## Wednesday, 17 February 2016

We were discussing Boyle’s law and Charles law in our previous post. Today we will try to understand here the first law of thermodynamics.

If a system follows a complete thermodynamics cycle, there will be a net work which will be done on the system or done by the system.

Let us consider one thermodynamic system which is following a complete thermodynamic cycle and net work is done by the system.

As we are aware with law of conservation of energy that energy can’t be created or destroyed but it could be converted from one form of energy to another form of energy. Therefore system will require a mechanical energy for doing this work.

Now let us concentrate over the system that we have assumed above. System is following a complete thermodynamic cycle i.e. system has reached to its original state or we may say that internal energy of the system will be constant. Therefore system will not provide the mechanical energy by itself for doing the work. For doing the work, system will require here heat energy.

According to law of conservation of energy, net work done by the system will be equivalent to the net heat supplied to the system.

### Now, we can define here the first law of thermodynamics

When a closed system follows a thermodynamic cycle, net work done by the system on its surrounding will be equal to the net heat supplied to the system from its surrounding.
Therefore, for a closed system that follows a thermodynamic cycle, we will have following relationship as per first law of thermodynamics.

Figure shows a thermodynamic cycle
∑d Q = ∑ d W

∑d Q: Net heat supplied to the system from its surrounding
∑d W: Net work done by the system on its surrounding

First law of thermodynamics is applicable to reversible as well as irreversible processes also.

### First law of thermodynamics for a non cyclic process

When we consider first law of thermodynamic process for a non cyclic process, there will be one more term involved i.e. internal energy of the system.

Let us consider a system is undergoing with change of state in a non cyclic process and heat and work transfer, both are involved in this process. Net transferred energy will be accumulated within the system.
Figure showing a system, where Q is the heat energy supplied to the system from surrounding and work energy W is the work which is done by the system on surrounding. Net transferred energy i.e. (Q-W) will be accumulated within the system. This accumulated energy will be termed as change in internal energy of the system.

According to first law of thermodynamics, we will have

Let us consider a system where there is more energy transformation takes place as shown in figure

According to first law of thermodynamics, we will have following equation here

Q2 + Q3 – Q1 = ∆E + (W2+ W3-W1-W4)

∆E or ∆U indicates change in internal energy of the system

### Sign convention for heat and work

Understanding of sign convention for heat and work is quite important to secure the correct equation.

Heat supplied to the system will be taken as positive and heat rejected from the system will be considered as negative.

Work done by the system will be considered as positive and work done on the system will be considered as negative.

We will discuss the second law of thermodynamics in our next post

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