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FIRST LAW OF THERMODYNAMICS FOR A CLOSED SYSTEM UNDERGOING A CHANGE OF STATE

We were discussing various basic concepts of thermodynamics such as work energy transfer in thermodynamics in our recent post. We have also discussed the concept of enthalpy in the field of thermal engineering.

Today we will see here the first law of thermodynamics for closed system undergoing a change of state with the help of this post.

Let us see here first basics of first law of thermodynamics

As we have already discussed that first law of thermodynamics deals with the law of conservation of energy and according to law of conservation of energy, energy can’t be created or destroyed but also it could be converted from one form of energy to another form of energy.

We can also say that energy will always be conserved. Heat and work, these are two different forms of energy. If heat energy is provided to the system by the surrounding then system may provide the work energy to the surrounding and similarly if work is being done upon the system then system may deliver the energy in terms of heat to the surrounding.

Hence, this is the basic concept of first law of thermodynamics. Now we will see here the first law of thermodynamics for a closed system undergoing a change of state. After that, in our next post, we will see first law of thermodynamics for an open system or control volume.

As we have discussed in our previous post “first law of thermodynamics for a closed system undergoing a cycle” that the algebraic summation of all energy transfer i.e. heat energy transfer and work energy transfer across the system boundaries will be zero. Or we can say that for a closed system undergoing a cycle, we will have following equation according to the first law of thermodynamics.
First law of thermodynamics for a closed system undergoing a cycle
But we must note it here that if system is not undergoing in a cyclic process but also we have one system which is undergoing a change of state then in that case above equation will not be applicable as above equation is only valid for a system which is going under a cycle.

So, what will be the equation of energy transfer across the system boundary according to the first law of thermodynamics for a closed system under a change of state?

Let we have one closed system which is undergoing a change of state and energies i.e. work energy and heat energy both are crossing the system boundaries. Net energy will be stored within the system and we must note it here that energies stored within the system will be termed as internal energy of the system or also termed as energy of the system.

Let us see the following figure, Q heat energy enters the system from surrounding and work energy W leave the system or we can also say that system is doing work W on the surrounding by taking Q amount of heat energy from the surrounding.
So, what will be the net amount of energy stored within the system during this process?

(Q-W) will be the net energy and it will be accumulated within the system and hence there will be increment in internal energy of the system as (Q-W) amount of energy will be stored within the system during this process.

Therefore according to the first law of thermodynamics, we have following equation for above thermodynamic process where system is under a change of state.

Q-W = ∆E

Where, ∆E is increase in internal energy of the system during the process

Let we have one system where multiple energies are crossing the system boundary in a process as shown in following figure,
Now we will remind here the concept of “Sign convention used for heat and work energy transfer across the system boundary “and we will write here the equation of energy transfer across the system boundary according to the first law of thermodynamics for this process.

Q1 + Q2-Q3-W1+W2+W3 = ∆E

We must note it here that we will not secure the absolute internal energy for this process, but also we will always secure here the change in internal energy or change of system energy.

We will see another topic “Energy a property of the system” in our next post.
Do you have suggestions? Please write in comment box

Reference:

Engineering thermodynamics by P.K. Nag
Image courtesy: Google

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