Finding the formula of the second volume theorem of physics in junior high school

Finding the formula of the second volume theorem of physics in junior high school

1. Ohm's law I = u / R2. Joule's law q = i2rt formula: 1. Electric work w = uit (w = i2rt, w = u2t / R can be deduced from Ohm's law) 2. Electric power P = w / T, P = UI (P = I2R, P = U2 / R can be deduced from Ohm's law) 3. Series circuit I = I1 = I2, u = U1 + U2, r = R1 + R24. Parallel circuit I =

Laws of physics in the second year of junior high school

Outline of physics review in grade two
1、 Measurement of length
1. Measurement of length
Length measurement is the most basic measurement, the most commonly used tool is the scale
2. Unit and conversion of length
The international unit of length is meter (m). The commonly used units are kilometer (km), decimeter (DM), centimeter (CM), millimeter (mm), micron (UM) and nanometer (nm)
1Km 103 m 10 m 10 dm 10 cm 10 mm 103um 103 nm
In the conversion of length units, small units are multiplied by large units, and large units are divided by small units
3. Correct use of scale
(1) Pay attention to observe the zero scale mark, range and graduation value before use
(2) Pay attention when using
① The ruler should be placed along the measured length and aligned with the measured object
② Do not use the worn zero mark. If another whole mark is taken as the zero mark due to the wear of the zero mark, never forget to subtract the scale value of the substituted zero mark from the final reading
③ The thick ruler should be placed vertically
④ When reading, the line of sight should be perpendicular to the ruler surface
4. Record the measured values correctly
The measurement results consist of numbers and units
(1) It is meaningless to write only numbers without units
(2) When reading, the next digit of the graduation value of the scale should be estimated
5. Error
Difference between measured and true values
Error can not be avoided, can be minimized, error can not be avoided
The basic method to reduce the error is to calculate the average value by multiple measurements. In addition, selecting precision instruments and improving the measurement method can also reduce the error
6. Special method measurement
(1) Cumulative method
Such as measuring the diameter of the fine wire or the thickness of the paper
(2) Caliper method
(3) Substitution method
2、 Simple exercise
1. Mechanical movement
The change in the position of an object is called mechanical motion
All objects are in motion, and there is no absolutely immobile object. That is to say, motion is absolute. We usually say that motion and stillness are relative to another object (reference object). Therefore, the description of motion is relative
2. Reference object
The object selected as the standard in the study of mechanical motion is called reference object
(1) The reference objects are not all stationary objects relative to the ground. We just assume that the objects are stationary when we choose which object as the reference object
(2) The reference object can be selected arbitrarily, but the description of the same object's motion may be different with different reference objects
3. Relatively static
Two objects moving at the same speed and in the same direction, or the position between them remains unchanged, are relatively stationary
4. Uniform linear motion
Constant speed, through the line is a straight line of motion, called uniform linear motion
Uniform linear motion is the simplest mechanical motion
5. Speed
(1) Speed is a physical quantity that indicates the speed of an object
(2) In the uniform linear motion, the speed is equal to the distance of the moving object in unit time
(3) Velocity formula: v = st
(4) Unit of speed
International unit: M / s common unit: km / h 1m / S = 3.6 km / h
6. Average speed
The ratio of the time it takes for a moving object to pass a certain distance is called the average speed of the object on this distance
Leveling speed must indicate the average speed of the distance or time
7. Measuring average speed
Principle: v = s / T
Measuring tools: scale, stopwatch (or other timer)
3、 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 objects, but not all vibrations will produce sound
2. Sound transmission
The transmission of sound needs medium, but vacuum cannot transmit sound
(1) The sound is transmitted by all kinds of gas, liquid and solid 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) The propagation speed of sound is different in different media
3. Echo
In the process of sound transmission, when encountering obstacles and being reflected back, the sound that people hear again is called echo
(1) The condition of distinguishing echo from original sound: the echo reaches the ear of the target more than 0.1 second later than the original sound
(2) When it is less than 0.1 second, the reflected sound can only strengthen the original sound
(3) The echo can be used to measure the depth of the sea or the distance between the sounding body and the obstacle
4. Tone
The higher the frequency, the higher the tone
