On the question of molecular motion, how do molecules move

The average velocity of the motion of the molecules can be zero, and the instantaneous velocity cannot be zero. Why is this sentence wrong.

This statement is completely true. It would be a mistake to say that the average velocity (i.e., the magnitude of the velocity) is likely to be zero. But the first two examples you gave are not appropriate.

When there are only one or two molecules in a container, for example, their motion is regular, and it does not belong to thermal motion, so there is no such statistical law as "the average velocity of molecular motion may be zero, and the instantaneous velocity cannot be zero" (note that this law is not a mechanical law, but a statistical law. A small number of molecules obey the laws of mechanics, have a definite trajectory, and can infer all subsequent changes based on the initial conditions. A large number of molecules obey statistical laws).

The pressure of an ideal gas of a certain mass on each surface of the container wall is equal to the pressure, and because the pressure is the magnitude of the impulse of gas molecules per unit time and unit area of the vessel wall, that is to say, in the ideal gas of a certain mass, there are molecules with velocity in this direction in all directions, and in any pair of opposite directions, the sum of the impulses of the molecules as a whole is zero, so the average velocity can be zero.

This is a more correct understanding. However, it is also slightly flawed, "molecules with velocity exist in all directions" is not strict enough, and should be understood to mean that there are velocity components of molecules with various velocities (including different directions) in each direction, and the average value of these components is zero. Since the average value of this component in any direction is zero, the mean value of the velocity is zero.

A condition where the average velocity is not zero only exists when the object is in motion relative to the specified frame of reference. This average velocity is the macroscopic velocity of the object.

If you have any questions, you are welcome to ask further.

The average velocity here generally refers to the average of the instantaneous velocity. It can't be zero.

Matter is in motion and never stops, stillness is relative, motion is absolute.

Molecular motion theory is a theory that expounds the law of thermal phenomena from the microstructure of matter, for example, it clarifies that the temperature of a gas is a sign of the average translational kinetic energy of molecules, and the pressure on the container wall is generated by the collision of a large number of gas molecules on the container wall.

First, there are various thermal phenomena in nature: changes in the temperature of objects, changes in the state of matter, thermal expansion and contraction of objects, etc. What is the nature of these thermal phenomena?

It was not until the 17th and 18th centuries that people began to understand that thermal phenomena are caused by the movement of a large number of particles inside matter, and this understanding gradually developed into a scientific theory. By the 19th century, the concept of energy had been established, and people gradually recognized that the energy associated with thermal phenomena was internal energy.

Second, molecules and substances.

We live in the material world, and we are surrounded by matter: water, air, stones, metals, animals, plants, etc. are all matter. As for the ancient topic of how matter is constituted, there have been various speculations for a long time, some assert that the source of all things is "qi", and some claim that the source of all things is "fire".

In the 5th century B.C., Mozi proposed that the smallest unit of matter is the "end", and in the 4th century B.C., Democritus of ancient Greece believed that everything in the universe is composed of atoms of different sizes and masses, which are inaccessible and constantly moving. After nearly 2,000 years of exploration, it was not until the end of the 17th century that it was scientifically recognized that matter is composed of molecules.

Matter is made up of molecules, which are extremely small particles. If you look at the molecule as spherical, it has a diameter of about 10-10 meters, which is an extremely small length that is not only invisible to the naked eye, but also difficult to see with a modern microscope. Because the molecules are extremely small, the number of molecules contained in an object is surprisingly large.

Normally, there is about one molecule in 1 centimeter3 of air, and if the speed of counting people can reach 10 billion per second, it will take more than 80 years to count this number.

The molecules that make up matter are in constant motion. Because the molecule is too small, it is not possible to directly observe the behavior of the molecule, but we can judge the behavior of the molecule from the macroscopic experimental phenomena.

a. Seeing fine dust flying and laughing in the sunlight is the movement of solid particles under the flow of air, not Brownian motion; therefore a error;

b. Compressed gas needs to do work because it needs to overcome gas pressure; Therefore berror;

c. During the melting process of the crystal, all the heat absorbed is used to destroy the spatial lattice and increase the molecular potential energy, while the average kinetic energy of the molecule remains unchanged, so the temperature of the crystal can remain unchanged during the melting process; There is a fixed melting point, so c is correct

d, the molecule A and B are far apart (the force between them can be ignored at this time), let A be fixed, B gradually approach to A, until it can no longer get closer, the molecular force decreases first and then increases, and the molecular potential energy decreases first and then increases, so D is wrong

Therefore, c

Molecule is a microscopic concept, it can be said that matter is composed of molecules, but it can never be said that matter is composed of molecules, and the composition is an element.

