THE FIRST LAW OF THERMODYNAMICS

 

THE FIRST LAW OF THERMODYNAMICS

 

Introduction:

The newton laws of motion are also similar to first law of thermodynamics. These are laws demonstrating universal truth. There is no any proof and no derivation of these laws. No evidence of violation or contradiction has been observed in these laws over the centuries. The first law of thermodynamics, also known as the conservation of energy principle, provides a sound basis for studying the relationship among the various forms of energy and energy interactions. We all known that a rock at some elevation possesses some potential energy, and part of this potential energy is converted to kinetic energy as the rocks falls. Experimental data shows that the decrease in potential energy (mg, delta Z) exactly equals the increase in kinematics energy [ m (v²₂-v²₁)/2] when the air resistance is negligible, thus confirming the conservation of energy principle for mechanical energy.

             

First law of thermodynamics:

The first law of thermodynamics is essentially the law of conservation of energy and is related to the equivalence of heat and work. The first law of thermodynamics was given by physicist James Prescott Joule and is based on his Experiments. Because of this, there is no mathematical proof of this law.

Statement of first law of thermodynamics:

The First Law of Thermodynamics states that heat is a form of energy, and thermodynamic processes are therefore subject to the principle of conservation of energy. This means that heat energy cannot be created or destroyed. It can, however, be transferred from one location to another and converted to and from other forms of energy.

What is the first law of thermodynamics in simple way:

                               

            

 

A thermodynamic system in an equilibrium state possesses a state variable known as the internal energy(E). Between two systems the change in the internal energy is equal to the difference of the heat transfer into the system and the work done by the system. The first law of thermodynamics states that the energy of the universe remains the same. Though it may be exchanged between the system and the surroundings, it can’t be created or destroyed. The law basically relates to the changes in energy states due to work and heat transfer. It redefines the conservation of energy concept.

 

 

Equation of first law of thermodynamics:

The equation of first law of thermodynamics is as given below:

                                                   

                                                          ΔU = q - W

      where,

·       ΔU = change in internal energy of the system

·       q = algebraic sum of heat transfer between system and surrounding.

·       W = work interaction of system with surrounding.

                      

First law of thermodynamics for closed system:

For a closed system undergoing a cyclic process , net heat interaction is equal to the network interaction.

 Consider the following example,

Consider the closed thermodynamic system there are two thermodynamic process.

1.     Reversible Process 

1-A-2-B-1 is a Reversible cycle,

According to First law of Thermodynamics for a cycle, For a cycle,

ƩQ = ƩW

ƩQ = Net Heat Interaction

 

ƩW = Net Work Interaction

Q 1-A-2 + Q 2-B-1 = W 1-A-2 + W 2-B-1

2.     Irreversible Process

Similarly, 1-A-2-C-1 is a Irreversible cycle because 2-C-1 is an irreversible cycle as it is shown with dotted line.

According to First law of Thermodynamics for a cycle,

ƩQ = Ʃ Similarly, 1-A-2-C-1 is a Irreversible cycle because 2-C-1 is an irreversible cycle as it is shown with dotted line.

According to First law of Thermodynamics for a cycle,

ƩQ = ƩW

 ƩQ = Net Heat Interaction

 

ƩW = Net Work Interaction

Q 1-A-2 + Q 2-C-1 = W 1-A-2 + W 2-C-1

 

Conclusion:

first law of thermodynamic remains same for both reversible and irreversible process.

 

1.     For power producing cycle:

                                     

Consider, a closed thermodynamics system. There are two thermodynamic processes.

 

1-A-2-B-1 is a Reversible cycle.

 

According to First law of Thermodynamics for a cycle,

ƩQ = ƩW

 

On P-V graph, cycle is clockwise. Thus, New work interaction is positive. So we have power producing cycle.

Ʃ W = Positive

 

For Power Producing Cycle, Net Heat Interaction is always Positive.

 

  ƩQ = Positive

 

Total Heat Supplied – Total Heat Rejected = Positive

 

 

 

 

 

2.     For power consuming cycle:

                                 

 Consider, a closed thermodynamics system. There are two thermodynamic processes.

1-A-2-B-1       is a Reversible cycle.

 

According to First law of Thermodynamics for a cycle,

 

ƩQ = ƩW

On P-V graph, cycle is Anti-clockwise. Thus, New work interaction is negative. So , we have power consuming cycle .

Ʃ W = Negative

 

For Power Consuming Cycle, Net Heat Interaction is always Negative.

 

ƩQ = Negative

Total Heat Supplied – Total Heat Rejected = Negative

 

Conclusion:

·       For Power Producing Cycles both Net Work Interaction & Net Heat Interaction are Positive.

·       For Power Consuming Cycles both Net Work Interaction & Net Heat Interaction are Negative.

Consequences of First Law of Thermodynamics:

·       Heat Interaction is a Path Function

·       Energy of the System is the Property of the System

·       Energy of an isolated system is constant

·       Perpetual Motion Machine of first kind (PPM – 1) is impossible

 

 

1.     Heat interaction in path function:

                            

1-A-2-B-1 is a Reversible cycle.

