Thermodynamics

Topic for collage is Themodynamic ---Basic Concept

What is thermodynamics?

Thermodynamics is the branch of science which deals with the interconversion of heat energy and mechanical energy. All those problems that are related to the interconversion of heat energy and work done are also studied in thermodynamics

Basic for thermodynamics are the concepts of system and surroundings.

A system is any chosen object or group of objects. Often in thermodynamics, the system is the atoms (or molecules) in a gas that is enclosed by a cylinder. The cylinder is not part of the system, it is the system's contact with the outside world—system surrounding (Shown in figure below)

Systems can be classified as open, closed, or isolated.

Open systems allow energy and mass to pass across the system boundary

The ocean is an example of an open system.The absorbed energy evaporates water from the ocean. As the water vapor (mass) enters the atmosphere it carries with it the heat used to evaporate the water (called latent heat). When the water vapor enters the air it raises the air's humidity. If the humidity is high enough, condensation occurs, latent heat is released, and clouds are created. Continued condensation creates precipitation (mass) that falls back into the ocean. Hence, energy and heat (solar radiation, latent heat) as well as mass (water vapor and precipitation) passes across the boundary between the atmosphere and hydrosphere.

A closed system allows energy but not mass across its system boundary. Closed systems exchange energy but not matter with an outside system. The Earth(shown in figure below) is essentially a closed system; it obtains lots of energy from the Sun but the exchange of matter with the outside is almost zero.

An isolated system allows neither mass or energy to pass across the system boundary. Isolated systems can exchange neither energy nor matter with an outside system.. The physical universe is an isolated system; a closed thermos bottle(shown in figure below) is essentially an isolated system (though its insulation is not perfect).

Heat can be transferred between open systems and between closed systems, but not between isolated systems.

Thermodynamics describes how systems change when they interact with one another or with their surroundings

There are two fundamental kinds of entity in thermodynamics, states of a system, and processes of a system.

The thermodynamic state of a system is defined by specifying values of a set of measurable properties sufficient to determine all other properties. For gaseous systems, typical properties are pressure, volume and temperature

There is a change in the state of a physical system because of heat transfer and performance of work

A thermodynamic system can also be defined in terms of the processes which it can undergo.

The first law of thermodynamics is the application of the conservation of energy principle to heat and thermodynamic processes

The internal energy of a system is the sum of all kinetic and potential energies of all components of the system; we call it E.

When energy is exchanged between the system and the surroundings, it is exchanged as either heat (Q) or work

The change in internal energy of a system is equal to the heat added to the system minus the work done by the system

That is, ΔE = Q - W.

For Q : + means system gains heat , - means system loses heat

For W : + means work done on the system, - means work done by system

The + ∆E means net gain of energy by system, – means net loss of energy by system

When work is done by a thermodynamic system, it is ususlly a gas that is

doing the work. The work done by a gas at constant pressure is: W = PDV

Fiqure above . Shows ∆V

Fiqure above :shows area under the P vs V curve showing +W and –W.

SIGN CONVENTION:

Q= positive if heat is added to a system

Q= negative if heat is released from a system

W = positive if work is done by the system

W = negative if work is done on the system

Work done by a system decreases the internal energy of the system is also indicated in the First Law of Thermodynamics.

Heat can be supplied to thermodynamic system under following condition

isothermal process, adiabatic process, isobaric process, isochronic process

Heat is supplied to the system at constant pressure, dp =0, and this process is called Isobaric process.

For an isobaric process Graph between P & V is a straight line which is parallel to V-axis(P₁,V₁,T₁--à(P₁,V₂,T₂)

An isobaric process occurs at constant pressure, dp =0, ∆Qp = ∆E +P∆ V (Bottom of collage)

Heat is supplied to the system at constant volume , dV=0 ,and this process is called isohoric

Process where (P₁,V₁,T₁---àP₂,V₁,T₂).

PV Diagram:Graph between P & V for an isochoric process is a straight line which is parallel to P-axis(P₁,V₁,T₁---àP₂,V₁,T₂), Delta ∆V =0, ∆Qv= ∆E+P(0), ∆Qv = ∆E(Left of collage)

Heat supplied under isochoric process is consumed in increasing the internal energy of the system but no work is performed.

Heat is supplied to the system at constant temperature, and process is called Isothermal process, since T is constant ,therefore delta ∆E=0, ∆Q= 0+ W = +ve . since T is constant ,P vs V is curve, , PV=constant; graph between P vs 1/V is a straight line.(shown in figure below)

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.A process that occurs without heat exchange with the surroundings is said to be adiabatic

A process that occurs without heat exchange(∆Q =0) with the surroundings is said to be adiabatic. delta ∆E =W and PV diagram is a curve

condition for adiabatic process PV^γ _{= Constant, where ,} γ = Cp/Cv

where Cp and Cv are heat capacities of gas at constant pressure and volume respectively.

Summary

The main points covered in present collage are surrounding,boundary,system

Heat can be supplied to thermodynamic system under following condition

isothermal process, adiabatic process, isobaric process, isochronic process

Ocean is open system, earth is closed system

First law of thermodynamics states that the change in internal energy of a system is equal to the heat added to the system minus the work done by the system

∆ E = Q –W

Change in internal energy = Heat added to the system –work done by the system

Internal energy is defined as the energy associated with randomness,disordered motion of molecules.It is separated in scale from the microscopic ordered energy associated with movingobjects;it refers to the invisible microscopic energy on the atomic and molecular scale.

Isobaric process occurs at constant pressure, (Bottom of collage)

Isothermal process occurs at constant temperature ,

isochoric process occurs At constant volume . (Left of collage)

Adiabatic process occurs without heat exchange(∆Q =0) with the surroundings

Figure above shows 1.A-B is Isobaric process

2.A-E is Isohoric process

3.A-C is Isothermal process

4. A-D is Adiabatic process

Condition for Adiabatic process PV^γ _{=Constant Where ,}γ = C_p/C_v

We see in figure above that curve for adiabatic process is steeper than that for isothermal process.Question is why it is so.

Answer is in isothermal process heat is absorbed to make up for the work done by the gas in expansion and the temperature remains unschanged. On the other hand ,adiabatic expansion takes place at the expense of internal energy which decreases and the temperature falls. For the same reason the curve for adiabatic process is steeper than that for the isothermal process.

Figure below shows difference between isothermal and Adiabatic process

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Take Home message ---Collage has one bigger confused face and one smaller happier face showing that before studying thermodynamics through this collage, you may be confused person but after seeing this collage carefully you will be happiest person

References

http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node11.html