First Law of Thermodynamics

A First Law of Thermodynamics is physical phenomena law which states the energy of an isolated system is always constant.

AKA: Thermodynamics Conservation of Energy Law.
Context
- It can be expressed as [math]\displaystyle{ \Delta U = Q \pm W }[/math] where [math]\displaystyle{ \Delta U }[/math] is the change of internal energy and H the heat added to system. Using either sign depends on convention how work is defined[math]\displaystyle{ + W }[/math] is the work done on the system while [math]\displaystyle{ - W }[/math] is the work done by system.
- For a quasistatic process, [math]\displaystyle{ d U = \delta Q \pm P DV }[/math] where [math]\displaystyle{ \delta Q }[/math] denotes is infinitesimal increment of heat added to the system, P is the pressure and V is volume change, dV. The work done by the is the [math]\displaystyle{ +PdV }[/math] while the work done "on" the system is [math]\displaystyle{ -P dV }[/math]
Example(s)
- Conservation of Energy.
Counter-Example(s)
See: Laws Of Thermodynamics, Perpetual Motion Machines, Thermodynamics, Physical System, Maxwell's Demon Thought Experiment, Adiabatic Process.

References

2015

(Wikipedia, 2015) ⇒ http://en.wikipedia.org/wiki/First_law_of_thermodynamics Retrieved:2015-12-19.
- The first law of thermodynamics is a version of the law of conservation of energy, adapted for thermodynamic systems. The law of conservation of energy states that the total energy of an isolated system is constant; energy can be transformed from one form to another, but cannot be created or destroyed. The first law is often formulated by stating that the change in the internal energy of a closed system is equal to the amount of heat supplied to the system, minus the amount of work done by the system on its surroundings. Equivalently, perpetual motion machines of the first kind are impossible.

2010

(Halliday et al., 2010) ⇒ David Halliday, Robert Resnick, and Jearl Walker. “Fundamentals of physics extended". John Wiley & Sons, 2010.

2005

(Hyperphysics Encyclopedia, 2005) ⇒ http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/seclaw.html#c1
- The first law of thermodynamics is the application of the conservation of energy principle to heat and thermodynamic processes:

The change in internal energy of a system is equal to the heat added to the system minus the work done by the system.

[math]\displaystyle{ \Delta U = Q - W }[/math]

The first law makes use of the key concepts of internal energy, heat, and system work. It is used extensively in the discussion of heat engines. The standard unit for all these quantities would be the joule, although they are sometimes expressed in calories or BTU.

It is typical for chemistry texts to write the first law as [math]\displaystyle{ \Delata U=Q+W }[/math]. It is the same law, of course - the thermodynamic expression of the conservation of energy principle. It is just that W is defined as the work done on the system instead of work done by the system. In the context of physics, the common scenario is one of adding heat to a volume of gas and using the expansion of that gas to do work, as in the pushing down of a piston in an internal combustion engine. In the context of chemical reactions and process, it may be more common to deal with situations where work is done on the system rather than by it.

1963

(Feynman et al., 1963) ⇒ Richard P. Feynman, Robert B. Leighton and Matthew Sands (1963, 1977, 2006, 2010, 2013) "The Feynman Lectures on Physics": New Millennium Edition is now available online by the California Institute of Technology, Michael A. Gottlieb, and Rudolf Pfeiffer ⇒ http://www.feynmanlectures.caltech.edu/
- Heat put into a system+Work done on a system=Increase in internal energy of the system:

[math]\displaystyle{ dQ+dW=dU }[/math]