Physics questions about molecular kinetic theory, physical molecular kinetic theory

The landlord's question focuses on the understanding of molecular kinetic theory, and I will talk about it together.

The gravitational and repulsive forces between molecules always exist at the same time, but the external performance is determined by the magnitude relationship between the two. Both the intermolecular gravitational and repulsive forces decrease as the molecular spacing increases, but the repulsion decreases faster than the gravitational force!

Therefore, when the distance between the molecules is relatively close, the repulsion force is still greater than the gravitational force, which is manifested as repulsion force externally; When the molecular spacing increases to a certain value r0, the gravitational repulsion force will be equal; When it is greater than r0, the repulsion force decreases to less than gravity, and the external force is gravity.

Generally speaking, the molecular spacing between solids and liquids can be around r0, while gases are generally much larger than r0, so for gases, such a distant molecular spacing will reduce the gravitational repulsion of molecules to 0, so the third problem can be explained.

After talking about the intermolecular forces, let's talk about molecular energy. The internal energy of an object is defined by us as the sum of all the kinetic energy and potential energy of the molecule.

The temperature you are talking about is the macroscopic representation of the kinetic energy (ek) of the molecule. ek = ncvt, then the formula for calculating the kinetic energy of the ideal gas molecule, cv is called the isochoric molar heat capacity, n is the amount of matter, t is the absolute temperature (k).

Whereas, molecular potential energy is the energy produced due to the presence of forces between molecules. I have already said that the spacing between the molecules of the gas is very large, so we also have the definition of an ideal gas, for an ideal gas, the molecular potential energy is 0

There is a first law of thermodynamics in thermals, where the change in the internal energy of an object is equal to the sum of the value of the work done and the value of heat absorption and release. I don't know what to describe, such as what the work is done, how to calculate the heat absorption and release, the landlord can refer to any university physics book thermal section.

According to the above law, if an object absorbs heat, we cannot tell whether there is a process of doing work, so we cannot conclude the change in the internal energy of the object. Even if the internal energy of the object changes, how can we conclude that it is not only the molecular potential energy that has changed? In the case where only the potential energy of the molecule changes, it is clear that the temperature does not change!

The landlord can hi me.

1. Not necessarily.

Both the "gravitational force" and the "repulsive force" between molecules decrease as the "distance increases".

Externally, it is the combined force of "gravity" and "repulsion".

Before equilibrium position (f-citation = f-repulsion), the greater the distance, the smaller the repulsive force expressed. After the equilibrium position, the gravitational force exhibited first increases and then decreases.

2. The temperature does not change (the molecular kinetic energy does not change), and the internal energy changes due to the change of "volume", which leads to the change of "molecular potential energy".

The intermolecular forces of the gas are very small (for ideal gases, the intermolecular forces are zero), and when compressing, the main consideration is the distance between the molecules.

For solids and liquids, the molecules were originally in the equilibrium position, but now the compression makes the distance between the molecules smaller, and the molecules show a "repulsive force" to the outside, and this force increases with the decrease of the distance.

1. Starting from the spacing is 0, both the repulsive force and the gravitational force decrease the resultant force, first decrease, then increase, and then decrease to 0

2 Internal energy includes molecular kinetic energy and potential energy, and temperature is the macroscopic realization of molecular kinetic energy! Do you understand? That is, it is possible that the potential energy of the molecule changes and the kinetic energy does not change, so the temperature does not change.

3. When the intermolecular difficulty is greater than a certain value, and the value is very small, the force between the fractions can be disregarded.

So the distance between the molecules of the gas is very large, much greater than that value, so the repulsion between the fractions is not taken into account, and the molecular spacing between the liquid and the solid is very small, and the distance is very close to that particular value, as the book says

So monetization is hard to compress.

1。There's a diagram that is roughly a hook, not necessarily bigger or smaller, depending on the intermolecular distance, and when it's greater than the critical distance, the greater the intermolecular distance, the smaller the gravitational force, and when it's below the critical distance, and vice versa, and it's also caused by repulsion.

2。Internal energy is caused by the movement of molecules, for example, when the pressure changes, the ice melts into an ice-water mixture, which is exothermic and the volume changes, but the temperature does not necessarily change from 0 degrees to 0 degrees.

3。It's the role of intermolecular repulsion, and the first question is a principle that can be explained by that diagram.

To correct a mistake: when an object absorbs or emits heat, its internal energy does not necessarily change.

This problem should be solved in this way: calculate the volume of space occupied by each oxygen molecule according to the number of oxygen molecules in 1mol of oxygen, and then look at each oxygen molecule as a bench to grind a cube, and find the side length of the cube, that is, the average distance between oxygen molecules. Divide this average distance by the diameter of the molecule to calculate the multiple.

If you calculate the specific jujube teasing process yourself, you will be more impressed. Hope it helps!

