thermal energy

Examples of thermal energy in the following topics:

• 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.
  • By far the largest fraction goes to thermal energy, although the fraction varies depending on the type of physical activity.
  • Energy consumed by humans is converted to work, thermal energy, and stored fat.

• Other Forms of Energy

  • Thermal, chemical, electric, radiant, nuclear, magnetic, elastic, sound, mechanical, luminous, and mass are forms that energy can exist in.
  • Thermal Energy: This is energy associated with the microscopic random motion of particles in the media under consideration.
  • An example of something that stores thermal energy is warm bath water.
  • Electric Energy: This is energy that is from electrical potential energy, a result of Coulombic forces.
  • For example, luminous energy is radiant energy.

• Elastic Potential Energy

  • If a force results in only deformation, with no thermal, sound, or kinetic energy, the work done is stored as elastic potential energy.
  • If the only result is deformation and no work goes into thermal, sound, or kinetic energy, then all the work is initially stored in the deformed object as some form of potential energy.
  • Elastic energy of or within a substance is static energy of configuration.
  • Thermal energy is the randomized distribution of kinetic energy within the material, resulting in statistical fluctuations of the material about the equilibrium configuration.
  • For example, for some solid objects, twisting, bending, and other distortions may generate thermal energy, causing the material's temperature to rise.

• What is Power?

  • It is the amount of energy consumed per unit time.
  • Power implies that energy is transferred, perhaps changing form.
  • It is never possible to change one form completely into another without losing some of it as thermal energy.
  • For example, a 60-W incandescent bulb converts only 5 W of electrical power to light, with 55 W dissipating into thermal energy.
  • The remainder becomes a huge amount of thermal energy that must be dispersed as heat transfer, as rapidly as it is created.

• Humans and Electric Hazards

  • The hazards from electricity can be categorized into thermal and shock hazards.
  • There are two known categories of electrical hazards: thermal hazards and shock hazards.
  • A thermal hazard is when excessive electric power causes undesired thermal effects, such as starting a fire in the wall of a house.
  • Electric power causes undesired heating effects whenever electric energy is converted to thermal energy at a rate faster than it can be safely dissipated.
  • Thermal energy delivered at this rate will very quickly raise the temperature of surrounding materials, melting or perhaps igniting them.

• Thermal Instability

  • So far we have examined instabilities where energy does not leave or enter the fluid.
  • In general hot gas emits and absorbs radiation; it may release energy through nuclear or chemical reactions as well.

• Thermal Stresses

  • Solids also undergo thermal expansion.
  • What are the basic properties of thermal expansion?
  • An increase in temperature implies an increase in the kinetic energy of the individual atoms.
  • In a solid, unlike in a gas, the atoms or molecules are closely packed together, but their kinetic energy (in the form of small, rapid vibrations) pushes neighboring atoms or molecules apart from each other.
  • Thermal stress is created by thermal expansion or contraction.

• Thermal Radiation

  • If it didn't, we could set up an adjacent blackbody enclosure at the same temperature and energy would flow between them.
  • $\displaystyle \text{Another Kirchoff's Law: }S_\nu = B_\nu(T) \text{ for a thermal emitter}$
  • Because $I_\nu=B_\nu(T)$ outside of the thermal emitting material and $S_\nu=B_\nu(T)$ within the material, we find that $I_\nu=B_\nu(T)$ through out the enclosure.
  • If we remove the thermal emitter from the blackbody enclosure we can see the difference between thermal radiation and blackbody radiation.
  • A thermal emitter has $S_\nu = B_\nu(T)$,$B_\nu(T)$ so the radiation field approaches $B_\nu(T)$ (blackbody radiation) only at large optical depth.

• Thermal Bremsstrahlung Emission

  • The most important case astrophysically is thermal bremsstrahlung where the electrons have a thermal distribution so the probablility of a particle having a particular velocity is
  • We know that radiation comes in bunches of energy $\hbar \omega$ so for a particular frequency $mv^2/2 > h\nu$ for the electron to have enough energy to emit a photon.
  • ${\bar g}_{ff}$ is the thermally averaged Gaunt factor.
  • Thermal bremsstrahlung spectra for two temperatures that differ by a factor of ten

• Thermal Bremsstrahlung Absorption

  • If we assume that the photon field is in thermal equilibrium with the electrons and ion we can obtain an expression for the corresponding absorption,
  • We can also integrate $\alpha_\nu^{ff}$ over all photon energies to get the Rosseland mean absorption coefficient which is