al zami

why after throtlling steam gets superheated???any explanation???

adibetic curve in pv will become isentropic curve in ts. then in Ts what will be the constant pressure curve of pv ??

what is the relation between pv and ts diagram.what i trying to say is if a pv diagram is given then how can i assume the ts diagram from it?????

sorry for the delay. been too busy for exam it becomes impossible to make time to visit the forum.

ok that gives the intuation .

 

entropy was originally introduced for engineers to calculate energy changes

as entropy means energy is used and we are not getting the energy back.so entropy is increasing.

previously you compered entropy with voltage difference.as we know voltage difference helps to move electrons from lower to higher voltage.does entropy do similar or such thing.i mean does entropy is responsible for the flow of something.if not then from where this comparison is derived from ????

I have asked several time why we are justified in using this equation ?

i didnt get the question.sorry for that.i dont know the answer.if i dont give answer to your questions i am sorry for that.entropy is not in my syllabus.i know maths will lead to better understanding.but classes are going in a full swing and its hard to make time for out of the syllabus topics.the basic thing which i wanted to know from where the formula dQ/T came from and what it signifies???what leads the scientists to introduce a concept like ENTROPY???

In order to freeze the water, q is negative thus entropy is negative.

 

Are you trying to tell me I can't freeze water?

dQ/T is always greater than or equal zero .it is said to be the fundamental law of the universe.entropy was not in my syllabus.i am trying to understand the topic because i have admiration towards astronomy and trying to understand the facts.if we freeze water entropy is negative.why this shows contradiction to dQ/T>=0;

actually i am not worried about calculations.i am trying to grab the concept.entropy of each thermodynamic process is always equal or greater than zero.if i melt ice that will increase entropy .and if in regain ice from water that should increase entropy too.how this happen ??

temp for melting q₁=m*l_f

=1*2100

temp for taking the temp upto 20 degree =m*c*t

=1*4200*20

total heat q=q1+q2

here q is the enthalpy

entropy S=intrigating q/T within 0 to 20

and s is the entropy

isn't that right?

actually

Why on earth do you think this process is irreversible? What happens if you cool it down again?

i tried to say if i put a piece of ice in water and let it melt .we cant get the exact piece of ice back from the water.if we freeze the water then the the whole water will turn into ice .but the exact piece can't be brought back,in that case it is irreversible

s=q/dT means heat added per degree of temperature change.how this is related to the terms "disorder" or with "arrow of time" such types of concepts ??????

This gives you entropy of fusion.

 

Can you say why?

it confuses me.this is the amount of heat taken for melting.here entropy changes because the molecules of ice gets disordered and gets mixed in water,and we cant get the actual amount ice back from melted ice and water mixture,isn't it?its irreversible

but if we apply this equation for entropy change s=q/T then we get

s=q/0 [temp at o degree]

s=undetermined

doesn't making any sense.

i dont understand the question.is it water and ice are in same container or in seperate container??

and what is the mass of ice .as i need it to calculate latent heat m*l_f

We deduce another physical quantity we call enthalpy from these same observables, can you recall the equation?

edit: It contains more than just heat transferred.

you've said It contains more than just heat transferred

but if it is constant pressure process

then we can say h=u+pdv

=>h=q-w+pdv

=>h=q-pdv+pdv

=>h=q

so

here enthalpy change is equal to heat transferred,isn't it??,But in

adiabatic process we cant say enthalpy depends only on heat added.

consider two system where one has volume two tymes than the other one but of same temperature then we can say the larger system has more heat content than the other one .so we can say the larger system has more entropy than the small one???

but

can we say the larger one has more enthalpy than the small one????so if

we can or can't then what is the difference between these two terms

????

and

what is the difference between enthalpy change and entropy change?and

what physical phenomena these two terms describe about a system .?

I note you are studying engineering. You should look up indicator diagrams. They are much used by engineers.

 

There are several pairs of quantities that when multiplied together have the units of energy or work.

 

Force x distance

Pressure times volume

Surface tension times area

EMF times charge

Magnetic field times magnetic moment.

 

If you plot one of these pairs on the x axis and the other on the y axis of a graph the area enclosed or between a line and a curve will 'indicate' the work done or energy transferred.

 

But you will note there is something missing from my list

 

Heat energy.

 

Entropy was introduced to pair with temperature in the same way so heat transferred, q = TdS

and the area enclosed by a line or curve on a T-S indicator diagram 'indicates' the heat transferred.

There are many published tables and diagrams of entropy v temperature as it makes a very convenient form of calculation.

 

The link to statistical mechanics was established later in history and has proved to be one of the most intriguing correspondences in science.

