The First Law of Thermodynamics and The Conservation of Energy

The First Law of Thermodynamics and The Conservation of Energy

The title sounds familiar?

I have no doubt that you all must have learned this topic while you were too young. But how many of you can exactly explain if you were asked it now? Here I’m making an attempt to polish your knowledge along with teaching many who don’t understand the basics.

To begin with, I would like to quote that familiar law,The law of conservation of Energy-

Energy can neither be created nor be destroyed; rather it can only be transformed from one form to another.

It is as simple as it sounds.We cannot create energy, but all that we can do is, we can transfer the form of energy from one to another.Many beginners may naturally question this as for them it may seem conflicting with what they are seeing in the real world.One can argue with me with the backing of many examples. Let me explain the simple example of producing electricity from hydropower plants.
What happens here is, the hydropower plant captures the energy of falling water to generate electricity.

Let’s consider a system of confined water like this

Picture Source-Pixabay

This is confined, the water cannot move to anywhere.The quantity of water is very huge! It has unimaginable potential energy that we cannot see unless we provide it with a way to flow freely.We can calculate the PE(Potential Energy) it holds by simply applying values in this formula.



m=mass in Kg,g=acceleration due to gravity(9.80665 m/s2) & h=height of the water from the bottom level

You can see The interesting maths related to this here

Now the water flows out of this dam like this

Image Source – Pixabay

Now what happens is that the Potential energy the water had when it was in the confined state gets converted into Kinetic Energy.As the name suggests it is the form of energy that is resultant of the motion of water.We have the formula to find the kinetic energy.


Image Source

The turbine in the power plant starts rotating due to this water flow


Source – Pixabay

The Kinetic Energy of water gets converted into Mechanical Energy by means of the rotation of this turbine.The turbines are connected with generators where the mechanical energy get converted into electric energy.

I think you have recalled what you have studied a long ago by now.Let me summarise the convertions

Potential Energy—->Kinetic Energy—->Mechanical Energy—–>Electrical Energy

You can see, only forms are changed but the energy remains constant.It is said that the total Energy of this entire universe is a big ZERO!!
Let me now come to the topic that I wish to focus on- First Law of Thermodynamics

To understand the laws of thermodynamics, first, you need to know the meaning of ‘System’ and ‘Surroundings’

A thermodynamic system is a quantity of matter of fixed identity, around which we can draw a boundary. The boundaries may be fixed or moveable. Work or heat can be transferred across the system boundary. Everything outside the boundary is the surroundings.


So a thermodynamic system can be anything that we pick to observe.It can be the glass containing water, A flask of warm water, The earth, The universe…etc anything. Everything lies outside this system is referred to as surrounding.Now, It is also necessary to know the definition of the Thermodynamics.

Thermodynamics is the branch of physical science that deals with the relation between heat and other forms of energy (Such as Mechanical, Electrical or Chemical Energy) and, by extension, of the relationships between all forms of energy.


The first law of thermodynamics is an application of ‘law of conservation of energy’ that we have discussed before.Here is it’s definition-

The First Law says that the internal energy of a system has to be equal to the work that is being done on the system, plus or minus the heat that flows in or out of the system and any other work that is done on the system


This diagram was drawn by me. The circle in it represents a ‘system’ and everything out of it’s drawn boundary is ‘surroundings’. We are talking here only about Closed System wherein matter cannot jump out of it.Only energy transfer takes place.

Internal energy denotes all forms energy available in a, It is the summation of possible energies that can co-exist in a system like Potential&Kinetic energies, Heat energy, Mechanical energy..etc..Certain forms of energies are absent as we consider the pure closed system.

The work done on the system has a key role to play here as it’s application increases the total internal energy.As the thermodynamics deals with the heat transfer, the heat escaped from the system has a detrimental effect on the same.

All that I can summerize is again this ‘*Energy can neither be created nor be destroyed.“‘

But a few factors seem to be contradicting this belief.for example, we can convert matter into energy provided enough antimatter is available.Fission,Fusion..etc being some examples.Let us talk about them later.

Note: Images that I have used here are mainly from pixabay and I have properly cited the sources too.References are quoted at the places itself.


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