First Law of Thermodynamics

The first law of thermodynamics states that the energy of the universe remains constant. Energy may exchanged between the system and its surroundings, but it can neither be created nor destroyed. This means that the total amount of energy in the universe remains constant.

For closed systems, the first law was originally induced from empirically observed evidence.

Adiabatic Processes

There is transfer of energy as work but not as heat in an adiabatic process. Irrespective of how the work is done, for all adiabatic process that takes a system from a given initial state to a given final state the respective eventual total quantities of energy transferred as work are the same. 

The work done on the system is defined and measured by changes in mechanical or quasi-mechanical variables external to the system. 

Physically, adiabatic transfer of energy as work requires the existence of adiabatic enclosures. Adiabatic work takes the system either from a reference state O with internal energy U (O) to arbitrary one A with internal energy U (A), or state A to the state O :

For all adiabatic processes between two specified states of a closed system of any nature, the net work done is the same regardless the details of the process, and determines a state function called internal energy, U.

History

The first full statements of the law came in 1850 from Rudolf Clausius and from William Rankine. 

 

William Rankine

Rudolf Clausius in 1850 said: “There is a state function E, called ‘energy’, whose differential equals the work exchanged with the surroundings during an adiabatic process.”

However, it was Germain Hess (via Hess’s Law), and later by Julius Robert von Mayer who first formulated the law. In 1840, Germain Hess stated a conservation law (Hess's Law) for the heat of reaction during chemical transformations. This law was later recognized as a consequence of the first law of thermodynamics, but Hess's statement was not explicitly concerned with the relation between energy exchanges by heat and work.

 

Clausius also stated the law in another form, referring to the existence of a function of state of the system, the internal energy, and expressed it in terms of a differential equation for the increments of a thermodynamic process.

In a thermodynamic process involving a closed system, the increment in the internal energy is equal to the difference between the heat accumulated by the system and the work done by it.
ΔU = Q – W, where ΔU denotes the change in the internal energy of a closed system, Q denotes the quantity of energy supplied to the system as heat, and W denotes the amount of thermodynamic work done by the system on its surroundings.

Sign conventions

Change in the internal energy of a system is the total energy added as heat to the system minus the thermodynamic work done by the system in non-cyclic process, both being measured in mechanical units.

Taking U as a change in internal energy, one writes

Nowadays, we use the IUPAC convention in which the first law is formulated with thermodynamic work done on the system by its surroundings being positive.
With this now often used sign convention for work, the first law for a closed system may be written:
 
  

In the Clausius sign convention for work, when a system expands in a fictive quasistatic process, the thermodynamic work done by the system on the surroundings is a product, of pressure and volume change whereas the thermodynamic work done on the system by the surroundings is –PdV.

Using either sign convention for work, the change in internal energy of the system is: 

Advantages

The total energy of the universe remains constant and different forms of energy are inter-convertible. Another great advantage of conservation laws such as the first law of thermodynamics is that they accurately describe the beginning and ending of complex processes without regard to the complications in between.

The first law of thermodynamics is so general that its predictions cannot all be directly tested. In many properly conducted experiments it has been precisely supported, and never violated. 

Indeed, within its scope of applicability, the law is so reliably established, that, nowadays, rather than experiment being considered as testing the accuracy of the law, it is more practical and realistic to think of the law as testing the accuracy of experiment.

Limitations

The first law does not say whether a process will occur or not or will the process occur on its own.
It also does not tell us about the entropy of system. It does not tell about the final temperature of two bodies when they are in direct contact nor the direction.

Direction

Spontaneity

 

 

Solution – Second law of thermodynamics overcomes the limitations of the first law by telling us about the feasibility of a process and the direction by two well statements

1.    Kelvin Planck Statement
2.    Clausius Statement

Real life examples

The heat energy of the fuel is converted into mechanical energy. When the fuel is ignited inside the cylinder, it produces heat energy, higher temperature is created inside the cylinder. As a result, the expansion of the piston takes place which in turn produces the rotational effect.

Steam engines convert thermal energy into mechanical energy in a train.