APPLIED THERMODYNAMICSEL 325 (3+0)

Topic 3Laws

1. LAW OF CONSERVATION OF MASS

dtdmmm

mmm

systemoutin

systemoutin

/

Conservation of Mass for General Control Volume

The conservation of mass principle for the open system or control volume isexpressed as

or

Steady state ?Change of Any quantity with time = 0

outin

systemoutin

mm

dtdmmm

0/

Reversible & Irreversible Process

A reversible process is one the direction can be reversed by an infinitesimal change ofvariable.

If a block of material (at T) is in contact with surrounding at (TT), then ‘heat will flow’into the surrounding. Now if the temperature of the surrounding is increased to (T+T),then the direction of heat flow will be reversed.

Irreversible processes

generally non-equilibrium processes

have a preferred direction (towards increasing “).

T

Heat flowdirection

T+T

T

Heat flowdirection

TT

Reversible process

Reversible process

3. Equilibrium:In thermodynamics equilibrium is a term used to determine

whether there is a process taking place in a system. Ifthere is no changes in states of a system, then it is said tobe in equilibrium

4. THE ZEROTH LAW OF THERMODYNAMICS

If two bodies are separately in thermal equilibrium with a thirdbody then they must be in thermal equilibrium with each other.

The zeroth law of thermodynamics is an observation. When two objects areseparately in thermodynamic equilibrium with a third object, they are inequilibrium with each other. Three objects as shown on the slide. Object #1and object #2 are in physical contact and in thermal equilibrium. Object #2 isalso in thermal equilibrium with object #3. There is initially no physical contactbetween object #1 and object #3. But, if object #1 and object #3 are broughtinto contact, it is observed that they are in thermal equilibrium.

The Zeroth Law of Thermodynamics

If object A is in thermal equilibrium withobject C, and object B is separately inthermal equilibrium with object C, thenobjects A and B will be in thermalequilibrium if they are placed in thermalcontact.

5. FIRST LAW OF THERMODYNAMICS

•Energy can be neither created nor destroyed but only transformed

•In thermodynamics cycles , if work is transferred during the cyclethen, since there is no final change in the properties of the workingsubstance, the energy to provide a work must have been transferred asheat and must exactly equal to work.

Thus, for a cycle, there is no net property change:

Net heat transfer = Net work transfer

First law of thermodynamics:

Energy In = Energy Out

For open system: W = QFor closed system which does not execute acycle: Q = W + U (If heat and work transfers

are not equal)

ENTROPY

It is the measure of the disorder in the system.

As the change in entropy can be described as the heat addedper unit temperature

ΔS = Q/Twhere S is the change in entropy,

Q is the heat flow into or out of a system, and T is theabsolute temperature in degrees Kelvin (K).

Entropy:a state variable whose change is definedfor a reversible process at T where Q is theheat absorbed.

Entropy: a measure of the amount of energy whichis unavailable to do work.

Entropy: a measure of the disorder of a system.

Entropy: a measure of the multiplicity of a system.

THE SECOND LAW OF THERMODYNAMICS

5. THE SECOND LAW OF THERMODYNAMICS5.1.CLAUSIUS STATEMENT

The second law of thermodynamics describes the flow ofenergy in nature in processes which are irreversible.The second law of thermodynamics may be expressed inmany specific ways.

Second Law and Refrigerator

It is not possible for heat to flow from a colderbody to a warmer body without any work havingbeen done to accomplish this flow. Energy willnot flow spontaneously from a low temperatureobject to a higher temperature object.

5.2. KELVIN-PLANCK STATEMENT

Second Law and Heat Engine

It is impossible to extract an amount of heat froma hot reservoir and use it all to do work. Someamount of heat must be exhausted to a coldreservoir.

It means that the efficiency of a heat engine cycleis never 100%.

Thus ,

Net work transfer = Net heat Transfer

But generally,

Net work transfer is less than Net heat transfer.

This means that some heat transfer must be rejected and is lost.

HEAT ENGINE

Qin is the heat flow from the hot reservoir to the engineQout is the heat flow from the engine to the coldreservoir.The work done by the heat engine is the differencebetween Qin and Qout.Heat engine efficiency:

in out

in

Q Q

Q

output

heat engineinput

w

q

5.1. PERPETUAL MOTION OF THEFIRST KIND

An engine which could provide work done without heattransfer would run forever; in other words, it would haveperpetual motion. IMPOSSIBLE!

WHAT IS “PERPETUAL MOTION”?“Perpetual Motion”

• Describes HYPOTHETICAL MACHINES thatoperate or produce useful work indefinitely &,more generally, hypothetical machines that

producemore work or energy than they consume,whether they might operate indefinitely or not.

• There is undisputed scientific consensus that

Perpetual motion would violateeither the 1st Law or the 2nd Lawof Thermodynamics, OR BOTH!

“Perpetual Motion”• Describes a theoretical machine that, without anylosses due to friction or other forms ofdissipation of energy, would continue to operateindefinitely at the same rate without any externalenergy being applied to it.

OVERBALANCED WHEEL