Shanghai Grade 8 Volume 2 physics knowledge points The first chapter is machinery and work The second chapter is heat and energy Just these two chapters The trial edition of Education Press

Shanghai Grade 8 Volume 2 physics knowledge points The first chapter is machinery and work The second chapter is heat and energy Just these two chapters The trial edition of Education Press

The third year of junior high school physics review simple mechanical work, mechanical work, mechanical energy, internal energy Jiangsu Science and Technology Edition 1
Review simple mechanical work, mechanical work, mechanical energy and internal energy
1. Simple machine changes the magnitude and direction of force;
2. The condition and application of lever balance;
3. Mechanical work;
4. Power
5. The concept of mechanical efficiency;
6. Application of power in practice
Key points and difficulties of teaching
Key points:
1. The condition and application of lever balance;
2. Mechanical work;
3. Power;
4. The concept of mechanical efficiency;
5. Application of power in practice
Difficulties:
1. Drawing of force arm;
2. The condition and application of lever balance;
3. Mechanical work;
4. Power;
5. Mechanical efficiency of pulley block;
III. key points of knowledge
(1) leverage
1. Definition of lever: in physics, a hard rod that rotates around a fixed point under the action of force is called a lever. The lever can be straight, curved, or some other deformation, such as a round ball
2. Five elements of leverage:
A. fulcrum: the point around which the lever rotates, usually indicated by the letter "O"
B. power: the force that makes the lever rotate, usually expressed by the letter "F1";
C. resistance: the force hindering the rotation of the lever, which is usually represented by the letter "F2";
D. power arm: the vertical distance from the fulcrum to the dynamic action line, usually represented by the letter "L1"
E. resistance arm: the vertical distance from the fulcrum to the resistance action line, usually represented by the letter "L2"
3. The condition of lever balance A. language expression: power by boom is equal to resistance by boom, which is expressed by formula as follows:
　　F1×L1＝F2 ×L2
4. Classification of leverage fulcrum
A. labor saving lever: the power arm is greater than the resistance arm, and when the lever is balanced, the power is less than the resistance
B. strenuous lever: the power arm is less than the resistance arm. When the lever is balanced, the power is greater than the resistance
C. equal arm lever: the power arm is equal to the resistance arm, and when the lever is balanced, the power is equal to the resistance
(2) pulley
1. Fixed pulley
A. definition: when in use, the pulley whose axis is fixed is called fixed pulley
B. It is a kind of deformed equal arm lever in essence
C. features: the use of fixed pulley does not save labor or distance, but can change the direction of force
2. Movable pulley
A. definition: the pulley whose axis moves with the object being pulled is called moving pulley
B. essence: the essence of movable pulley is a kind of labor-saving lever
C. features: the use of moving pulley can save labor, but can not change the direction of force
3. Pulley block
A. the machine is composed of movable pulley and fixed pulley. The pulley block can change the direction of force and save labor
B. when using the pulley block, the moving pulley is pulled by several sections of rope, and the force used to lift the object is a fraction of the gravity of the object
(3) merit
1. The two necessary factors for doing work are (1) the force acting on the object and (2) the distance the object passes in the direction of the force
2. The formula of work is w = FS, and the unit of work is 1J = 1nm
(4) power
1. In physics, the work done per unit time is called power. The formula is. The unit of power is w
2. The work with utility value is called active work, the work without utility value that has to be done is called extra work, and the work done by external force to machinery is called total work. The relationship is wtotal = wthe useful + wthe extra work
(5) mechanical efficiency
1. The ratio of active work to total work is called mechanical efficiency, expressed by the letter η, and the formula η =
2. In the experiment of measuring the mechanical efficiency of the pulley block, the experimental principle is the formula η =. The measuring tool used is a dynamometer, and the physical quantities to be measured are the material weight and the pulling force. When using the spring dynamometer, it should pull vertically and uniformly
Review outline of "re He Neng" (1)
Molecular thermal motion
1. Matter is made up of molecules. If molecules are spherical, their diameters are measured by 10-10m
2. The molecules of all objects are moving irregularly
Diffusion: the phenomenon that different substances enter each other when they contact each other
(2) the diffusion phenomenon shows that: A. There are gaps between molecules. B. the molecules are in constant and irregular motion
(3) the purpose of putting nitrogen dioxide under the device in the textbook is to prevent the diffusion of nitrogen dioxide from being mistaken for the result of gravity. Experimental phenomenon: two bottles of gas are mixed together and the color becomes uniform. Conclusion: the gas molecules are constantly moving
(4) solid, liquid and gas can diffuse, and the diffusion speed is related to temperature
(5) molecular motion and object motion should be distinguished: diffusion and evaporation are the results of molecular motion, while flying dust, liquid and gas convection are the results of object motion
3. There are mutual attraction and repulsion between molecules