5. Loudness
The size of sound is called loudness. Loudness is related to the amplitude of the vibration of the speaker and the distance from the sound source to the human ear
6. Timbre
The quality of the sound produced by different speakers is called timbre
7. Noise and source
From the physical point of view, noise refers to the sound produced by the speaker when it vibrates irregularly and disorderly. From the perspective of environmental protection, any sound that hinders people's normal rest, study and work belongs to noise
8. 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
9. Ways to reduce noise
It can be weakened at the sound source, in the process of transmission and at the human ear
4、 Thermal phenomenon
1. Temperature
The degree of heat and cold of an object is called temperature
2. Centigrade temperature
The temperature of ice water mixture is 0 ℃ and the temperature of boiling water at 1 standard atmospheric pressure is 100 ℃
3. Thermometers
(1) Principle: the liquid is made of the nature of heat expansion and cold contraction
(2) Construction: glass shell, capillary, glass bubble, scale and liquid
(3) Use: before using the thermometer, pay attention to observe the range and recognize the division value
Do the following three things with a thermometer
① The thermometer is in full contact with the object to be measured
② Wait until the indication is stable before reading
③ When reading, the line of sight should be level with the upper surface of the liquid level, and the thermometer should still be in close contact with the object to be measured
4. Main differences between thermometer, experimental thermometer and thermometer
How to construct the scale value
There is a necking above the glass bubble of the thermometer, 35-42 ℃ 0.1 ℃ (1) it leaves the human body to read
② Need to throw before use
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
(1) Solids can be divided into crystalline and amorphous
(2) Melting point: crystals have a certain melting temperature, called melting point
Freezing point: the crystal has a certain solidification temperature, called freezing point
The freezing point of a substance is the same as its melting point
7. The change of a substance from a liquid state to a gaseous state is called vaporization. There are two different ways of vaporization: evaporation and boiling, both of which are endothermic
8. Evaporation phenomenon
(1) Definition: evaporation is the phenomenon that liquid can vaporize at any temperature and only on the surface of liquid
(2) Factors affecting evaporation speed: liquid temperature, liquid surface area, liquid surface air flow speed
9. Boiling phenomenon
(1) Definition: boiling is the intense vaporization of liquid inside and on the surface at the same time
(2) The conditions of liquid boiling are as follows: (1) temperature reaches boiling point; (2) continue to absorb heat
10. Ascending and condensing phenomena
(1) 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
(2) Sublimation and sublimation in daily life
11. Sublimation endothermic, sublimation exothermic
5、 Reflection of light
1. Light source: the object that can emit light is called light source
2. Light propagates along a straight line in a homogeneous medium
The atmosphere is uneven, and when light hits the ground from outside the atmosphere, it bends
3. Speed of light
The speed of light propagation in different substances is generally different, the fastest in vacuum,
The propagation velocity of light in vacuum: C = 3 × 108 M / s, which is close to that in air, 3 / 4C in water and 2 / 3C 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 on one side, two lines separated, two corners equal"
reason
(1) The reflected light is determined by the incident light, and the word "reverse" should be used in the narration
(2) Conditions of reflection: the junction of two media; occurrence: the point of incidence; result: return to the original medium
(3) 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
(1) Specular reflection: the parallel light is reflected by the interface and is emitted in a certain direction, and can only receive the reflected light in a certain direction
(2) Diffuse reflection: parallel light reflected by the interface is reflected in different directions, that is, reflected light can be received in different directions
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 size of the image and the object. (3) 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 actual light, which can be received by the screen, and of course can be seen by 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, which can only be seen by the eyes, but not by the screen
13. Application of plane mirror
(1) Reflection in water (2) plane mirror imaging (3) periscope
6、 Refraction of light
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, both refraction and reflection occur
2. Refraction law of light
When light slants from air into water or other medium, the refracted light is at the same level as the incident light and normal