How to judge whether a gas does negative or positive work to the outside world?

The expansion of the gas and the increase in the volume of the gas are to do work on the outside world, and it is said to be doing positive work on the gas. The decrease in the shrinkage volume of the gas is the work done by the outside world on the gas, and the negative work is done on the gas.

However, when substituting u w+q, it should be noted that the gas expands and the volume increases, and the gas does positive work on the outside world, and when substituted into w, it is necessary to substitute negative work, because the left side is the change in the internal energy of the object. When the gas does positive work, it will reduce the internal energy of the object, and Q is also, the gas is exothermic; Q is a negative value.

The specific situation is analyzed on a case-by-case basis, but generally: the volume is reduced, and the outside world does positive work on the gas; As the volume increases, the gas does positive work on the outside world.

There is no such thing as negative merit (I specifically asked the teacher).

It is necessary to consider both volume change and temperature pressure.

I'll send you a picture for you to see, and if you see it clearly, you'll pick it.

The book says that the work done by the outside world on the gas is endothermy.

The gas is exothermic to the outside world.

1. There are two ways to judge positive and negative work:

1. Look at the effect. For example, if you let go of an object in the air and let it fall freely, then in the process of falling, only gravity does the work, what is the result?

The velocity of the object is increasing, that is to say, the kinetic energy is increasing. So where does this increased kinetic energy come from? The positive work done by gravity is transformed.

On the contrary, if an object is thrown vertically, the velocity of the object will slowly decrease under the action of gravity, that is, the kinetic energy will become smaller and smaller, which is because the negative work done by gravity cancels out the kinetic energy.

2. Look at the angle. This angle is the angle between the direction of the force and the direction of the object's motion, if the angle is greater than 90, negative work is done; If it is less than 90, do positive work. Take the above example to say that in the free fall motion, the direction of motion of the object is downward, and the direction of gravity is also downward, then the angle is 0 degrees, so it is a positive work.

The vertical upward throwing motion, the direction of movement is upward, the gravity is downward, and the angle is 180 degrees, so negative work is done.

Also, when you talk about friction, it's a very special force. The definition of this force is that the direction is opposite to the relative motion trend (that is, it is a force that wants to hinder the motion of the object), so the direction of the frictional force is (in general) opposite to the direction of the object's motion, that is, the angle is 180 degrees, so (in general) the frictional force is done negative work.

Second, the above analysis is whether it is positive or negative work, and the following is an analysis of what circumstances do not do work.

There are also two possibilities:

1. There is no friction or zero friction, which means that the contact surface is smooth and the like.

2. There is friction but no relative motion, for example, if you push a big box, but you don't push it, why don't you push it, because there is friction between the box and the ground, but the box doesn't move, so friction doesn't do work.

One more thing to add: Regarding the problem of force not doing work, in the high school range, it is generally necessary to pay attention to: when the angle is 90 degrees.

For example, the centripetal force in circular motion and the direction of motion of the object are always 90 degrees, so the centripetal force does not do work. And the friction force you are talking about generally does not have an angle of 90 degrees (as mentioned above, the special thing about friction is that its force direction is always opposite to the relative movement trend, that is, it is at an angle of 180 degrees).

Gas compression is a common industrial process that can make the gas smaller in size and more pressurized. In this process, the gas compressor exerts a force on the gas, which reduces the volume of the gas, does this mean that the gas compression does positive work?

Gas compression does negative work.

In fact, gas compression is by no means a positive process, but a negative work. This is because, when a gas is compressed, it will rebel against the force exerted on it, forming a counterforce. This reaction force is applied in the opposite direction and is proportional to the pressure of the compressed gas.

According to the law of conservation of energy, the work done by an object on its surroundings should be equal to the energy of the external force it absorbs, minus the total energy it possesses. When a gas is compressed, the force exerted on the gas produces negative work, which is consumed to maintain the molecular motion inside the gas during compression.

It is important to note that the reaction force of gas compression does not only come from the movement of molecules inside the gas, but is also constrained by the force object of the gas. These constraints resist the compression of the gas, causing the gas to react more. Therefore, the negative work done by gas compression is much greater than that of other objects.

Impact on compressors.

Since the gas compression does negative work, the energy required by the compressor in the working process is relatively high. Especially when dealing with high-density gases, higher pressures are required and higher energy is required. This is a question worth considering for industries that often require gas compression.

At the same time, the efficiency of the compressor is also affected by the compression ratio. This is because, as the compression ratio increases, the temperature of the gas also rises, resulting in more energy loss. Therefore, it is necessary to carefully select the compression ratio when compressing the gas to ensure that the energy loss does not overgrow.

The heat generated by the compression of gases.

Since the gas absorbs a lot of energy during compression, it tends to get very hot. This is because, when a gas is compressed, the molecules inside it are in closer contact and hence more molecular energy is formed, which increases the temperature of the gas.

This phenomenon is very important for some industries. For example, air jujube compressors produce a high-temperature gas during operation, which can be used to heat other objects or directly used as a drying gas. Therefore, we need to pay attention to the temperature change of the gas when doing gas compression in order to make the best use of the heat it generates during the treatment process.