 

Does this help?

but for w=F*x means because of F force a particle covers x distance .here i can measure the force and distance x .

in here

q=Tds because of temperature T .......... what can i say in the dash to make it a perfect definition for q as i did it for work done w.

and if so , what is the difference between entropy and enthalpy

Imagine floating in space with a balloon full of smoke, you pop the

balloon, and watch what happens to the smoke. It spreads out in all

directions, thinner and thinner and thinner.

what is meant by this example???it is not clear to me ?can you explain this to me??

Well the definition of entropy change is (assume constant temperature for simplicity of explanation)

 

dS = q/T

 

But the definition of adiabatic is

 

q = 0

 

So dS = 0

 

In other words there is zero entropy change.

 

Does this help?

thanks!it clears the mathemetical part.i wanted to know the actual meaning of increasing and decreasing entropy?what happens when entropy increase or decrease?how it is related to heat content and temperature??

what this equation means

h=h_f+x*h_fg

what is the physical meaning of dryness fraction.

let me ask you what is Entropy and who it is defined?

actually how can i physically define entropy???

i know that change in position.disorder etc means there is a change in entropy,isn't it?

why adiabatic process is called isentropic. we know disorder or any change makes entropy to change.then how adiabatic process keeps entropy constant .a clear explanation will be appreciated.

how in case of compressor "work is done on it"//???

in case of turbine it is said the work is done by the system and for compressor it is said that the work is done on the system.in case of turbine steam moves the blades and in case of compressor air moves the piston.then where is the difference in regard to work done or work by???

got it.this entire discussion was very helpful for me.thanx a lot to you .

i solved the problem following your hints .adiabatic means there will be no heat transfer between the system and surrounding.neither take nor give away.

It is a good exercise to describe what happens during the process.

temperature isn't given here.but if in this problem the mass m and molecular mass M of gas was given,we can find temperatures T₁ and T₂ from PV=mRT₁

if that so can we assume the process adiabatic and so we can use c_p/c_v=y; which is 1.4

but its incorrect for this problem

how can i tell from a given problem the process is adiabatic or not.its not clear to me.

are we assuming it is not adiabatic for our own convenience ???or there is exact reason that the process is not adiabatic?

as i made a mistake solving the problem assuming it adiabatic and taking y=1.4

here c_v is given 3.175. so it is not air

actually i am following thermodynamics by hollman

in books usually there are big mathemetical proofs but no explanation in plain english which leads to better understanding the concepts

 

But we are not told the quantity of gas nor the starting or finishing temperatures.

 

So we cannot assume an adiabatic or isothermal process

and here though there is no mention of temperature how that means it will not be adiabetic.as we found m(T2-T1) there is temperature involved in two state for which it can be a adiabetic curve ,isn't it?( here confusion arrises)

so for that we can use cp/cv=y (if we assume adiabetic)

from cv value we can find

1)it is not for air;

2)from its unit ,we find we are dealing with per kg of substance

what other we can assume from the value of C_p or C_v????i mean its significance

You were not justified using gamma for this ratio that is for reversible adiabatic expansion, which this is not

how can i assume this is reversible of irreversible ???

this leads to another question that in the book we always find reversible process is that which returns to its initial state on contrary to irreversible.not more than that

but what are the actual physical significance of those two process ??i mean basic difference other than ''returns to its original state or not"

as you told we can not use gamma here as it is not reversible? how can i make sure that it is reversible or irreversible.

You were not justified using gamma for this ratio that is for reversible

adiabatic expansion, which this is not. Do you know why not?

actually i don't know.it would be very helpful if you explain it to me?

how i can can know the process is reversible or not from any problem given??

Remember that you are working per kg

there was no mention that i am working with per kg or not?? how can i come by this information???

Check whether there is temperature change ,you have ideal gas equation.

 

Now.......

in question there is no mention of its being ideal gas or not. do enthalpy change means temperature change??

i ve done the math in comments section.please check it whether it is ok or not.

are you saying

H₁ = U₁ + P₁V₁

_H2=U2+P2V2

_{H2-H1=(U2-U1)+(P2V2-P1V1)}

_{dH=dU+(P2V2-P1V1)}

_{so from here we can find dU.right?}

and relation between cp and H is dH=m*Cp*(T2-T1)

and we can find m(T2-T1) from p1v1=mRT1 and p2v2=mRT2

and then cp-cv=R

how i did it is a little bit different from this ,so please check it,especially the R value ,i am confused about its value

i've solved this like this

we know y=cp/cv

1.4=cp/3.175
cp=4.445
R=cp-cv
=1.27
as for ideal gas
p1v1=mRT1
and
p2v2=mRT2

from here
m(T2-T1)=-20130.24
from here
dU=m*Cv*(T2-T1)
=-89478.92

but i have some question

1) ive never seen a R (characterstic gas constant)value 1.27 before.is it ok?

Do you know ( can you state) the relationship between enthalpy and internal energy?

actually i dont know.can you explain ?

then how can i do it

i cant proceed any further than the equations i wrote above