What is the First Law of Thermodynamics

Thermodynamics is a branch of physics which deals with the heat energy, its transfer, and its relation with other forms of energy. In thermodynamics, there are 4 basic laws that govern the nature of particles when they undergo heat transfer. Here we will focus on the first law. In simple words, the first law of thermodynamics is the law of conservation of energy. It states that the internal energy of a system is always equal to the sum of work done on the system and heat flowing in or out of the system.  

Definition of the First Law of Thermodynamics:

Any thermodynamic system in its equilibrium state contains a certain amount of energy known as the internal energy(E). For two systems in equilibrium, the change in the internal energy is equivalent to the difference of the heat transfer into the system and the work done by the system. According to the first law of thermodynamics’ definition, the energy of the system and the surroundings remain the same, i.e. it can change its form from one type of energy to another but it can’t be created or destroyed. 

Let’s try to understand the first law of thermodynamics with a common example. Internal combustion engine, which is used in automobiles, is a device that converts heat energy into mechanical energy and vice versa. When a fluid is heated, its volume is increased and on cooling the pressure of fluid increases. Internal combustion (IC) engines leverage this relationship between heat, volume and pressure of a fluid to convert thermal energy into mechanical energy. 

In IC engines, the liquid fuel is burnt in a closed chamber in the presence of air. One end of the chamber is fitted with movable pistons, which is again connected with the gearbox and, in turn with the wheel of a vehicle. On burning of fuel, the temperature of the gas increases along with its volume which pushes the pistons. This movement of pistons can then be used to move the wheels of the vehicles. 

Equation of the First Law of Thermodynamics

The mathematical equation for the First Law of Thermodynamics can be summarized as: 

                                                          ΔU = Q+ W

     Where,

                 ΔU: Signifies the change in internal energy(E) of the thermodynamic system.

                   Q: Depicts the sum of heat transfer between the system and its surroundings.

                  W: Total work interaction of the system with its surroundings.

Thermodynamic systems

As per the law of heat transfer, there are three types of thermodynamic systems:

Open systems: An open system freely exchanges energy and matter with its surroundings. For example, a pan of boiling water over a stove receives thermal energy from the fuel of the stove. The radiating heat from the pan boils the water and emits matter along with heat into surroundings in the form of steam. 

Closed systems: A closed system exchanges energy but not matter with its surroundings. For example, suppose we put a lid on the pan from the above example. The pan will radiate energy into the surroundings but it won’t allow steam to go out so no matter is exchanged with the surroundings. 

Isolated systems: A system that does not exchange energy or matter with its surroundings is called an isolated system. For example, if you pour that hot water into a thermos bottle and close the lid, it will cut its connection with the surroundings and the thermos bottle becomes an isolated system. 

In the physical world, a perfectly isolated system can’t exist. Each and every system exchanges energy to and from their environment. That’s why the hot water that is kept in the thermos will come to room temperature in a few hours. Another such example is white dwarf star. White dwarf stars are the hot residue of burned-out stars which don’t produce energy. They remain in the perfect vacuum of space but eventually lose energy by radiation and cool down to absolute zero from several tens of thousands of degrees. 

Limitations of the First Law of Thermodynamics

Some of the limitations of the first law of thermodynamics includes:

1. Inability to explain the change in the energy of a system when it undergoes change of state i.e. from one form of matter to another form. The change in state of matter emits or intakes energy for changing the kinetic energy of the particles which can’t be explained by the first law of thermodynamics. 
2. Again, the first law is not able to explain the phenomena of heat transfer from the hot end to cold end of a metallic rod when it is heated at one end. 
3. It doesn’t explain anything about the direction of flow of the thermal energy.
4. The first law of thermodynamics doesn’t explain the spontaneity of a process.  

Conclusion:

The internal energy of a system which is introduced by the first law of thermodynamics is similar to the potential energy of a body kept at any height above the ground or the kinetic energy of an object in motion. The internal energy of a body undergoing thermal exchange is conserved in the same way as the kinetic or potential energy. The first law of thermodynamics allows particles to exist in n-number of states. However, there are only 5 states of matter that are known to us till date. There are many devices like heat engines, heat pumps, refrigerators, air conditioners and so on that work on the principle of the first law of thermodynamics.