, which is indicated by "F".

Each lens has two focal points, a focal length and an optical center.

6. The influence of lens on light

Convex lens: convex light.

Concave lens: It diverges light.

7, convex lens imaging law

The object distance (U) should be used for imaging size, virtual image position and image distance (V).

U & gt2f reduces both sides of the real-image lens F.

U = 2f, and both sides of the large real-image lens v = 2f.

f & ltu & lt2f。 V & gt2f slide projector on both sides of magnifying real image lens

U = f is not like a picture.

U<f magnifying virtual image lens on the same side v > U magnifying glass

Oral memory method of convex lens imaging law

"One focus is virtual reality, and the other focus is size; The same side of the virtual image is positive, and the far image of the object becomes larger; The real image is on the opposite side and the object is much smaller. "

8. In order to make the image on the screen "vertical" (upward), the slide should be inserted backwards.

9. The lens of a camera is equivalent to a convex lens, and the film in the camera box is equivalent to a light screen. We adjust the focus ring, not the focal length, but the distance from the lens to the film. The farther the object is from the lens, the closer the film is to the lens.

Chapter IV Changes in the State of Matter

1, temperature: the degree of heat and cold of an object is called temperature.

2. Celsius temperature (symbol: T unit: Celsius)

Shertius of Sweden stipulates: ① The temperature of pure ice-water mixture is set at 0℃; ② The boiling temperature of pure water at 1 standard atmospheric pressure is set to100℃; ③ Divide the temperature of 0 ~ 100℃ into 100 equal parts, each equal part is 1℃.

3. Thermometer

Principle: It is made by using the property of liquid expanding when heated and contracting when cooled.

Structure: glass shell, capillary, glass bubble, scale and liquid.

Usage: Before using the thermometer, pay attention to the measuring range and recognize the dividing value.

When measuring the liquid temperature with a thermometer, the following three points can be achieved:

(1) The glass bulb of the thermometer should be completely immersed in the measured object; (2) Wait until the numbers are stable before reading; (3) When reading, do not take the thermometer out of the liquid, and the line of sight should be flush with the upper surface of the liquid level.

4. The main differences between thermometers, experimental thermometers and thermometers.

A construction method of range division value

When the thermometer is at 35-42℃ and 0. 1℃, there is a mark on the top of the glass bulb, which is convenient for reading when leaving the human body. It should be thrown away before use.

The experimental thermometer must be -20- 100℃, 1℃. You can't read without the measured object, and you can't throw it away.

The thermometer is not -30-50℃ 1℃.

5, melting and solidification

The change of a substance from solid to liquid is called melting, and melting requires heat absorption.

When a substance changes from liquid to solid, it is called solidification, and solidification gives off heat.

6. Melting point and freezing point

Solids can be divided into crystalline and amorphous.

Melting point: all crystals have a certain melting temperature, which is called melting point; Amorphous crystals have no melting point.

Freezing point: the crystal has a certain freezing temperature, which is called freezing point; Amorphous crystals have no freezing point.

The freezing point and melting point of the same substance are the same.

Conditions for crystal melting: ① reaching the melting point temperature; ② continuing to absorb heat from the outside.

Conditions for liquid to solidify into crystals: ① reaching freezing point temperature; ② Continuously release heat to the outside.

Memorize some common crystals and amorphous.

7. Vaporization and liquefaction

When a substance changes from a liquid state to a gas state, it is called vaporization. There are two different ways of vaporization: evaporation and boiling, both of which absorb heat.

When a substance changes from gas to liquid, it is called liquefaction. There are two different ways of liquefaction: lowering the temperature and compressing the volume, both of which give off heat.

8, evaporation phenomenon

Definition: Evaporation is a phenomenon that liquid can occur at any temperature and only occurs on the surface of liquid.

The factors that affect the evaporation rate are: liquid temperature, liquid surface area and air velocity on the liquid surface.

9, boiling phenomenon

Definition: Boiling is a violent vaporization phenomenon both inside and on the surface of liquid at a certain temperature.

Conditions for liquid boiling: ① The temperature reaches the boiling point; ② Continue to absorb heat.

10, sublimation and solidification

The direct change of matter from solid to gas is called sublimation, and the direct change from gas to solid is called sublimation.

Sublimation and condensation in daily life (frozen wet clothes dry, frost in winter)

Sublimation absorbs heat and condensation releases heat.

mnemonics

Steaming and boiling

discrepancy

Influencing factors of temperature change under severe temperature conditions in the place where it occurs

similar

sublimate

┌—————————┐

Melting vaporization

Solid → liquid → gas (endothermic)

- - - - - - - - - - -

Gas → liquid → solid (endothermic)

Liquefaction and solidification

└—————————┘

sublimate

Chapter V Current and Circuit

Simple electrical phenomenon circuit

1, charge charge is also called electricity, which is a property of matter.

(1) only positive and negative charges. The same charge as the glass rod rubbed by silk is called positive charge; The charge that is the same as that of the rubber rod rubbed by fur is called negative charge.

(2) The same charges repel each other, and the different charges attract each other.

(3) charged body has the characteristics of attracting light and small objects.

(4) The amount of charge is called electricity.

⑤ Electroscope: an instrument used to check whether an object is charged. Its working principle is that like charges repel each other.

2. Conductors and insulators are easily conductive objects, called conductors. Metal, human body, earth and aqueous solution of acid, alkali and salt are all common conductors. Objects that do not conduct electricity easily are called insulators. Rubber, plastic, glass and ceramics are common insulators.

Understanding: The division between conductor and insulator is not absolute. When conditions change, insulators can also become conductors. For example, glass is a good insulator at room temperature and becomes a conductor at high temperature. For another example, under normal circumstances, there are few charged particles (free electrons, positive ions and negative ions) that can move freely in gas, so gas is a good insulator. However, under the action of strong electric field force or when the temperature rises to a certain extent, gas discharge occurs due to the ionization of gas, and then the gas changes from insulator to conductor. Therefore, there is no absolute boundary between conductor and insulator. When conditions change, insulators and conductors can be transformed into each other.

3. The circuit will be a current path connected with electrical appliances, power supplies and switches by wires.

Three states of a circuit: a circuit that is connected everywhere is called a path or a closed circuit, at which time a current flows; Open circuit is called open circuit, when there is no current in the circuit; A circuit that directly connects the two poles of the power supply with wires is called a short circuit.

4. Circuit connection mode Series circuit and parallel circuit are the basic ways of circuit connection.

Understanding: The basic method to identify a circuit is the current method, that is, when the current passes through various components on the circuit, there is no shunt phenomenon, and the connection relationship of these components is in series. If there is shunt phenomenon, the connection relationship between elements on several shunt branches is parallel.

5. The circuit diagram symbolically represents the graph of circuit connection.