Physics Problems Middle School Stage Physics Middle School Questions

1. The conductivity of copper is stronger than that of iron.

2. It is right that the resistance of a superconductor is zero.

Atoms are still divisible.

Semiconductors can conduct electricity as long as the conditions are right.

Nanometers are by far the smallest of the units of length.

3. The white gas seen is the liquefaction of water vapor in the air when it is cold.

4. Oxygen, ultraviolet rays, water, gravity, etc.

1、c2、b

3. It is a liquefaction phenomenon.

4. Air and water.

If you don't know the reason for any question, you can ask it.

1c 2b 3 water vapor in the air turns into small droplets when cold, that is, white gas, and crosses.

4。There is no air, gravitational acceleration is too small, the temperature difference between day and night is too large, there is no food**, and there is no water source.

CB, the atoms upstairs can also be divided into nuclei and electrons, and the nuclei can also be divided into neutrons and protons. 3.Because the water vapor in the air condenses into small water droplets, it is the phenomenon of liquefaction 4

Without an atmosphere, it is difficult to resist a large number of rays in the universe, there is no water, the temperature difference between day and night is large, and gravity is too small to affect the function of the human body.

1,c2,a

3. The gas is invisible, and the "white gas" seen is the water vapor around the popsicle, and the wildflowers are small water droplets when they are cold.

4. Without oxygen, gravity is different.

C, B, the temperature of ice is low, liquefied when heated, forming water vapor, there is no air, the temperature difference between day and night is large, and there is no atmosphere.

1，c 2，b

3. Yes, the popsicle is white because the popsicle lowers the temperature around it, causing the water vapor in the surrounding air to liquefy.

4. Lack of oxygen and water.

I think it's right that the two balls are of equal quality.

Because D is cylindrical and the liquid mass is equal, according to the pressure p=mg s, it is concluded that the pressure at the bottom before the ball is equal, and the pressure is still equal after the ball, according to the pressure p=liquid density * g*h, the height of alcohol is higher than water, so the body of alcohol discharged by ball B must be larger than the volume of water discharged by ball A, and at the same time it is immersed, so D is selected

From the pressure p=pgh=pgsh s=g s and g and s are equal, it can be obtained that the pressure of water and alcohol on the bottom of the container before the ball is released, and the pressure of water and alcohol on the bottom of the container after the ball is also equal, it means that the liquid mass of the two submerged balls must be equal, and because (p water is greater than p alcohol), so the v water discharged is less than v alcohol, that is: the volume of ball A is less than the volume of ball B, it is certain, so the choice of D other c is wrong, A and B can not be determined.

A is the indicator of v1 and B is the indicator of v2.

r1＝10ω

r2=20ω

1. Find the gravitational force of the object: g=mg=20kgx10n kg=200n;

Second, find the thrust of the object: f=(1 2)g=(1 2)x200n=100n;

Third, the object moves in a straight line at a uniform speed in the horizontal direction, and the frictional force is balanced with the thrust force, and the frictional force is obtained

f=f=100n。

The object moves in a straight line at a uniform speed in the horizontal plane.

Then, the force on the horizontal plane is balanced, i.e., horizontal thrust = frictional force.

Suppose the acceleration due to gravity g=10

Gravity g=mg=20*10=200

Friction = Horizontal Thrust =

Solution: g = mg = 20 10n = 200n

Do a uniform linear motion on a horizontal plane.

f = f push = 1 2g = 1 2mg = 1 2 20 10n = 100n

Internal energy is the sum of all the molecular kinetic energy and molecular potential energy within an object. For a gas of a certain mass, when the volume remains unchanged and the temperature increases, the molecular potential energy does not change, and the total kinetic energy of the molecule increases, so the internal energy increases. When the temperature is unchanged and the volume increases, the total kinetic energy of the molecule remains unchanged, because the distance between the molecules increases in the gravitational range, the potential energy of the molecule increases, and the internal energy increases.

When the temperature of this question does not change, it should be endothermic.

A misunderstanding and two methods for judging the change of gas internal energy.

Determining the change in the internal energy of a gas is not a complicated problem, but students often make mistakes. This is because students first have a misunderstanding: temperature is linked to heat, and the two change in the same direction.

That is, when the temperature rises, it must absorb heat; When the temperature drops, the heat must be released. On the contrary, if the heat is absorbed, the temperature must rise; When the heat is released, the temperature must drop. In fact, there is no single functional relationship between temperature change and absorption and discharge.

An increase in temperature does not necessarily absorb heat; Absorbing the heat, the temperature does not necessarily rise. The correct point of view is that for non-ideal gases, temperature is related to molecular kinetic energy, and the higher the temperature, the greater the total kinetic energy of the molecule; For ideal gases, the temperature is directly related to the internal energy of the object, and the higher the temperature, the greater the internal energy of the object.

Two. Two methods.

After getting out of the misunderstanding, some students still can't answer the question of determining the change of gas internal energy, because they do not realize that there are two basic methods for determining the change of gas internal energy, and different methods should be used according to the different conditions provided in the question.

Method 1: Use the definition of internal energy.

The internal energy is the sum of all the molecular kinetic energy and molecular potential energy in the object, if we can determine the change in the total kinetic energy of the molecule, the change in the potential energy of the molecule, so that the change in the internal energy of the object can be further determined, then we can use Method 1.

Method 2: Apply the first law of force science.

The First Law of Thermodynamics,. This method is used to determine the change in the internal energy of a gas, i.e., to analyze the work done and the heat transfer.

Using method 2, in general, the work done by the gas on the outside or the work done by the outside world can be determined according to the volume expansion or compression of the gas, and the volume change is often determined according to the relationship between the state parameters. Endothermic and exothermic cannot be determined according to the change of temperature. For ideal gases, the change in internal energy can be determined directly based on the change in temperature.

When the heat absorption and release cannot be directly determined, it should be placed at the end and determined according to the first law of thermodynamics. This kind of problem to determine the change of internal energy is already a kind of thermal comprehensive problem. Most of the students are wrong in the thinking ideas and thinking procedures of such problems.

Unchanged, because in this case, only the volume changes, and everything else, including the mass, will not change OK??