## At A Glance

### First Law Of Thermodynamics

Check us out at http://www.tutorvista.com//videos The first law of thermodynamics, an expression of the principle of conservation of energy, states that energy can be transformed (changed from one form to another), but cannot be created or destroyed. The first law of thermodynamics says that energy is conserved in any process involving a thermodynamic system and its surroundings. Frequently it is convenient to focus on changes in the assumed internal energy (U) and to regard them as due to a combination of heat (Q) added to the system and work done by the system (W). Taking dU as an incremental (differential) change in internal energy, one writes where δQ and δW are incremental changes in heat and work, respectively. Note that the minus sign in front of δW indicates that a positive amount of work done by the system leads to energy being lost from the system. Depending on discipline, an alternative convention may be adopted for the "work" under consideration, leading to: where is the work done on the system by the surroundings. When a system expands in a quasistatic process, the work done on the system is ? PdV whereas the work done by the system while expanding is PdV. In any case, both give the same result when written explicitly as: Work and heat are due to processes which add or subtract energy, while U is a particular form of energy associated with the system. Thus the term "heat energy" for δQ means "that amount of energy added as the result of heating" rather than referring to a particular form of energy. Likewise, "work energy" for δw means "that amount of energy lost as the result of work". Internal energy is a property of the system whereas work done and heat supplied are not. A significant result of this distinction is that a given internal energy change (dU) can be achieved by, in principle, many combinations of heat and work. Informally, the law was first formulated by Germain Hess via Hess's Law, and later by Julius Robert von Mayer 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."
Length: 03:30

## Contact

Questions about First Law Of Thermodynamics