Work Done, by what on what? [electrical/mechanical]

[vis viva]

I have been reading older posts containing "work done", but they don't seem to answer my doubts, or maybe I don't understand the answers because I need some visual pedagogical aids.

I can sort of take the concept in, when we are dealing with a charge in an electric field, but when it comes to something more complex as the below setup then I'm not so sure anymore.
https://www.siyavula.com/read/science/grade-12/electrodynamics/images/DC_Generator.png
Would it be reasonable to state that the above can be construed as three separate systems i.e. 1. crank operator 2. stator and rotor 3. commutator and bulb ?

Seen from the crank operators point of view, what is doing work on what?

 

[Dale]

You can split it up into as many or as few systems as you like. The designation of a system is completely arbitrary.

What you want to do is to choose the boundary so that interesting things happen across the boundaries and uninteresting things happen inside the boundaries.

 

[vis viva]

Ok, thank you and duly noted.

So now I have arbitrarily decided to make it into these three systems.

Seen from the crank operators point of view, what is doing work on what? And can something be said about the sign +/- of the work?

 

[Dale]

System 1 is doing positive work on system 2. System 2 is doing negative work on system 1 and positive work on system 3. System 3 is doing negative work on system 2.

 

[Lnewqban]

You could also see it as several linked things that do not move unless they are forced to move by an external agent.
The movement itself is considered "work" or "energy", while the speed at which those things move is considered "power".
During the transfer of that energy through the several links of that chain, there is always a portion that "leaks out" or is lost in overcoming different forms of frictions and undesired transfers of heat.

 

[256bits]

Seen from the crank operators point of view, what is doing work on what? And can something be said about the sign +/- of the work?

Look at Fig 2 which shows a useful sign convention for direction of energy as work or heat into and out of a system. Note that for mechanical work you might see that designated as shaft work at times.
Look at Fig 3 which shows how changing the system boundary can affect whether you have Q or W crossing the boundary. Perhaps study that extensively to really comprehend the drawing of a system boundary.
http://www.sfu.ca/~mbahrami/ENSC 388/Notes/First Law of Thermodynamics_Closed Systems.pdf

 

[vis viva]

256bits, thank you for the nice paper, it is well written so I will keep it as a reference.

And yes, I assume the best way to view this would be through thermodynamics, and after reading further I see now that what I'm actually interested in; the 2nd law of thermodynamics "The Second Law of Thermodynamics is about the quality of energy. It states that as energy is transferred or transformed, more and more of it is wasted. The Second Law also states that there is a natural tendency of any isolated system to degenerate into a more disordered state. ". more precisely, how does the 2nd law manifest itself in system 2 i.e. where and what is it that is becoming increasingly disordered and wasting energy?

By the way, what's this? I'm quoting the paper (authors own emphasis)

The first law cannot be proved mathematically, it is based on experimental observations, i.e., there are no process in the nature that violates the first law.

I did not know that, I thought it was supposed to be the other way around, first you prove your theory mathematically, then confirm it experimentally, then you can make your claims e.g. "no process in the nature that violates the first law". So why is this law the exception?

System 1 is doing positive work on system 2. System 2 is doing negative work on system 1 and positive work on system 3. System 3 is doing negative work on system 2.

Thank you, that sounds logical. Now that I can formulate my query more precisely, should I start a new post?:

If you remove/disconnect system 3, how does the 2nd law of thermodynamics manifest itself in system 2 i.e. where and what inside system 2 is becoming increasingly disordered and wasting energy?

 

[Dale]

I thought it was supposed to be the other way around, first you prove your theory mathematically, then confirm it experimentally, then you can make your claims e.g. "no process in the nature that violates the first law". So why is this law the exception?

No, it is not the exception. All physical theories are like this. You make some unproven assumptions (called postulates), then you mathematically derive the experimentally measurable predictions, then you perform experiments. If the experiments match the predictions then you take that as validation of the assumptions.

Usually the starting assumptions come from generalizing observations already available.

If you remove/disconnect system 3, how does the 2nd law of thermodynamics manifest itself in system 2 i.e. where and what inside system 2 is becoming increasingly disordered and wasting energy?

If you disconnect 3 then the crank will spin almost freely. There will be very little work and what little there is will all go into frictional heating along the “shaft”.

 

[vis viva]

No, it is not the exception. All physical theories are like this. You make some unproven assumptions (called postulates), then you mathematically derive the experimentally measurable predictions, then you perform experiments. If the experiments match the predictions then you take that as validation of the assumptions.

That's more or less what I meant. It's just that when the author puts emphasis on "cannot be proved mathematically" which to me infers that it must be some kind of exceptional exception.

Or maybe it's just the mathematical fundamentalist' rhetoric that still echoes in my mind e.g. "If it can't be proven mathematically it doesn't exist" or "Mathematics is the language of God", and in this context the authors statement seemed somewhat unusual.

Anyway it's not that important to me, the below however is:

If you disconnect 3 then the crank will spin almost freely. There will be very little work and what little there is will all go into frictional heating along the “shaft”.

So absolutely no work is being done on the charges in this case? So this must be a very efficient power source?

The main reason I'm asking is because I noticed that when I use the electronic pendant of the hand crancked DC generator, the lab power supply, I can see that when I connect a load, the idle power consumption accounts for about half of the total consumption. I can't imagine it's all due to bad engineering, so doesn't the 2nd law of thermodynamics come into play here?

 

[Dale]

So absolutely no work is being done on the charges in this case? So this must be a very efficient power source?

Quite the opposite. System 2 is doing 0 output work for some small input work. So its efficiency is 0.

 

[vis viva]

Quite the opposite. System 2 is doing 0 output work for some small input work. So its efficiency is 0.

Sorry, I didn't express myself correctly, what I meant to ask was: So this must be a very efficient power source, when system 3 is connected again to system 2, compared to my lab power supply which is ~50% efficient? Or would system 2 exhibit same inefficiency as my lab power supply if system 2 also included system 1? If yes, would that be because of the 2nd law of thermodynamics? and if yes to that too, where does the disorder occur?

 

[anorlunda]

The whole field of thermodynamics is based on the average behavior of large numbers of particles. In other words statistics.

The setup in your OP with a motor and wires is very ill suited for understanding the laws of thermodynamics. May I suggest the following as a more productive means of self study of thermodynamics, than asking questions on the Internet.

https://www.khanacademy.org/search?page_search_query=thermodynamics

 

[Dale]

So this must be a very efficient power source, when system 3 is connected again to system 2, compared to my lab power supply which is ~50% efficient?

I have no idea about the loaded efficiency. The unloaded efficiency is obviously 0, but there is nothing to indicate the loaded power.

I don’t know if 50% is typical or not.

I don’t see the connection with thermodynamics. Are you considering friction and resistance to be part of thermodynamics?

 

[vis viva]

I think I may have mixed some concepts up somehow along the line, I suspected that the intrinsic losses in thermal power generation also applied to the setup in the OP. But I see now based on your replies that the losses in thermal power generation are purely of thermal nature e.g. ref this discussion: Almost 50% of the heat generated is lost at the condenser as heat rejection. It is unavoidable as without heat rejection it is not possible to convert heat energy into mechanical energy and drive the turbine without drop in temperature. (and maybe I should have started there, but that's how it goes sometimes).

I sincerely thank you for time and kind replies.

 

[Dale]

No problem. Sometimes it confuses me too. One thing that helped me is that someone told me that work doesn’t have entropy. Not that it has entropy of 0, but that entropy isn’t a relevant concept for work.