Transformation of energy

Thermal energy and combustion.
Effects of thermodynamics


Internal Energy

Internal energy

In thermodynamics,  internal  energy is the total energy that a thermodynamic system contains. It is the energy that is needed to create the system.

According to this definition, it is excluded:

  • energy to displace the system environment
  • any energy related to external force fields (potential energy, gravitational energy, etc.)
  • any energy associated with movement (for example, kinetic energy).

The internal energy of a system can be modified by working on it or by heating it (providing thermal energy).

The first law of thermodynamics postulates that the increase in internal energy is equal to the total added heat plus the work done by the environment. According to this principle of thermodynamics: if the system is isolated, its internal energy remains constant.

Internal energy is a state function of the system, since its value depends only on the current state of the system and not on the path chosen to reach it. It is an extensive property.

Units of measurement of internal energy

The unit of measurement, according to the International System, is July (J).

Sometimes specific internal energy is discussed. It is the internal energy per unit of mass; its unit is J / kg. You can also define the intensive internal molar energy property, which expresses the internal energy in relation to the amount of substance; its units are J / mol.

What are kinetic energy and internal potential energy?

Internal energy is the sum of two different types of energy:

  • internal kinetic energy.
  • the internal potential energy.

Internal kinetic energy

The internal kinetic energy is the sum of the kinetic energies of each element of a system with respect to its center of mass.

The internal kinetic energy is caused by the movement of the particles of the system (translations, rotations and vibrations).

Internal potential energy

Internal potential energy is the potential energy associated with each of the interactions of these elements.

The internal potential energy is associated with the static constituents of matter, the electrostatic energy of the atoms inside the molecules or crystals, and also includes the static energy of chemical bonds.

Internal energy of the ideal gas

Thermodynamics often uses the concept of the ideal gas. It is an approximation of the real systems used for educational purposes.

The ideal gas is a gas of particles considered as point objects that interact only by elastic collisions. They fill a volume such that their average free run between collisions is much larger than their diameter. Such systems are approximated by monatomic gases, helium, and the other noble gases. Here the kinetic energy consists only of the translational energy of the individual atoms. Monoatomic particles do not spin or vibrate, and are not electronically excited at higher energies except at very high temperatures.

Therefore, changes in internal energy in an ideal gas can only be described by changes in its kinetic energy. Kinetic energy is simply the internal energy of the perfect gas and is completely dependent on its pressure, volume, and thermodynamictemperature.

The internal energy of an ideal gas is proportional to its mass (number of moles) and its temperature T

U = c·n·T

where c is the dimensionless specific heat at constant volume of the gas. The internal energy can be written as a function of the three extensive properties S, V, n (entropy, volume, mass) as follows.

Internal energy as a function of entropy, volume and mass

where const is an arbitrary positive constant and where R is the universal gas constant.

How is internal energy measured?

Direct measurement is not possible.

Variation of internal energy

By cons, its variation can be measured. This change can be found by measuring thermal energy, light energy, or similar energies transmitted from the limits of internal energy to the environment, or by calculating the amount of work done by the system or work done on the system.

The variation of the internal energy is expressed by ΔU.

  • An absorption energy system occurs when an increase in domestic energy. Internal energy can be increased by doing work on the system.
  • The internal energy decreases when the system emits energy or works.

Changes in the thermal energy of a thermodynamic system

The system does not actually have thermal energy. It is transferred to the medium in the form of thermal energy. In cases where a change in the internal energy of the system occurs due to the transfer of thermal energy, a very small portion changes.

When a thermodynamic system transfers thermal energy to another system, in addition to varying the internal energy, the two systems may experience alterations in other of their state variables:

  • The pressure.
  • The volume.
  • The temperature.
  • The enthalpy.
  • Entropy.
  • ...

Published: April 4, 2017
Last review: May 3, 2020