studiot

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?

I don't know, you have not named any of your symbols.

So are you going to enlighten us?

Can you see the 'trivial solution' to your first question? I can immediately see one other solution. I cannot understand your second question, what do you mean?

Energy has mass. You guys might like to read Frank Wilczek on the subject. http://web.mit.edu/physics/news/physicsatmit/physicsatmit_03_wilczek_originofmass.pdf or get his book, "The lightness of being".

Oh dear, Oh dear, Oh dear. I thought we were making progress and has agreed to start at the beginning with basics. You have made a great fuss about not using mathematics so imagine my suprise when asked to tell me what a wave is you respond with mathematics. Is a sine curve not mathematics? How about A wave is a form of motion. It transports energy from one place to another. Now that presupposes we know what space, time (to have motion) and energy are. Can we take these given? Ther may be other properties of waves you feel are important to your analysis, the subject is open to discussion. I too have experiencing this difficulty and, really, all that typing on your part is wasted. In my case I would like you to say what you think a Field is, not repeatedly tell me there are no field equations. We can examine that second statement when we have the first. In my opinion, fields in physics (which is what I think we are discussing) refer to place so such a notion is not only useful for discussing my offering for waves but also for fields. However over to you.

Since this was posted in linear algebra, I actually wondered if Greylorn had heard of Frenet paramertisation and the 'speed of a curve' and was confusing this. Incidentally, Greylorn, if you crumple an object in n+1 dimensions, there is nothing to stop you doing it in n+2, n+3 etc.

So you don't like my comparison? Both energy and holes are nouns. Both are abstract nouns. But they also have differences. There are grey areas. The noun classification is not perfect and there is some overlap between the categories. Energy is definitely quantifiable. I know this was posted in the classical physics section but neither 'a thing' Humour is another abstract noun that is quantifiable, most would agree there is a lot of it in a Tom and Jerry cartoon. But would it not also be possible to consider the question the province of philosophy, rather than classical physics?

An allied question to ask is Is the hole in a polo mint real?

Now I hope we are ready to make a start. I think that in order to have a meaningful discussion we first have to agree on the meaning of the various terms we wish to employ. The fundamental ones at least, others can be developed along the way. You see I think that part of your difficulty communicating your ideas to others is that you and they mean different things by some basic terms. That is why I asked to start with the term 'wave'. So what do you mean by a wave (any wave, not just your TEW), how would I recognise one if I met one in the street? Armed with this I can compare against your 'basis for elementary waves' above and answer the question, and you will have communicated. Other important terms to agree are field, velocity as applied to waves, interference. Would that do for starters?

So where did you hear of this wondrous theorem?

Formally the statement: An omnipotent being can perform the task of setting himself an impossible task. leads to a paradox. There is a 'smoke and mirrors' philosophical resolution of this, which I don't buy into. So far as I am concerned that paradox is enough to demonstrate that an omnipotent God does not exist. As regards a more powerful being than myself, I prefer the baby looking at a university professor or F1 racing champion to phi for all's ant example. A cat can look at a king. So the question of does God exist reduces to No, an omnipotent being does not exist. Any less than omnipotent being is one I (Mankind) could aspire to one day. So the question becomes irrelevant.

Note it is not a good idea to only consider the work done on the gas since no machine is 100% efficient. Some of the work you have to input to the compressor in order to achieve compressed fluid is lost in the compressor as friction etc, some may be employed in changing the temperature.

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?

Well it's fairly simple. Every compressor has an input shaft that must be turned by an engine, which does work turning the shaft. This is true of a piston or fan bladed compressor. Conversely a turbine has an output shaft which can drive another machine ie do work when gas or liquid passes through the machine. So from the point of view of the machine, work is input to a compressor and output from a turbine. From the point of view of the working fluid, A compressor outputs work to the fluid and a turbine extracts work from the fluid. As Swansont said, you need to define your system and your book regards the machine as the system, not the working fluid. Does this help?

Thank you, there is no need for the rest of the fancy language. So what exactly do do mean by equality? This is not a trick question since opposing equal forces can lead to a turning moment or couple, which is what you are seeking. This is not, however, true in an inertial analysis where we consider equilibrium as a result of the opposing action of centripetal and centrifugal forces.

A substantial last part of my post#111 was a clear demonstration of a technical situation where mathematics cannot be employed to describe the technicalities of that situation. This was your reply, in post#112 How did you get from my comment to your reply? And in post#120 you are still promoting the fiction that all I want is mathematics I would say that my post#95 pretty much matched your specification above!! So how about a proper discussion?

Definitely IE8 see attachment. Thank you for the nod and wink about compatibility settings I have now set SF and will try that. Please delete the attachments when viewed if you want the space, they have served their purpose, unless they help others somewhere along the line.

Internet Explorer 8 as up to date as possible. IE is the one browser you cannot do without and I am used to the corporate environment. go well

A problem will make it clear if a process is adiabatic. Usually by stating this explicitly. In this case at the outset we do not have enough information to say if the process is adiabatic or if it is not so we have to allow that it may not be and not use properties specific to adiabatic processes. If our calculations later show it to be adiabatic, so much the better, but we can't 'guess'. The important thing with this problem is to realise that whatever the nature of the process differences between state variable values depend only on the state and not on the process by which that state was reached. So we can calculate dH, dU, d(PV), dT and so on. You complained earlier that your book has more maths than explanation in words. Well this thread is your opportunity for words. I really do recommend trying to work through what happens in the process as I suggested. It will definitely help.

Yes at leat one other forum I know does that (Vintage Radio). But as underlined it significantly adds to moderator burden, which was the point of this thread.

What do you understand adiabatic to mean? It is a good exercise to describe what happens during the process. By this I mean the energy transfers between the system and surroundings and the changes to the system internal energy, pressure, volume and temperature. You don;t need numbers. Try it and then I will say how I would describe it.

To have reached this problem you posted you must have been studying thermodynamics for some time, it is not at elementary level. But you need to revise some elementary concepts. A reversible process is one that can be reversed, not one that reverses of its own accord. Reversible means that the after reversal the system is put back to the same state it started in, and all the work done or heat transferred is also returned. None of the work done is lost in additional processes such as friction. An irreversible process is one which is not reversible. Which textbook are you using?

I wonder if that is to do with the way that different computers respond to the website due to their configuration. This PC is an old Dell running XP. Some of the SF functions do not work correctly or even at all on the max version of IE that I can use. For instance when posting if I choose the reply button I cannot type into the reply box or use the icons. But if I choose 'more reply options' everything is hunky dory. More and more websites are being redesiged without reference to their target audience these days and I am seeing this sort of thing increasingly. Perhaps your spammer has found a loophole in this aspect.

Hint 4 noted the units of Cv in post#5. This question is about the most general case where all three of the fundamental directly observable (variables, temperature, pressure and volume) vary. Further the system does both work on the surroundings and gains heat from it. Yes this could be the polytropic process if the same process took place throughout the change, but we are not told this. The general polytropic process is not reversible and has an index usually denoted by n. Gamma is reserved for the reversible adiabatic case. You should re read the derivations of the relationships to consolidate this. We are told that values of system variables at two defined states so we can use this to obtain changes in state variables. But we are not told the quantity of gas nor the starting or finishing temperatures. So we cannot assume an adiabatic or isothermal process. You are entitled to assume that the perfect gas laws hold because you are not told anything else and no information is supplied that would allow any other law to be employed. The property of the perfect gas we employ here is that both internal energy and enthalpy depend only on the temperature of the gas and lead to the heat capacity equations I gave. Does this help?