Bottom Science
The First Law of Thermodynamics is a form of the law of conservation of energy, it states that energy can only be converted from one form to another, but it cannot be created or destroyed.
Joule’s law gave us the relation between the work done and heat produced,
i.e. W = JH
Here J is the Joule’s mechanical equivalent of heat.
It is true when the whole work done is used to produce the heat but in actual practice, when heat is supplied to a system then whole heat energy may not be converted into work. Some part of heat energy may be used in increasing internal energy of the system.
Before we jump on the mathematical definition of the first law of thermodynamics, we must have some knowledge about the concept of internal energy.
Internal Energy
It is the stored energy of a system, which is revealed by the system during its phase transition. It’s basically the sum of – Kinetic Energy + Potential Energy + Other Electron & Neutron Interaction.
It changes due to:
- Thermal interactions (exchange of heat).
- Mechanical interactions.
- Diffusion Interaction.
Mathematical Expression
If dU is the change in the internal energy of the system, δW be the work done by the system and δQ is the change in heat, then mathematically the first law can be represented as:
dU = δQ – δW + μdN
Here, the quantity μdN is due to the number of particles in the thermodynamic system. This quantity can be neglected for a closed system.
Applications
1. Isothermal Process:
As you know that, a process in which temperature of the system remains constant is called Isothermal process. As the temperature is constant, the internal is also constant.
Hence in this process heat absorbed is entirely used for work done.
2. Adiabatic Process:
In the adiabatic process, there’s no exchange of heat between the system and surroundings.
i.e. δQ = 0
Hence, by using the first law
dU = 0 – δW
dU = -δW
Therefore, change in the internal energy of the system take place due to work done on the system or work done by the system.
3. Isochoric Process:
There is no change in volume during the isochoric process.
i.e. δV = 0 therefore, δW = 0
Hence from the first law, we get
dU = δQ – 0
dU = δQ
Thus the increase in internal energy of the system is due to the absorption of the heat from surroundings and the decrease of the internal energy is due to the release of heat from the system.