1）。Molecular formula onlooker concept.

2）。A molecule is composed of atoms (a molecule is the smallest particle that retains the chemical properties of a substance, and the molecule can be redivided, but the atom cannot be redivided).

3) Molecules are constantly moving irregularly (the higher the temperature, the faster the molecules move) (4) There are gaps between molecules (e.g., gases can be compressed) (5) Atoms are composed of atoms and electrons outside the nucleus, and the nucleus is composed of protons and neutrons, which in turn are composed of quarks.

Because the irregular motion of molecules is related to temperature, the irregular motion of a large number of molecules inside an object is called thermal motion; The higher the temperature, the more intense the molecules move

So the answer is: temperature, thermal motion

Temperature. The motion of the molecules is related to the temperature of the object (thermal motion is also done in the case of 0, and the internal energy is based on thermal motion), the higher the temperature of the object, the faster the motion of its molecules. For example, this movement can be seen when looking at Garcinia cambogia powder and pollen particles suspended in water under a microscope, and the higher the temperature, the more intense the movement.

It was 1827 by the botanist RBrown was the first to find out.

In diffusion motion, it is found that the higher the temperature, the faster the diffusion. When observing Brownian motion, it is also observed that the higher the temperature, the more pronounced the motion of suspended particles. These states of affairs show that the irregular movement of molecules is related to temperature, and the higher the temperature, the more intense this movement becomes.

To take a simple example, with a piece of gold and a piece of lead glued together, left at room temperature for 5 years, and then cut, you will find that they penetrate into each other 1mm deep, if you continue to add 100°C heat, they will fit faster.

Strictly speaking, the rate of molecular motion is related to factors such as molecular mass, intermolecular forces, and temperature.

In the case of the gas velocity rate, when the actual gas can be approximated as an ideal gas, the intermolecular forces are negligible, and the molecular velocity is only related to the molecular mass and temperature.

The irregular rate of movement of gas molecules affects the diffusion velocity of gases, and the ratio of diffusion velocities at the same temperature is equal to the inverse ratio of the square root of the relative molecular mass, which shows that the mass of gas molecules also affects the velocity of gas molecules, not just the temperature. In junior high school, the higher the temperature, the more violent the irregular motion of molecules means that the higher the rate of molecular motion for the same substance, and the word "vigorous" is more accurately understood to refer to the average kinetic energy of molecules, not to velocity.

The problem is actually very deep. Simply put, it is due to the presence of temperature. Temperature is microscopically caused by the movement of microscopic particles, which collide with each other, so the molecules are moving all the time. Only at that moment when the whole world stands still will it cease to move.

As for why microscopic particles move, it is difficult to answer. There are several ways to answer, but not completely. For example, in the existence of energy, there must be a balance between mass and energy, and there will also be a transformation between kinetic energy and potential energy; It can also be said that it is the cause of interaction.

In fact, movement is one of the nature of nature, as for why, it can only be said that this is an existence, but also a law, and this law of existence creates our observable nature, and also creates us. If there is no movement, nothing exists, nothing can be observed, and there is no meaning to understand it.

So physicists are cunning, there is a saying: only study what we can observe. This is actually the pragmatism of science.

The basic content of the theory of molecular motion is that an object is composed of a large number of molecules, the molecules are never going to move randomly, and there is an interaction force between the molecules. The irregular motion of a large number of molecules is called the thermal motion of molecules.

It can be seen that the movement of molecules is due to "heat", and the "heat" here refers to the internal energy of the molecule. We also know that objects with non-absolute zero have internal energy, and absolute zero is impossible to reach, so the molecules of all objects will have irregular thermal motion.

The fact that you can ask such questions shows that you are very brainy. Explaining why molecules move is probably going to have to be linked to explaining the formation of the universe.

From the point of view of classical physics, it is energy that causes molecules to vibrate constantly, for example, if the temperature of matter is higher than 0 K (it should not be written as 0° K), then the molecules will vibrate due to heat energy.

If the substance is crystalline, its vibrational mode will also vary depending on the crystal structure.

However, it is better to explain this problem in terms of quantum mechanics.

Because molecules have energy, and if the energy they possess is not canceled out by each other, there will be unbalanced forces that lead to the movement of molecules.

Because molecules have internal energy, i.e., kinetic energy and potential energy of molecules.

Motion is absolute, and rest is relative.

Don't choose the most satisfying first, the one is mine, typing is a little slow, wait for me to explain why. I'll see the answer in the supplement.