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ChemWiki: The Dynamic Chemistry E-textbook > Physical Chemistry > Thermodynamics > State Functions > Thermal Energy

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Thermal Energy

Thermal Energy, also known as random or internal Kinetic Energy, due to the random motion of molecules in a system. Kinetic Energy is seen in three forms: vibrational, rotational, and translational. Vibrational is the energy caused by an object or molecule moving in a vibrating motion, rotational is the energy caused by rotating motion, and translational is the energy caused by the movement of one molecule to to another location.

Thermal Energy and Temperature

Thermal energy is directly proportional to the temperature within a given system (recall that a system is the subject of interest while the surroundings are located outside of the systems and the two interact via energy and matter exchange.) As a result of this relationship between thermal energy and the temperature of the system, the following applies:The more molecules present, the greater the movement of molecules within a given system, the greater the temperature and the greater the thermal energy

+ molecules = +movement = + temperature = + thermal energy

As previously demonstrated, the thermal energy of a system is dependent on the temperature of a system which is dependent on the motion of the molecules of the system. As a result, the more molecules that are present, the greater the amount of movement within a given system which raises the temperature and thermal energy. Because of this, at a temperature of 0?C, the thermal energy within a given system is also zero. This means that a relatively small sample at a somewhat high temperature such as a cup of tea at its boiling temperature could have less thermal energy than a larger sample such as a pool that's at a lower temperature. If the cup of boiling tea is placed next to the freezing pool, the cup of tea will freeze first because it has less thermal energy than the pool.

To keep definitions straight, remember the following

  • Temperature: Temperature is the average kinetic energy within a given object and is measured by three scales of measurement (Fahrenheit, Celsius, Kelvin)
  • Thermal Energy: Thermal energy is defined as the total of all kinetic energies within a given system.
  • Heat: It is important to remember that heat is caused by flow of thermal energy due to differences in temperature (heat flows from object at higher temperature to object at lower temperature), transferred through conduction/convection/radiation. Additionally thermal energy always flows from warmer areas to cooler areas.

Thermal Energy and States of Matter

Matter exists in three states: solid, liquid, or gas. When a given piece of matter undergoes a state change, thermal energy is either added or removed but the temperature remains constant. When a solid is melted, for example, thermal energy is what causes the bonds within the solid to break apart.

Heat: the Transfer of Thermal Energy

Heat can be given off in three different processes: conduction, convection, or radiation. Conduction occurs when thermal energy is transferred through the interaction of solid particles. This process often occurs when cooking: the boiling of water in a metal pan causes the metal pan to warm as well. Convection usually takes place in gases or liquids (whereas conduction most often takes place in solids) in which the transfer of thermal energy is based on differences in heat. Using the example of the boiling pot of of water, convection occurs as the bubbles rise to the surface and, in doing so, transfer heat from the bottom to the top. Radiation is the transfer of thermal energy through space and is responsible for the sunlight that fuels the earth.

Thermal energy is a concept applicable in everyday life. For example, engines, such as those in cars or trains, do work by converting thermal energy into mechanical energy. Also, refrigerators remove thermal energy from a cool region into a warm region.

On a larger scale, recent scientific research has been aiming to convert solar energy to thermal energy in order to create head and electricity. For example,scientific research centers such as NASA explore the uses and applications of thermal energy in order to provide for more efficient energy production. In 1990, for example, NASA extensively researched and explored the potentials of a hybrid power system which made use of Thermal Energy Storage (TES) devices. This power system would convert solar energy into thermal energy which would then be used to produce electrical power and heat. However, converting solar energy to thermal energy has been found to be much easier and much more feasible when systems are not in a state of thermodynamic equilibrium. Rather, scientists have proposed, a moving object or a running fluid can allow the energy to be converted into thermal energy.

Thermal Energy and the 2nd Law of Thermodynamics The 2nd Law of Thermodynamics? states that whenever work is performed, the amount of entropy in the atmosphere is increased. Thus the flow of thermal energy is constantly increasing entropy.


  1. Kerslake, Thomas W. & Ibrahim, Mounir B. "Analysis of Thermal Energy Storage Material with Change-of Phase Volumetric Effects." NASA Technical Memorandum 102457 (February 1990). 30 May 2008. <external link: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov>
  2. Petrucci, et al. General Chemistry: Principles & Modern Applications: AIE (Hardcover). Upper Saddle River: Pearson/Prentice Hall, 2007.


  • Leah Hughes
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12:54, 28 May 2014



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