Conclusion Gas compression is a negative work process, and the energy required by the compressor to perform gas compression is relatively high. Understanding these characteristics of gas compression can help us better select the compression ratio and understand the operating condition of the compressor, and also use the heat generated by gas compression for other applications.

To judge whether the force does positive or negative work on the object, you only need to draw the starting point of the force and displacement vectors on the center of gravity of the object, and look at the angle between the force and the displacement vector. If it is an acute angle, it is positive; obtuse angle, negative work; Right angle, no work.

Work is the process by which energy is transformed from one form to another. There are two necessary factors for work to be done: the force acting on the object and the distance the object travels in the direction of the force.

Definition of classical mechanics: When a force acts on an object and causes the object to pass a distance in the direction of the force, it is said in mechanics that the force has done work on the object.

Different forms of energy in nature correspond to different forms of motion: the motion of objects has mechanical energy, the motion of molecules has internal energy, the motion of electric charges has electrical energy, the motion inside the nucleus of an atom has atomic energy, and so on.

Different forms of energy can be converted into each other: "Frictional heat generation is the conversion of mechanical energy into internal energy by overcoming friction and doing work; When the water in the kettle boils, the water vapor does work on the lid of the kettle to lift the lid up, indicating that the internal energy is converted into mechanical energy; The electric current does work through the heating wire, which converts electrical energy into heat energy, and so on." These examples illustrate that different forms of energy can be transformed into each other, and that this transformation process is accomplished by doing gong in the Zheng Guo Zen section, which is the amount of energy that is transformed.

To judge whether the force does positive or negative work on the object, you only need to draw the starting point of the force and displacement vector Hu Ming on the center of gravity of the object, and look at the angle between the force and the displacement vector. If it is an acute angle, it is positive; obtuse angle, negative work; The trousers are at right angles and do not work.

Positive work: Displacement is generated along the direction of action of the force, and the force does positive work. Negative work: The displacement is generated against the direction of action of the force, and the force does negative work. For example, if I press a spring, the spring does negative work on me because the direction of its force is against the direction of my hand's motion.

The meaning of positive work is: the force does positive work on the object, indicating that the effect of this force is to promote the motion of the object, which is the driving force.

The meaning of negative work is: the force does negative work on the object, indicating that the effect of this force is to carefully hinder the movement of the object, which is a resistance.

If the external force reduces the mechanical energy of the object, then the negative work will be written off and the positive work will be done on the contrary. If the external force decreases the velocity of the object, then the external force does negative work on the object, and the velocity of the object increases, then it does positive work.

What acute angles and obtuse angles, I won't either, our teacher said, if the force and velocity direction are consistent, it is positive work, if the force and velocity direction are opposite, it is negative work.

This hall orange boy question doesn't need to be positive and negative.

Use the whole process analysis to pretend to be Wang.

Gravity does positive work is mgh

It is w to set people to do work

Then use the kinetic energy theorem.

W+mgh = 1 2mv2 squared - 1 2mv1 squared.

1. To judge whether the force does positive or negative work on the object, you only need to draw the starting point of the force and displacement vector on the center of gravity of the object, and look at the angle between the force and the displacement vector. If it is an acute angle, it is positive; obtuse angle, negative work; Right angle, no work.

2. The total work needs to find the algebraic sum of each work.

3. Each work corresponds to different energy changes, for example: gravity does positive work, and gravity potential energy is carefully reduced; Gravity does the negative wide draft and the gravitational potential energy increases. The combined force does positive work, and the kinetic energy increases; The combined force does negative work, and the kinetic energy decreases.

<> conclusion satisfies the following conditions: there is no external force on the piston in the process from 1 to 2; From 2 to 1, the piston is subjected to a constant force f to the left, p2 is equal to the atmospheric pressure, and when Yu Sen tells the piston to move to 1, it is stationary; Both processes are carried out slowly.

First look at 1 to 2, the volume of the gas increases, and the positive work is done to the outside world, and when the gas pressure is the same as the atmospheric pressure, the volume stops increasing, that is, the 2 state. Let the cross-sectional area be s and the gas pressure is p when the volume is v, and under this condition, the piston does work when the piston moves dl vertically through a small section: dw=fdl=psdl=pdv, which is the derivation process of the purple formula in the figure above.

Integrating in the interval of 1 to 2, we find the area of the trapezoid-like pattern in which the positive work of the gas in the process from 1 to 2 is obtained.

Looking at 2 to 1 again, the volume of the gas decreases, the outside world does positive work on the gas, and the gas does negative work on the outside world. Let the spring or cross-sectional area be s, and from the state 1 at rest, we can get: f=(p1-p2) to do :

w=fl=(p1-p2)sl=(p1-p2)(v2-v1), the area of the rectangle enclosed by p1, p2, v1, and v2 of the image. The amount of negative work done by the gas to the outside world is equal to the amount of positive work done by the outside world to the gas.

If the conclusion is different from your teacher's, it may be that the conditions are different. You understand the process of deriving the conclusion from my condition, and then use your teacher's condition to deduce it yourself.