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Thermodynamics.
Transformation of energy

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Thermodynamic Properties

Thermodynamic Properties

A thermodynamic property is a characteristic or a particularity that allows the changes of the work substance, that is to say, changes of energy.

The thermodynamic properties can be classified as intensive and extensive. They are intensive those that do not depend on the amount of matter of the system (pressure, temperature, composition). Extensive ones depend on the size of the system (mass, volume).

Thermodynamic variables

The thermodynamic variables are the magnitudes that we consider necessary or convenient to specify to give a macroscopic description of the system. Most of these magnitudes come from other branches of physics.

A thermodynamic variable is a macroscopic physical magnitude that characterizes the state of a system in equilibrium. Then, by a certain number of state variables, the state of a thermodynamic system in equilibrium can be defined. In general, systems outside equilibrium can not be represented by a finite number of degrees of freedom, and their description is much more complex.

The value of a state function only depends on the current thermodynamic state of the system, no matter how it arrived at it. This means that if, at a given moment, we have two thermodynamic systems in equilibrium with n degrees of freedom and we measure the same value of n independent state functions, any other state function will have the same value in both systems, regardless of the value of the thermodynamic variables in previous instants.

Intensive thermodynamic properties

The intensive properties are dependent on the mass, they are characteristic of the system. The intensive ones do not depend on the size of the system. If a system is divided into two parts, an intensive property maintains the same value in each part. For example, the density of water is the same if it is concentrated in one liter than that which is concentrated in a huge deposit.

Within this set of properties we have all the specific values ​​as specific internal energy, specific enthalpy, specific entropy, temperature, pressure, specific volume, etc.

Here is a brief description of some of them:

  • Density: the density that exists between the mass that is occupying a total volume or, in other words, the mass per unit volume.
  • Specific volume: the specific volume is the relationship that exists between the total volume occupied by a mass. It can also be defined as the volume per unit mass.
  • Specific weight. The specific weight is defined as the relationship between the weight of the body and the total volume it occupies.
  • Pressure. Pressure is the force exerted by a body per unit area.
  • Temperature. Temperature is the thermal state of a substance that is considered to transmit heatThe temperature can be expressed in different scales: degrees Celsius, degrees Fahrenheit, Kelvin or Rankine.

Extensive thermodynamic properties

The extensive properties do not depend on the mass, but depend on the size of the system. So when the different parts of a whole come together, you get a total value. If a system is composed of different subsystems, then the value of the extensive property for the total system will be the sum of the value of the different subsystems.

Extensive properties become intensive if they are expressed per unit mass (specific property), moles (molar property) or volume (property density).

Among this group of properties we have the total values, such as total energies, volume, weight, amount of substance, etc.

Next, we describe some extensive thermodynamic properties:

Hot

The heat in thermodynamics is considered as the energy that flows when two substances that are at different temperatures come into contact. The heat always flows from the warm body to the cold body.

By convention the heat that leaves a system has a negative sign; while the heat that enters a system has a positive sign.

Temperature is an intensive property, while heat is an extensive property. Heat, in turn, is not a state function because it depends on the path traveled.

The units to express heat are the energy units, the most common are July (J), and calorie (cal).

Work

The work in thermodynamics always represents an exchange of energy between a system and its environment.

By convention, the work done by the environment on the system has a positive sign; while if the system that performs work on the environment has a negative sign.

Work is not a function of the state, it depends on the path traveled.

The units to express a job are those of energy, the most common are July (J), and calorie (cal).

Internal energy

The internal energy of a thermodynamic system is the sum of all the existing energies in the system (kinetic energy, thermal energy, potential energy, etc.). Some authors represent it with the letter U.

The internal energy is a type of energy that can not be determined, in absolute form, therefore, what is measured is the internal energy variation of the system.

The internal energy is an extensive thermodynamic property that depends on the quantity and quality of matter. In turn, the internal energy is a function of state, does not depend on the path traveled.

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Last review: April 17, 2019