Let each oxygen molecule be a cube with a side length of l. then there is l 3 =, l = can be found, and the ratio is a multiple).

Answer B Analysis and Answer: This question integrates physical knowledge into the natural phenomena of the four seasons of the year, giving people a sense of beauty The movements of catkins, yellow sand, and snowflakes are all mechanical movements, which are macroscopic phenomena, not microscopic molecular movements, so options A, C, and D are all wrong; "Acacia fragrance" refers to the fact that there are molecules of aromatic oil produced by locust flowers in the air around the locust tree, which is the result of the diffusion of the molecules of acacia aromatic oil due to irregular movements, option B is correct

To sum up, the answer to this question is B

Answer C Analysis: The ore in item A is crushed into powder, which is the particle of the material ore, not the molecule of the ore, and the particle is composed of a large number of molecules The cork of the thermos in item B is difficult to pull out, which is the effect of atmospheric pressure, not due to the gravitational force between the molecules Option D The compression spring needs to be forced, indicating that the force can change the shape of the object Only in option C, the sewage molecules and water molecules enter each other when they are in contact with each other, and the diffusion phenomenon occurs Therefore, the closed annihilation c is chosen

Answer C [Analysis] can preliminarily grasp the preliminary knowledge of molecular kinetic theory, and use the direct selection method to select The ore is crushed into powder by the powder plate, which is mechanical, and the physical reason makes its volume smaller, and the size of the ore molecule does not change, so item A is wrong; The cork of the thermos bottle is sometimes difficult to pull out, because the temperature inside the bottle decreases, the gas pressure becomes smaller, and the atmospheric pressure outside the bottle is greater than the air pressure of the gas in the bottle, so the cork is difficult to pull out, and the gravitational force between molecules cannot explain this phenomenon, so item b is wrong; Soon after the sewage is discharged into the pond, the entire pond water is polluted, indicating that the molecules of all objects are constantly moving irregularly, so the C term is right; The compression spring requires pressure to change the shape of the object, which is not caused by intermolecular repulsion, so the D term is wrong

Summary: To understand the motion of microscopic molecules and macroscopic objects, it is especially necessary to pay attention to the difference between the movement of molecules in the object and the movement of objects composed of molecules

Does equilibrium distance refer to the radius of a molecule? Yes, it is also called the effective Yinzhou bend radius.

Is the distance between the molecules of an object generally greater than the diameter of a molecule? That is, all objects generally exhibit gravitational attraction between their molecules? Yes, unless compressed.

When squeezing the plank with the hand, the plank behaves as repulsion? The solid structure is dense and the deformation is small.

The repulsion force is a short-range force, and when the intermolecular distance d is between r and 10r, the repulsion force has no effect, and when it is less than r, it acts and increases rapidly.

Is the "gravitational pull" greater at 3R or the gravitational force at 4R? The "gravitational pull" at 3R is large because the molecular potential energy curve is convex on the upper trace.

1 from. The spacing is 0

Be cautious. Repulsion.

and gravity. all reduce the resultant force.

First decrease, then increase.

Then decrease to 0

Internal energy includes: Molecular kinetic energy and potential energy, temperature is the kinetic energy of the molecule.

Macro realization! Do you understand? That is, it is possible that the potential energy of the molecule changes but the kinetic energy does not change.

So the temperature does not change.

3 when. Intermolecular daunting.

When it is greater than a certain value and "this value is small".

It is not possible to consider the forces between the parts.

So the distance between the gas molecules is large.

Much greater than that.

So the repulsion between the points is not considered.

And liquids and solids.

The molecular spacing is small, and the spacing is very close to that particular value.

What the book says. Therefore, it is difficult to suppress the first filial piety.

1。There's a picture.

It is roughly a hook shape.

If you don't call, it must be bigger, it must be thicker, it must be smaller.

It depends on the distance between molecules.

When greater than the critical distance.

The greater the intermolecular distance, the smaller the gravitational pull.

below the critical distance.

On the contrary, and will also be repulsed.

2。Internal energy is caused by the movement of molecules.

Like what. When the pressure changes, the ice melts into an ice-water mixture, which is exothermic and the volume changes, but the temperature does not necessarily change.

From 0 degrees to 0 degrees.

3。It is the action of intermolecular repulsion.

And the first chaotic branch Kai question is a principle.

It can be explained with that diagram.

The average translational kinetic energy of the equation of state of an ideal gas w=3kt 2, is only related to temperature, so both translational kinetic energy is equal. The average kinetic energy of the molecule = the average translational kinetic energy + the average translational kinetic energy, which is related to the degree of freedom of the molecule, the monoatomic molecule only has the translational degree of freedom i=3 , and the diatomic molecule has the degree of freedom i=5, so the average kinetic energy of helium he=3kt 2, and the average kinetic energy of helium o2=5kt 2The two are not equal.

So the answer is C