Internal energy
Internal energy changes

Thermodynamics is chiefly concerned with the changes in internal energy Δ U {\displaystyle \Delta U}.

For a closed system, with matter transfer excluded, the changes in internal energy are due to heat transfer Q {\displaystyle Q} and due to thermodynamic work W {\displaystyle W} done by the system on its surroundings. Accordingly, the internal energy change Δ U {\displaystyle \Delta U} for a process may be written Δ U = Q − W (closed system, no transfer of matter). {\displaystyle \Delta U=Q-W\quad {\text{(closed system, no transfer of matter)}}.}

When a closed system receives energy as heat, this energy increases the internal energy. It is distributed between microscopic kinetic and microscopic potential energies. In general, thermodynamics does not trace this distribution. In an ideal gas all of the extra energy results in a temperature increase, as it is stored solely as microscopic kinetic energy; such heating is said to be sensible.

A second kind of mechanism of change in the internal energy of a closed system changed is in its doing of work on its surroundings. Such work may be simply mechanical, as when the system expands to drive a piston, or, for example, when the system changes its electric polarization so as to drive a change in the electric field in the surroundings.

If the system is not closed, the third mechanism that can increase the internal energy is transfer of matter into the system. This increase, Δ U m a t t e r {\displaystyle \Delta U_{\mathrm {matter} }} cannot be split into heat and work components. If the system is so set up physically that heat transfer and work that it does are by pathways separate from and independent of matter transfer, then the transfers of energy add to change the internal energy: Δ U = Q − W + Δ U matter (matter transfer pathway separate from heat and work transfer pathways). {\displaystyle \Delta U=Q-W+\Delta U_{\text{matter}}\quad {\text{(matter transfer pathway separate from heat and work transfer pathways)}}.}

If a system undergoes certain phase transformations while being heated, such as melting and vaporization, it may be observed that the temperature of the system does not change until the entire sample has completed the transformation. The energy introduced into the system while the temperature does not change is called latent energy or latent heat, in contrast to sensible heat, which is associated with temperature change.
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