First law of Thermodynamics for a Closed System

 

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First law of Thermodynamics for a Closed System

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What is closed system?

It is a thermodynamic system, in which only allows the transfer of energy. That is no mass is transferred between the system.

Example of closed system: Sun rays coming to earth, Piston cylinder arrangement without valves.

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Fundamental statement for first law of thermodynamics

 

“Energy can neither be created nor destroyed, it can just transform from one form to another form.”

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1.     First law of Thermodynamics for closed system undergoing a thermodynamic cycle

Thermodynamic cycle: - It is the set of processes arranged in such a way that, the initial and final state are on the same point.

Experimental setup

The following setup contains mercury in it. In this type of system, Mechanical work is supplied externally. A stirrer is used which connected using a shaft, one end of the shaft is connected to the stirrer and other end fitted with pulley. Dead weight is used to drive the stirrer. A thermometer is used to measure the change in the temperature.

 

Weight (P) = m * g

 

 

T₁ = Initial temperature of the mercury

T₁ = T_atmospheric…...(Initial condition)

 

The weight is moved by certain distance& it will occupy the new position (That is, position number 2)

h = distance between initial position of weight to final position of weight

Due to this, change in potential energy takes place,

By using, work energy principal

W = m * g* h

W = T * θ …..(Shaft work)

 

Were,

T = Torque on shaft

θ = Rotation of shaft in degrees

 

W = T *  θ= m * g * h

 

T₂ = Final Temperature

 

T₂ > T₁

Plotting the experiment in the property diagram

 

Where,

J – Joule’s equivalent or Mechanical equivalent of heat

 

 

J = 1 N.m/J  ….

[Unit in S.I. system is Joule, N.m/J]

 W_Net = Q_Net

 ….. 

[Cyclic integral of dw = cyclic integral of dQ in cyclic process]

 

…..

[That is, Cyclic integral of any property is always zero.]

 

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1.     First law for a closed system undergoing a change of state.

If Q amount of heat is transferred to a system and w amount of work transferred from the system.

Then,

 

Net energy transfer = (Q – W) will be stored in the system.

 

The energy that stored in the system is neither heat or work, it is named as internal energy or energy of the system.

 

Where,

ΔΕ = Increase in the internal energy of the system

 

Or

 

Q =ΔΕ + W

The unit of Q, W and  is same. That is, in Joules.

If there are number of energy transfer quantities involved in process,

 

 

Then according to First law of Thermodynamics,

 

(Q₁ + Q₂ - Q₃) =ΔΕ+(W₁ + W₄ – W₂ – W₃ )

Energy is converted in the system during the process.

Therefore, the first law of thermodynamics is also called as the law of conversion of energy.

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Conclusion

The experiment performed proves the first law of thermodynamics, in this experiment external mechanical work is converted into heat energy using the stirrer used in the system.

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*Content context: Images used in the blog are not owned by us. Images are taken from the

1.     Book Engineering Thermodynamics by P. K. Nag.

2.     https://images.app.goo.gl/K5JiWJE3os5syj399

3.     https://images.app.goo.gl/Y7HXc76LesFV1vSd7

 

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·      Institute: - Vishwakarma Institute of Technology, Pune

    Under the Guidance of Prof. M. K. Nalawade

·      Group members: -

1.    Darshan Behere

2.    Manswi Bhelkar

3.    Amey Bhombe

4.    Tushar Bhosale

5.    Sunidhi Chaudante