Thermodynamics: Basic Topics & Laws

(Last Updated On: December 26, 2018)

What is thermodynamics?

A branch of physics which deals the relationship between heat and the other forms of Energy. In particular, we read about thermal energy and how it affects to atoms or molecules (matter) in thermodynamics. Thermal energy occur due to temperature of system.



Heat is a form of energy which transferred between substance or system due to difference in temperature. It transferred from high temperature to low temperature. It cannot be created or destroyed. We can transfer it and change into other forms.

Foe a example, Steam turbine can change heat into kinetic energy and generator can change it into electrical energy. Bulb can change electrical energy into electromagnetic radiation (light) which absorb by nature and it change back into the heat. So, we can change heat into other forms but we cannot create or destroyed it. It always conserve.


Heat Transfer

we can transfer the heat with 3 different ways:

  1. Conduction: We can transfer the heat via solid that called conduction.
  2. Convection: We can transfer the heat via fluid that called convection.
  3. Radiation: We can transfer the heat via photons (emission of electromagnetic energy) that called radiation.


Source – Google



it is a measurement of average kinetic energy. It tells that how much heat transferred or what is amount of transferred heat. Temperature depends on the speed and numbers of the molecules in motion. If the speed of molecules in any object is higher then maximum heat transfer and if the number of molecules is high then also maximum heat transfer means that the temperature will be high.

If the temperature of the system is 0K then it means that there is no any motion of molecules and total absence of heat.




Thermal conductivity

Thermal conductivity is a rate of heat energy transfer per unit time and per unit area. It shown by k. If value of k is high then heat transfer quickly.

Foe copper and silver (Cu and Ag) the value of k is high (401 and 428). So Cu and Ag usually uses in computer chips for cooling them.

The highest value of k is for diamond in the world and that is 2,200 w/m.k.


Source – Wikipedia


A measure of the disorder and randomness in a closed system is called entropy. Entropy is a waste energy means that it is a kind of energy that cannot use for any work. It always increases.



The Law of Thermodynamics

Newton’s Law of Cooling

Sir Issac Newton tell about cooling any system in 1701.

According to this law, the rate of change in the temperature is proportional to the difference between the temperature of the object and the temperature of surrounding environment.




The Four Law of Thermodynamics

1. The Zeroth Law

If two bodies are in thermal equilibrium with other third body then they are also in thermal equilibrium with each other, this is called the zeroth law.

Assume that there are two system A and B which are in thermal equilibrium with other third system C. Then A and B also will be in thermal equilibrium.




2. The First Law

According to the first law of thermodynamics, the energy always conserved in isolated system. Energy cannot be destroyed or cannot be created. It can changes in other forms but it always conserve. This law also called The Principle of Conservation.

If Q is given heat to system by surroundings then some part of it change in internal energy (U) and some part change in work done by system (W). So total energy always conserve.




3. The Second Law

According to the second law of thermodynamics, heat always transfer to low temperature from high temperature. Heat cannot transferred to high temperature from low temperature without any extra energy. In other words, there is not any kind of engine possible that convert whole energy into work.

There are two statements given which is based on the second law:

  1. Kelvin-Plank Statement
  2. Clausius Statement




4. The Third Law

According to this law, at 0k temperature, all pure crystalline solids have minimum entropy (0). Entropy cannot be in negative. It always in positive and it increases regularly in the universe.



Why heat transfer from high temperature to low temperature?

Just Imagine that you have a glass which is full of hot water. When you put it in the environment where is low temperature than temperature of water then after some time water give it’s own heat to environment and become normal. That means that heat transfer to low temperature from high temperature.

But why? Why heat transfer to low temperature from high temperature and why heat does not transfer to high temperature from low temperature? Why water not become more hotter than it was before?


Source – Google



Heat transfer from high temperature to low temperature due to motions of molecules.

Motion of molecules depends on Temperature. If there is temperature is high then molecules have high kinetic energy that means molecules have high velocity and they move faster. But if there is low temperature then the condition is opposite. In this condition, molecules move slower than they move at high temperature because their kinetic energy will slow down and they move slower.

So molecules will move from high temperature to low temperature due to difference in their motion and they carry heat to low temperature from high temperature.



One thought on “Thermodynamics: Basic Topics & Laws

  • September 18, 2016 at 6:33 am

    This is very good and helpfully for basic knowledge….


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