Why is pressure an intensive property?

In summary, the conversation discusses the difference between intensive and extensive properties. An intensive property is independent of mass, while an extensive property varies with mass. The definitions given in the lecture notes and by the speakers are slightly different, with the latter specifying that other properties may also change when mass is changed. The conversation also mentions that the definitions given in the notes are the usual ones used in thermodynamics courses. Additionally, it is noted that intensive properties do not change with the amount of matter present, while extensive properties do. Examples of both types of properties are given.
  • #1
AAMAIK
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Here is how I thought about it
Consider a surface on which atoms bump into, and if I increase the number of atoms and at the same time allow the surface area to increase as well the pressure is still the same because these atoms occupy have size and thus occupy a certain area , and If they are distributed equally and their size remains fixed namely (made from the same substance).
I also thought to myself that If I consider a rigid tank with a valve that allows mass to enter, then as mass flows into the rigid tank, pressure must increase. this led to me thinking that in classifying of whether a certain property is intensive or extensive I should not put constraints such as my previous example that volume normally would increase as well and in that way pressure would remain the same.
 
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  • #2
Are you clear on the definitions of "intensive" and "extensive"? A property is "intensive" if it can be defined at each point on a surface or each point of a three dimensional region. It is "extensive" if it must be defined over a surface or three dimensional region. The total force on a surface is "extensive", the pressure, the "force per cc" at each point on the surface, is "intensive".
 
  • #3
HallsofIvy said:
Are you clear on the definitions of "intensive" and "extensive"? A property is "intensive" if it can be defined at each point on a surface or each point of a three dimensional region. It is "extensive" if it must be defined over a surface or three dimensional region. The total force on a surface is "extensive", the pressure, the "force per cc" at each point on the surface, is "intensive".
Based on the definition given in the lecture notes I am studying from, an intensive property is independent of mass and an extensive property varies with mass. The definitions you gave for intensive and extensive properties are more reasonable to me.
 
  • #4
The definition in your notes is a poor one, because it does not specify what else changes when you change the mass. Implicitly it is assuming that the substance is in the same state (defined by P, Vm, T), and you just have "more of the same". If you have twice the mass in twice the volume, you have the same density and pressure. If you double the mass in the same volume (halving Vm), you double the density and pressure, but you have a different thermodynamic state.
 
  • #5
mjc123 said:
The definition in your notes is a poor one, because it does not specify what else changes when you change the mass. Implicitly it is assuming that the substance is in the same state (defined by P, Vm, T), and you just have "more of the same". If you have twice the mass in twice the volume, you have the same density and pressure. If you double the mass in the same volume (halving Vm), you double the density and pressure, but you have a different thermodynamic state.
The definition in the notes is the usual one given out and is correct. for any thermodynamic course.
An intensive property does not change due to the amount of matter present. They can be considered as any bulk property of the material, such as temperature, density, color to name a few, or any other property than can be used to describe a particular item without changing the item.
An extensive property does. Examples here would be weight, volume, mass.
 
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Related to Why is pressure an intensive property?

1. Why is pressure considered an intensive property?

Pressure is considered an intensive property because it does not depend on the size or amount of the substance. It only depends on the state of the substance, such as its temperature and volume. This means that the pressure of a substance remains constant regardless of the amount of the substance present.

2. How is pressure different from extensive properties?

Unlike extensive properties, which depend on the amount of substance present, pressure is a characteristic of a substance that remains the same regardless of the amount of the substance. This means that the pressure of a substance can be used to identify and compare different samples of the same substance, regardless of their size or amount.

3. What are some examples of substances with high pressure?

Substances with high pressure include gases in a compressed tank, liquids in a hydraulic system, and solids under high stress. For example, the pressure inside a scuba tank can reach up to 3000 psi, and the pressure at the bottom of the Mariana Trench can reach over 1000 times the atmospheric pressure at sea level.

4. Why is pressure important in thermodynamics?

Pressure is an important concept in thermodynamics because it is a fundamental property that helps us understand the behavior of substances. It is used in equations such as the ideal gas law and the van der Waals equation to describe the relationships between pressure, temperature, and volume. Pressure also plays a crucial role in phase changes and chemical reactions.

5. How does pressure affect the properties of a substance?

Pressure can affect the properties of a substance in various ways. For example, increasing pressure can decrease the volume of a gas, causing it to condense into a liquid. It can also affect the melting and boiling points of substances, as well as their solubility and reactivity. In addition, pressure can also influence the speed of chemical reactions and the stability of different phases of a substance.

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