Q 1-A-2 + Q 2-B-1 = W 1-A-2 + W 2-B-1------------------- (1)

1-A-2-C-1 is a Reversible cycle

Q 1-A-2 + Q 2-C-1 = W 1-A-2 + W 2-C-1------------------ (2)

Subtracting equation (1) from equation (2)

Q 2-C-1 - Q 2-B-1   = W 2-C-1 - W 2-B-1----------------- (3)

W _2-C-1 is not equal to W _2-B-1 (because Work is Path Function.

 

Let’s Say,

W _2-C-1 - W _2-B-1= X (X is non-zero)                                               ------------ (4)

From Equations (3) & (4)

Q 2-C-1 - Q 2-B-1 = X

Q 2-C-1 = X + Q 2-B-1

Thus, Q _2-C-1 is not equal to Q _2-B-1

Here we are going from state 2 to state 1 in both cases, Since the paths are different the heat interactions are different

             Thus, the heat interaction is path function heat

 

 Interaction has following Properties:

·       Path function

·       Inexact Differential

·       Energy in Transit

·       Boundary Phenomena

 

2.     Energy of the system is the property of the system:

      

    From equation (3)

Q 2-C-1 - Q 2-B-1   = W 2-C-1 - W 2-B-1

Q 2-C-1 - W 2-C-1 = Q 2-B-1   - W 2-B-1

(Q – W) _2-C-1 = (Q – W) _2-B-1

Where, Q – W = ΔE (ΔE) 2-C-1 = (ΔE) 2-B-1

Here, ΔE is representing change in property and E is representing property

 

Energy has the following property:

·       Property

·       Point function

·       Exact differential

First law of Thermodynamics for a Process:

For a cycle, ƩQ = ƩW for a process, Q – W = ΔE

Q = W + ΔE

δQ = δW + dE

ΔE = ΔK.E. +ΔP.E.

+ ΔU

For Stationary System,

 

ΔK.E. = 0

 

ΔP.E. = 0

Thus, Q = W + ΔU

 

A)   For Simple Compressible and Stationary System, W = W_d (Only displacement work)

Thus, Q = W_d + ΔU

B)    For Simple Compressible and Stationary System undergoing internally reversible process

For Internal reversible process , W_d = pdv

Q =  pdv + ΔU

C)    For a cyclic process , ΔE = 0

 

3.     Energy of an isolated system is constant:

 

                                     

       For isolated system           

              Q = 0

W = 0

Q = W + ΔE

Thus , ΔE = 0 

Thus , E = Constant

 

4.     Perpetual Motion Machine of first kind ( PPM – 1 ) is impossible:

·       PMM-1 is fictitious machine which is continuously produces the mechanical work without consuming any form of Energy .

·       PMM-1 is impossible as it violets first law of thermodynamics for a cycle .

 

Applications of first law of thermodynamics:

 

1.     Melting process:

When a solid melts to liquid, its internal energy increases. Let m = mass of liquid and L = latent heat of the solid. Amount of heat absorbed by the system, DQ = mL

A small amount of expansion occurs, i.e., ΔV = 0 

⇒ DW = PΔV = 0 

So, 

DQ = DU + DW 

⇒ DU = mL

Thus, internal energy increases during the melting process.

2.    Heat engine:

The heat engine is the most common practical application of the First Law. Thermal energy is converted into mechanical energy via heat engines and vice versa.

 

3.  Refrigerators, air conditioners, and heat pumps:

Refrigerators and heat pumps are mechanical energy converters that convert mechanical energy to heat. The majority of these are classified as closed systems. When a gas is compressed, its temperature rises. This hot gas can then radiate heat into its surroundings. When the compressed gas is allowed to expand, its temperature drops below what it was before compression because some of its heat energy was removed during the hot cycle. After then, the cold gas can absorb heat energy from its surroundings. This is the operating principle of an air conditioner. Air conditioners do not generate cold; rather, they remove heat. A mechanical pump transports the working fluid outside, where it is compressed and heated. The heat is then transferred to the outside environment, typically via an air-cooled heat exchanger. Then it is delivered indoors to expand and cool before taking heat from the internal air via another heat exchanger.

A heat pump is basically a reverse-cycle air conditioner. The compressed working fluid’s heat is used to warm the building. It is then moved outdoors, where it expands and cools, allowing it to absorb heat from the outside air, which is normally warmer than the chilly working fluid even in winter. 

 

Limitations of first Law of thermodynamics:

·       1^st Law doesn’t talk about the details of non-work interactions.

·       1^st Law tells us only about the Quantity and doesn’t talk about the

Quality.

·       1^st Law doesn’t dictate the direction of interaction

·       1^st Law fails to tell us about the feasibility of the interaction.

·       1^st Law fails to tell us about the limiting range of feasible interaction.

 

 

Conclusion:

We conclude that the energy can’t be created or nor be destroyed it only transfer from one form to another form. we analyze  an any open or closed system by using first law of thermodynamics, the energy in to the system is equal to energy leaving to the system.

Contributors:

1. Karan Sanap

2. Vikas Sanap 

3. Sagar Sanas 

4. Durgesh Sandhan 

5. Aditya Saravade