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

• 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.

• Heat as Energy Transfer

  • How is this energy transferred?
  • Work can transfer energy into or out of a system.
  • Heat added or removed from a system changes its internal energy (a concept we will discuss in the following section) and thus its temperature.
  • Work done on the system or by the system can also change the internal energy of the system.
  • A system has a well-defined internal energy, but we cannot say that it has a certain "heat content" or "work content."

• Work

  • For closed systems, energy changes in a system other than as work transfer are as heat.
  • Heat transfer (often represented by Q) and doing work (W) are the two everyday means of bringing energy into or taking energy out of a system.
  • Heat transfer and work are both energy in transit—neither is stored as such in a system.
  • However, both can change the internal energy of a system.
  • Internal energy is a form of energy completely different from either heat or work.

• 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).

• Inelastic Collisions in One Dimension

  • In an inelastic collision the total kinetic energy after the collision is not equal to the total kinetic energy before the collision.
  • If two objects collide, there are many ways that kinetic energy can be transformed into other forms of energy.
  • For example, in the collision of macroscopic bodies, some kinetic energy is turned into vibrational energy of the constituent atoms.
  • When this happens, kinetic energy is often exchanged between the molecules' translational motion and their internal degrees of freedom.
  • The kinetic energy is used on the bonding energy of the two bodies.

• Humans: Work, Energy, and Power

  • The human body converts energy stored in food into work, thermal energy, and/or chemical energy that is stored in fatty tissue.
  • Conservation of energy implies that the chemical energy stored in food is converted into work, thermal energy, or stored as chemical energy in fatty tissue, as shown in .
  • Energy consumed by humans is converted to work, thermal energy, and stored fat.
  • Useful work requires a force exerted through a distance on the outside world, and so it excludes internal work, such as that done by the heart when pumping blood.
  • Energy consumed by humans is converted to work, thermal energy, and stored fat.