internal energy

Examples of internal energy in the following topics:

• Internal Energy

  • The internal energy of a system is the sum of all kinetic and potential energy in a system.
  • However, a system does contain a quantifiable amount of energy called the internal energy of a system.
  • The equation describing the total internal energy of a system is then:
  • The kinetic energy portion of internal energy gives rise to the temperature of the system.
  • Express the internal energy in terms of kinetic and potential energy

• Comparison of Enthalpy to Internal Energy

  • In thermodynamics, the total energy contained by a given thermodynamic system is referred to the internal energy (U).
  • Because the internal energy encompasses only the energy contained within a thermodynamic system, the internal energy of isolated systems cannot change.
  • Enthalpy (H) encompasses both the internal energy of a system and the energy associated with displacing the system's surroundings.
  • Sometimes, measuring the internal energy of a system may be an inaccurate gauge of the change in energy.
  • Therefore, to account for both the possible volume change at constant pressure and the internal energy, enthalpy is used.

• Internal Energy of an Ideal Gas

  • In thermodynamics, internal energy is the total energy contained by a thermodynamic system.
  • Therefore, we will disregard potential energy and only focus on the kinetic energy contribution to the internal energy.
  • Therefore, practical internal energy changes in an ideal gas may be described solely by changes in its translational kinetic energy.
  • Therefore, the internal energy for diatomic gases is U=52NkTU = \frac{5}{2}NkT.
  • Determine the number of degrees of freedom and calculate the internal energy for an ideal gas molecule

• The First Law

  • The 1st law of thermodynamics states that internal energy change of a system equals net heat transfer minus net work done by the system.
  • It is usually formulated by stating that the change in the internal energy of a closed system is equal to the amount of heat supplied to the system, minus the amount of work done by the system on its surroundings.
  • Here ΔU is the change in internal energy U of the system, Q is the net heat transferred into the system, and W is the net work done by the system.
  • Note also that if more heat transfer into the system occurs than work done, the difference is stored as internal energy.
  • The change in the internal energy of the system, ΔU, is related to heat and work by the first law of thermodynamics, ΔU=Q−W.

• Human Metabolism

  • Thus, in such situations the body loses internal energy, since ΔU=Q−W is negative.
  • Eating increases the internal energy of the body by adding chemical potential energy.
  • Our body loses internal energy, and there are three places this internal energy can go—to heat transfer, to doing work, and to stored fat (a tiny fraction also goes to cell repair and growth).
  • If you eat just the right amount of food, then your average internal energy remains constant.
  • If you overeat repeatedly, then ΔU is always positive, and your body stores this extra internal energy as fat.

• Free Energy and Work

  • The Gibbs free energy is the maximum amount of non-expansion work that can be extracted from a closed system.
  • The work is done at the expense of the system's internal energy.
  • Energy that is not extracted as work is exchanged with the surroundings as heat.
  • ΔG is the maximum amount of energy which can be "freed" from the system to perform useful work.
  • The impossibility of extracting all of the internal energy as work is essentially a statement of the Second Law.

• Energy Transformations

  • Energy transformation occurs when energy is changed from one form to another.
  • For example, an internal combustion engine converts the potential chemical energy in gasoline and oxygen into heat energy.
  • For example, the theoretical limit of the energy efficiency of a wind turbine (converting the kinetic energy of the wind to mechanical energy) is 59%.
  • This corresponds to zero kinetic energy and thus all of the energy of the pendulum is in the form of potential energy.
  • These figures illustrate the concepts of energy loss and useful energy output.

• Conservation of Mechanical Energy

  • Conservation of mechanical energy states that the mechanical energy of an isolated system remains constant without friction.
  • Though energy cannot be created nor destroyed in an isolated system, it can be internally converted to any other form of energy.
  • The work-energy theorem states that the net work done by all forces acting on a system equals its change in kinetic energy (KE).
  • This equation is a form of the work-energy theorem for conservative forces; it is known as the conservation of mechanical energy principle.
  • The total kinetic plus potential energy of a system is defined to be its mechanical energy (KE+PE).

• The First Law of Thermodynamics

  • Thermodynamics is the study of heat energy and other types of energy, such as work, and the various ways energy is transferred within chemical systems.
  • Energy exists in many different forms.
  • For instance, light bulbs transform electrical energy into light energy, and gas stoves transform chemical energy from natural gas into heat energy.
  • Another useful form of the first law of thermodynamics relates heat and work for the change in energy of the internal system:
  • Plants can convert electromagnetic radiation (light energy) from the sun into chemical energy.

• Food Energy and ATP

  • Animals use energy for metabolism, obtaining that energy from the breakdown of food through the process of cellular respiration.
  • Animals need food to obtain energy and maintain homeostasis.
  • Homeostasis is the ability of a system to maintain a stable internal environment even in the face of external changes to the environment.
  • ATP stores energy in phosphate ester bonds, releasing energy when the phosphodiester bonds are broken: ATP is converted to ADP and a phosphate group.
  • ATP is the energy molecule of the cell.