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## First law of thermodynamics
In thermodynamics, the ## Additional recommended knowledge
## StatementsSuccinctly, the first law of thermodynamics states: - The increase in the internal energy of a system is equal to the amount of energy added by heating the system, minus the amount lost as a result of the work done by the system on its surroundings.
## DescriptionThe first law of thermodynamics basically states that a thermodynamic system can store or hold energy and that this where The δ's before the heat and work terms are used to indicate that they describe an increment of energy which is to be interpreted somewhat differently than the The first explicit statement of the first law of thermodynamics was given by Rudolf Clausius in 1850: "There is a state function E, called ‘energy’, whose differential equals the work exchanged with the surroundings during an adiabatic process." Note that the above formulation is favored by engineers and physicists. Chemists prefer a second form, in which the work term δ ## Mathematical formulationThe mathematical statement of the first law is given by: where An expression of the first law can be written in terms of exact differentials by realizing that the work that a system does is equal to its pressure times the infinitesimal change in its volume. In other words δ In the case where the number of particles in the system is not necessarily constant and may be of different types, the first law is written: where A useful idea from mechanics is that the energy gained by a particle is equal to the force applied to the particle multiplied by the displacement of the particle while that force is applied. Now consider the first law without the heating term: It is useful to view the Similarly, a difference in chemical potential between groups of particles in the system forces a trasfer of particles, and the corresponding product is the amount of energy transferred as a result of the process. For example, consider a system consisting of two phases: liquid water and water vapor. There is a generalized "force" of evaporation which drives water molecules out of the liquid. There is a generalized "force" of condensation which drives vapor molecules out of the vapor. Only when these two "forces" (or chemical potentials) are equal will there be equilibrium, and the net transfer will be zero. The two thermodynamic parameters which form a generalized force-displacement pair are termed "conjugate variables". The two most familiar pairs are, of course, pressure-volume, and temperature-entropy. ## Types of thermodynamic processesPaths through the space of thermodynamic variables are often specified by holding certain thermodynamic variables constant. It is useful to group these processes into pairs, in which each variable held constant is one member of a conjugate pair. The pressure-volume conjugate pair is concerned with the transfer of mechanical or dynamic energy as the result of work. - An
**isobaric**process occurs at constant pressure. An example would be to have a movable piston in a cylinder, so that the pressure inside the cylinder is always at atmospheric pressure, although it is isolated from the atmosphere. In other words, the system is**dynamically connected**, by a movable boundary, to a constant-pressure reservoir.
- An
**isochoric**process is one in which the volume is held constant, meaning that the work done by the system will be zero. It follows that, for the simple system of two dimensions, any heat energy transferred to the system externally will be absorbed as internal energy. An isochoric process is also known as an**isometric**process. An example would be to place a closed tin can containing only air into a fire. To a first approximation, the can will not expand, and the only change will be that the gas gains internal energy, as evidenced by its increase in temperature and pressure. Mathematically, δ*Q*=*d**U*. We may say that the system is**dynamically insulated**, by a rigid boundary, from the environment.
The temperature-entropy conjugate pair is concerned with the transfer of thermal energy as the result of heating. - An
**isothermal**process occurs at a constant temperature. An example would be to have a system immersed in a large constant-temperature bath. Any work energy performed by the system will be lost to the bath, but its temperature will remain constant. In other words, the system is**thermally connected**, by a thermally conductive boundary to a constant-temperature reservoir.
- An
**isentropic**process occurs at a constant entropy. For a reversible process this is identical to an adiabatic process (see below). If a system has an entropy which has not yet reached its maximum equilibrium value, a process of cooling may be required to maintain that value of entropy.
- An
**adiabatic**process is a process in which there is no energy added or subtracted from the system by heating or cooling. For a reversible process, this is identical to an isentropic process. We may say that the system is**thermally insulated**from its environment and that its boundary is a thermal insulator. If a system has an entropy which has not yet reached its maximum equilibrium value, the entropy will increase even though the system is thermally insulated.
The above have all implicitly assumed that the boundaries are also impermeable to particles. We may assume boundaries that are both rigid and thermally insulating, but are permeable to one or more types of particle. Similar considerations then hold for the (chemical potential)-(particle number) conjugate pairs.
## See also- Conservation of energy
- Laws of thermodynamics
- Perpetual motion
Categories: Laws of thermodynamics | Atmospheric thermodynamics |
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "First_law_of_thermodynamics". A list of authors is available in Wikipedia. |