What are the Laws of Thermodynamics?

Thermodynamics is that branch of physics which discusses about the relationship between heat and other forms of energy.

Thermal energy in a substance or system is there due to its temperature, i.e., due to the energy of moving or vibrating molecules. Thermodynamics is used to measure this energy.



Thermodynamics is concerned with many properties of matter. One of them is heat. Heat is defined as the energy which is transferred between substances or systems as a result of the temperature difference between them. As a form of energy, heat can neither be created or destroyed, it can only be conserved. But it can be transferred from one place to another. Heat

can also be changed to other forms of energy.



The amount of heat which is transferred by a substance is dependent on the speed and number of atoms or molecules in motion. When the atoms or molecules move faster, then the temperature is higher. When there are more atoms or molecules in motion, then the heat transferred will be in greater quantity.


Specific heat

The amount of heat which is needed to increase the temperature of a certain mass of a substance with a certain amount is called specific heat. The unit is calories per gram per kelvin.


The three laws of thermodynamics


The first law of thermodynamics

This law is also known as Law of conservation of energy. It states that energy can neither be created nor be destroyed but can be transformed or changed from one form to another. For example, while we turn on a light, it produces energy. But it is the electrical energy that is converted.


The first law can be expressed as any change which occurs in the internal energy of a system and can be denoted by the sum of the heat (q) that flows through the boundaries and the work (w) performed on the system by the surroundings.

According to this law, there are two kinds of processes, heat and work, which can cause a change in the internal energy of a system.


The second law of thermodynamics

According to the second law of thermodynamics, the entropy of any isolated system must increase. Isolated systems make thermal equilibrium which is a state of maximum entropy of the system.

A simple way of explaining the second law of thermodynamics is that, a room which is not cleaned will become more dirtier with time.

When the room is tidied, its entropy decreases. But the effort which is needed to clean the room leads to an increase in entropy outside the room.


The third law of thermodynamics

According to the third law of thermodynamics, the entropy of a system reaches a constant value as the temperature reaches absolute zero.

However, in Physics, there are four laws of thermodynamics

The 4th law is as follows:

Zeroth law of thermodynamics

When two thermodynamic systems are in thermal equilibrium with a third, then they are said to be in thermal equilibrium with each other. This law   consider temperature as a fundamental and measurable property of matter. This law is very vital. That is why, it has been placed before all the other 3 laws.

On in other words, if A=B and C=B then A=C.


The first law

According to the first law of thermodynamics, energy can neither be created nor destroyed. It can only change from one form to another. Energy is conserved.


The energy in a system can be changed to heat or work or other things. But the total energy will be the same. Heat is a form of energy which is under the principle of conservation.

The second law

This is one of the most famous and important laws of science. According to the second law, it means that the entropy of the universe goes to a maximum.


The third law

The third law gives a reference point for measuring entropy. It states that as the temperature of a system reaches absolute zero (-273.15 degree C, 0 K), then the value of the entropy reaches a minimum.

The entropy of a pure crystal at absolute zero is zero. Entropy is ‘Waste energy”, i.e., energy which cannot do any work. As there is no heat energy at absolute zero, there will be no waste energy. Entropy is also used to measure the disorder in a system. A perfect crystal is perfectly ordered. Any positive value of temperature means that there is motion in the crystal thus causing disorder. Due to these reasons, there are no physical system with lower energy. That is why, entropy is always positive.


Can you solve this:

During an adiabatic process, the pressure of a gas is found to be proportional to the cube of its absolute temperature. The ratio C_P//C_V for the gas is:

  1. A) 3/2
  2. B) 2/3
  3. C) 4/3
  4. D) 5/3


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