When the distance between molecules d = the equilibrium distance r, gravity = repulsive force
When d < R, gravity < repulsion, repulsion plays a major role, solid and liquid are difficult to be compressed because: repulsion between molecules plays a major role
When D ＞ R, gravitation ＞ repulsion, and gravitation plays a major role. It is difficult for solids to be broken. The reason why pens write and glue stick to things is that the gravitation between molecules plays a major role
When d > 10R, the interaction between molecules is very weak and can be ignored
The reason why the broken mirror can't be round is that the distance between the mirror blocks is far greater than the action range of the intermolecular force, and the mirror can't be combined because of the intermolecular force
2. Internal energy
1. Internal energy: the sum of kinetic energy and molecular potential energy of all molecules in an object, which is called the internal energy of the object
2. An object has internal energy under any circumstances: since the molecules in the object are constantly moving and interact with each other, the internal energy exists unconditionally, whether it is hot metal or cold ice
3. Factors influencing the internal energy of objects: ① temperature: when the mass, material and state of objects are the same, the higher the temperature, the greater the internal energy of objects; ② mass: when the temperature, material and state of objects are the same, the greater the mass, the greater the internal energy of objects; ③ material: when the temperature, mass and state of objects are the same, the materials of objects are different, The internal energy of the object may be different; 4. State of existence: when the temperature and material quality of the object are the same, the internal energy of the object may also be different when the state of existence of the object is different
4. Internal energy is different from mechanical energy
Mechanical energy is macroscopical, it is the energy that the body moves as a whole, its size is related to mechanical motion
The internal energy is microscopic, which is the sum of the energy of all the molecules in the body doing irregular motion. The size of the internal energy is related to the speed of the molecules doing irregular motion and the molecular action. This kind of irregular motion is the movement of the molecules in the body, not the overall movement of the body
5. Thermal motion: the irregular motion of a large number of molecules in an object is called thermal motion
The higher the temperature, the faster the diffusion
Change of internal energy
1. External manifestation of internal energy change
Increase (decrease) of body temperature - increase (decrease) of body internal energy
Change of state of existence (melting, vaporization, sublimation) - change of internal energy
On the contrary, it can not be said that the change of internal energy will inevitably lead to the change of temperature
2. Ways to change internal energy: work and heat transfer
A. work changes the internal energy of an object
(1) work can change the internal energy: when work is done on an object, the internal energy will increase. When work is done on an object, the internal energy will decrease
The essence of work changing internal energy is the mutual transformation of internal energy and other forms of energy
(3) if the internal energy is changed only by doing work, the amount of work can be used to measure the change of internal energy. (w = △ E)
(4) explain the example: Fig. 15.2-5 a shows that the cotton burns, because the piston compressed air works, so that the internal energy of the air increases and the temperature rises to reach the ignition point of the cotton, so that the cotton burns. Drilling wood for fire: the wood rubs against each other, and people work on the wood, so that its internal energy increases and the temperature rises to reach the ignition point of the wood, so that the cotton burns. Fig. 15.2-5 B shows that when the plug jumps up, Fog appeared in the container, because the air in the bottle pushed the stopper to do work on the stopper, the internal energy decreased, the temperature decreased, and the water vapor liquefied and condensed into small water droplets
B. heat transfer can change the internal energy of an object
Heat transfer is the phenomenon of heat transfer from high temperature object to low temperature object or from high temperature part of the same object to low temperature part
(2) the condition of heat transfer is temperature difference, and the transfer mode is conduction, convection and radiation. The heat transfer is internal energy (heat), not temperature
In the process of heat transfer, the internal energy increases with the increase of temperature and decreases with the decrease of exothermic temperature
In the process of heat transfer, the amount of energy transferred is called heat, and the unit of heat is Joule. The essence of heat transfer is the transfer of internal energy
C. The difference between work and heat transfer in changing internal energy: because they have the same effect in changing internal energy, work and heat transfer are equivalent in changing internal energy. But the essence of work and heat transfer in changing internal energy is different. The form of the former energy has changed, while the form of the latter energy remains unchanged
D. difference of temperature, heat and internal energy:
Indicate the meaning of "heat" in the following physical terms:
"Heat" in heat transfer refers to heat;
"Heat" in thermal phenomenon refers to: temperature;
"Heat" in thermal expansion refers to temperature;
"Heat" in friction heat generation refers to internal energy (heat energy)