What is the Difference Between First Law and Second Law of Thermodynamics

The key difference between first law and second law of thermodynamics is that the first law of thermodynamics states that energy cannot be created or destroyed, and the total quantity of the energy in the universe stays the same, whereas the second law of thermodynamics describes the nature of energy.

Thermodynamics refers to the branch of physical science that deals with the relations between heat and other forms of energy such as mechanical, electrical, or chemical energy.

1. Overview and Key Difference
2. What is the First Law of Thermodynamics
3. What is the Second Law of Thermodynamics
4. First Law vs Second Law of Thermodynamics in Tabular Form
5. Summary – First Law vs Second Law of Thermodynamics

What is the First Law of Thermodynamics?

The first law of thermodynamics describes that the internal energy of a system is the difference between the energy it absorbs from the surroundings and the work done by the system on the surrounding. This is a version of the law of conservation of energy that is adapted for thermodynamic processes. It distinguishes three kinds of transfer of energy: heat, thermodynamic work, and internal energy.

We can give the first law of thermodynamics without mass transfer as follows:

ΔU = Q – W

In this expression, ΔU refers to the change in the internal energy of a closed system, while Q denotes the quantity of energy supplied to the system as heat, while W is the amount of thermodynamic work done by the system on the surrounding.

Moreover, the first law of thermodynamics with mass transfer needs involves further conditions; with due account of the corresponding reference states of the system, when two systems are separated only by an impermeable wall, they are combined into a new system by thermodynamic operation of removal of this wall, which leads to the following expression:

U0 = U1 + U2

Where U0 is the internal energy of the combined system, U1 and U2 are the internal energies of the corresponding systems.

What is the Second Law of Thermodynamics?

The second law of thermodynamics describes that the heat cannot flow from a colder location to a hotter area spontaneously. It is the physical law of thermodynamics that describes heat and loss in the conversion. The simplest way of expressing the second law of thermodynamics is “not all heat energy can be converted into work.”

According to the other versions of this law, the concept of entropy is established as a physical property of a thermodynamic system. We can formulate the second law of thermodynamics through the observation “the entropy of isolated systems left to spontaneous evolution cannot decrease because they always arrive at a state of thermodynamic equilibrium (this happens where the entropy is highest at the given internal energy).

What is the Difference Between First Law and Second Law of Thermodynamics?

Thermodynamics refers to the branch of physical science that deals with the relations between heat and other forms of energy such as mechanical, electrical, or chemical energy. The key difference between first law and the second law of thermodynamics is that the first law of thermodynamics state that energy cannot be created or destroyed and the total quantity of the energy in the universe stays the same, whereas the second law of thermodynamics describes that the heat cannot flow from a colder location to a hotter area spontaneously.

The below infographic presents the differences between first law and the second law of thermodynamics in tabular form for side by side comparison.

Summary – First Law vs Second Law of Thermodynamics

The first law of thermodynamics describes that the internal energy of a system is the difference between the energy it absorbs from the surroundings and the work done by the system on the surrounding. The second law of thermodynamics describes that the heat cannot flow from a colder location to a hotter area spontaneously. So, this is the key difference between first law and second law of thermodynamics.