JOULE’S EXPERIMENT || INTERNAL ENERGY || FIRST LAW OF THERMODYNAMICS

●  JOULE’S EXPERIMENT

Today’s concept of heat developed by the crucial experiments carried out in the 1840s by James P. Joule.

At that time Joule has performed multiple experiments with water, oil and mercury.  In that experiments he took known amounts of water, oil or mercury in an isolated vessel and agitated the fluid with stirrer.

He noted the power given to fluid by the rotating stirrer. He noticed that the temperature of the fluid was increased. He precisely measured the amount of work done on the fluid by the stirrer (Power input to the Stirrer) and the resulting temperature changes in the fluid (T_final – T_intial).

He showed that for each type fluid certain amount of work per unit mass of fluid was required to raise the temperature by 1 degree by stirring. And the previous temperature of fluid is restored by the simple heat transfer with cooler object.

These experiments demonstrated the existence of a quantitative relationship between work and heat, and thereby showed that heat is a form of energy.

 ●  INTERNAL ENERGY:

In experiments like those performed by the James P. Joule, Energy added to the fluid as a work by the agitator and transferred to the cooler object is later known as the “HEAT”.

A general question is: Where does the energy stored after its addition to the fluid and before its transfer from the fluid?

Answer to this question is that it is contained within the fluid in another form of energy which is known as “INTERNAL ENERGY”.

Internal energy of a substance does not count energy that is may possess as a result of the gross movement or position as a whole. It refers to energy of the molecules comprising the substance. Due to ceaseless motion all molecules comprising the substance possess the kinetic energy of translation.

The addition of heat to a substance increases molecular motion and thus causes an increase in the internal energy of the substance. Work done on the substance also have the same effect as shown by James P. Joule. Potential energy associated with the intermolecular forces are also counted as internal energy. Energy is associated with the interactions of electrons and nuclei of atoms on a sub-molecular scale, which includes the energy of chemical bonds that hold atoms together as molecules.

Name internal energy distinguishes from external forms the potential, static or kinetic energy associated with a substance because of its macroscopic motion, position or configuration which are considered as external forms of energy.

There is no precise thermodynamic definition of internal energy. Measurement of internal energy of any thermodynamic system cannot be measured. There is no instrument available to measure internal energy. Absolute value of internal value are unknown. This is not disadvantage in thermodynamic analysis because the absolute value of internal energy of system is not required only changes in internal energy are required.

●  THE FIRST LAW OF THERMODYNAMICS

The first law of thermodynamics is also known as principle of conservation of mechanical energy. It also includes heat and internal energy in addition to work and external potential and kinetic energy. In generally, it can be extended to other forms of energy like surface energy, electrical energy and magnetic energy.

“Although energy assumes many forms, the total quantity of energy is constant, and when energy disappears in one form it appears simultaneously in other forms.”

To apply this law to a particular process, the process is divided into two parts, the system and its surrounding. Region in which process occurs is known as system and everything outside the system is known as surroundings. System and surrounding is separated by boundary, boundary may be rigid or flexible, real or imaginary. The first law of thermodynamics applies to system and its surroundings; not to the system alone. For thermodynamic process, the first law requires:

∆ (Energy of the system) + ∆ (Energy of the surroundings) = 0

(Note: ∆ is Difference Operator)

In thermodynamic language, heat and work represent energy in transit across the boundary dividing the system from its surroundings, and never stored in system nor contained in the system. On the other hand, potential, kinetic and internal energy stored with matter. Heat and Work represents energy flows from or to a system.

The sum of change in energy of system and change in energy of surrounding is zero. This is known as first law of thermodynamics.