<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Relevant discussion may be found on the talk page . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (September 2014) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Relevant discussion may be found on the talk page . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (September 2014) (Learn how and when to remove this template message) </Td> </Tr> <P> In thermodynamics, thermal energy refers to the internal energy present in a system due to its temperature . The concept is not well - defined or broadly accepted in physics or thermodynamics, because the internal energy can be changed without changing the temperature, and there is no way to distinguish which part of a system's internal energy is "thermal". Thermal energy is sometimes used loosely as a synonym for more rigorous thermodynamic quantities such as the (entire) internal energy of a system; or for heat or sensible heat which are defined as types of transfer of energy (just as work is another type of transfer of energy). Heat and work depend on the way in which an energy transfer occurred, whereas internal energy is a property of the state of a system and can thus be understood even without knowing how the energy got there . </P> <P> Heat is energy transferred spontaneously from a hotter to a colder system or body . Heat is energy in transfer, not a property of the system; it is not' contained' within the boundary of the system . On the other hand, internal energy is a property of a system . In an ideal gas, the internal energy is the statistical mean of the kinetic energy of the gas particles, and it is this kinetic motion that is the source and the effect of the transfer of heat across a system boundary . The internal energy of an ideal gas can in this sense be regarded as "thermal energy". In this case, however, thermal energy and internal energy are identical . </P>

The external energy cycle includes which of the following