Why isn't anyone answering me? Does no one here know this?

Question

In Thermodynamics, Heat and work are path functions. Can you explain how with an example please. I know that path functions depend on the path followed and state functions only on the initial and final states... But what I'm looking for is an explanation. Only serious answers please... Thankyou

Answer 1

Essentially you are correct to say that path function depend on the path followed.

I assume you are caught up on exactly what that means... so, I'll help ya out.

In the case of work. The work done by a system over a period of time cannot be defined by the beginning and endpoints alone. Take as an example, if you were to lift a ball into the air and then bring it back to the same point, and you wanted to know the work done during that motion. Of course it won't be zero... it will be dependent on the path the object took, how much force was applied to lift it, how high it was lifted, how much force resisted gravity when it was dropped, and how far it was dropped to. All this must be taken into account in order to fully define the work done. If you were to use the start and end point only it would be as if you did nothing... I hope that made sense.

With heat it is much the same. If you say "Hey I have a system that started at 0 deg. F and went to 32 deg. F, How much energy (heat) was added to or subtracted from the system over the heating time" Well, you could say that you added enough energy to raise the temp of the system to 32 deg. But then you are assuming it was constant heating, which can happen (Then you are talking about an isentropic, isothermal, isobaric system, which in praticality is impossible). But what if heat was added at a greater rate over a certain time interval, and the heat surpassed 32 deg, and then you had to pull heat off the system to bring it to 32? Or if you take into account thermal inefficiency, then the system is entirely path dependent because you would have to have data for the entire time interval to actually define the heat added or subtracted from the system.

A path function also cannot be simply reversed, meaning that you cannot just reverse the actions performed on the system to return to the initial state usually due to thermal inefficiencies and such.

I hope this helped. Probably wasnt too clear, but I'm no teacher. Just an aerospace engineer :)

Answer 2

Ok I did some research and this is what I got ( I just did it cuz I like your witch hat and according to your avatar u r cute so I hope u send me a message later):

There are two functions, state and path. State relates to a begining and and end of a function whose change from goint from initial to final is independent of the route taken, conversely, if it IS deoendent then it is a path function. A state function would be like temperature rising from 0-100 degrees. No matter if you heat it first or cool it first, if you go form 0-100 the result is the same and the result is independent of the route taken.

A path function using work and heat wpuld be like this. Let a block fall form an incline, the heat is created due to the friction ( this would be th epotential energy ) . In here no work has been done. But if you attach a pulley system to the block, because of the attached mass at the other end of th epulley,the block moves more slowly and less heat is disipated as friction. This energy was now used as work to raise the object's potential energy which is in the other end of the pulley.
In both experiments the initial and final states are the same. block falling down the incline, the change in the internal energy has also been the same but the heat and energy are different in each experiment. But in both cases the sum of heat and work will be the same.

Hope it helps!!

Answer 3

Thermodynamics , is the study of energy transformation which constitutes the subject, the laws of thermodynamics have led to results of fundamental importance in Chemistry , physics and engineering . The first law of thermodynamics stated , the total energy of a system and its surrounding must remain constant , although it may be changed from one form to another.notice that the energy change independent of the path, while work depend on path.
The second law of thermodynamics stated that heat cannot be completely converted into an equivalent amount of work without causing other changes in some part of the system and its surroundings.
The fraction of the heat absorbed at higher temp.that is conver ted in the work is called the efficiency of the machine and no machine has yet been made that has the efficiency of unity , 100 percent.

Answer 4

For example,

Take a car batter and hook it up to an electric motor.......
the electrical energy in the battery does work by spinning the fan but there is also a small amount of energy which also goes into heat energy since the process is not 100 efficient.

Take a car battery and short circuit it by placing a wrench between the terminals (don't really do this, it is quite dangerous). The electrical energy within the battery is converted entirely into heat, not work is done.

Depending on the "path" in which the electrical energy is used, the amount of heat releases and work preformed differs...making these path functions. In the end of both paths, the state of the battery is the same...drained, empty, out of energy, but the heat/work is different.

Answer 5

Work ,defined by any part of physics has to be the same .
Assume this example from mechanics to show that work is a path function :- Suppose you are pushing a cart around your neighborhood , the amount of work that you do will depend on the distance you travel [because work is a scalar quantity] .The distance you travel depends directly on the route you take around your neighborhood. If you were to take a short-cut you would be doing lesser amount of work . else you would do more work [remember, work=force * distance]
This is the meaning of the path function . it depends on the route you take the longer the route the more the work is done/to be done .

It's the same thing with heat .Heat flow depends on how it flows through the bodies . If you place thick slabs with varying materials ,heat flow would slow down a bit . It depends on the path it is taking through the materials through which it has to flow.

Answer 6

Ok layman's terms...

if I ask you to move a pile of rocks from over there to over here and then back again you have done work. You have to look at the path you took with the rocks while you were carrying them to calculate the entire work... was it up hill, through mud and snow etc... your total work in the end depends on that path taken. Path function

If a balloon is sitting there and then you squeeze it and make it bulge and then let go and let it go back to the original shape and volume... the particles inside the balloon changed volume but came back to the original shape so dV=0. This is a state function... the particles are back to the same energy and state they were in to begin with...it does not matter one bit that you 'messed' with them.

Answer 7

heat and work are functions that follow a path like when a cannon ball is shot.

but other functions are called position functions such as with speed and time. at one spot in time, it goes this speed or this much distance

well goodluck

Answer 8

patience young one. if you want an immediate answer go talk to your prof. i'm sure they'd love to see that you actually want to learn something.

path functions are like directions. you take a different route, and although you might end up at the same place the scenery was different therefore the journey was different

Answer 9

Some questions - such as this one require more work than it is worth for 2 or 10 points. I used to do this kind of thing in college.

Answer 10

Too much brain work for me, I'm only a system engineer after all - not a rocket scientist. Good luck in finding your answer.