The h Bar

ZeroTheHero

2:02 AM

@JohanLiebert did you get your answer on moments of inertia?

Johan Liebert

@ZeroTheHero no

ZeroTheHero

the answer is no: it is not possible.

Johan Liebert

@ZeroTheHero yes I concluded that by making a counter example. What you did for getting the answer?

ZeroTheHero

as an example the cube and the sphere have the same principal moments of inertia so moments about any other axes will be the same.

(if I understand your question correctly).

In fact shape is related multipole expansion of the surface, not to the moments of inertia.

There is some fancy word around solids with same moments of inertia... it has the word “similarity” in it but I don’t remember right now...

”rotationally similar” or something like this... I just don’t remember...

Johan Liebert

@ZeroTheHero yes in my case I did it with a solid and a hollow sphere (both have the same inertia). So I concluded that there must not be a unique solution. Though I would like to see a mathematical treatment of the case.

Shootforthemoon

6:56 AM

Hey, can I spam this group with a question of mine (boutnied since 5 days, no new answers from reputable source...)? XD

Yuvraj Singh...

No, you can, t advertising the question is not allowed in the room. @Shootforthemoon

Shootforthemoon

I know, I was kidding @Yuv

But at max I would advertise knowledge >:P

John Rennie

7:12 AM

@Shootforthemoon hi. This question?

Shootforthemoon

@JohnRennie yep

John Rennie

@Shootforthemoon I'm not sure why you're ignoring the battery.

When a battery is present electrons don't travel round the circuit. Inside the battery a chemical reaction produces free electrons at the anode and they travel round the circuit back to the cathode where the reaction consumes the electrons.

If you don't include the energy of the reaction in the battery then it does look as if the energy isn't conserved, but why would you ignore the energy change in the battery?

Shootforthemoon

Yes, it's quite obvious that a battery is non conservative XD. But in general an emf can be any work that creates a separation of charges, so I don't get why it needs to be conservative

sorry, non conservative

Especially my problem is that closed integral

I don't see how the emf is present all around the circuit, or in some cases (electromagnetic induction) even delocalized

Yuvraj Singh...

7:46 AM

@Shootforthemoon emf is not a mechanical force like you are comparing it.

Electromotive force, abbreviated as E.M.F and denoted by ε, is not a force. It is defined as the energy utilized in assembling a charge on the electrode of a battery when the circuit is open.

Emf is not f it is $E$, it very confusing to me too when I first saw that word and from the user of this website only I got know emf is not a force.

Shootforthemoon

You are right (in fact I said mechanical work :p)

Yuvraj Singh...

So can I paste this as answer.

Shootforthemoon

I saw that also in the comments or answers of some users, that's one reason why I did not consider them reputable sources

@YuvrajSingh... ?

No wait, when I talk about a force in the question I put it in latin to underline that a work (EMF) is done by some hypothetical force and that force must be non conservative as well

Yuvraj Singh...

@Shootforthemoon I ask are you OK with answer like that.

Too extended my above answer , emf a is a measure of the amount of energy added to a coulomb by something. If a battery has an emf of 2V, then it adds 2J to each coulomb flowing through it. If the battery is perfect, then that also means that the potential difference between the two ends is also 2V. If the battery is imperfect, then some of that extra energy may be lost again in the internal resistance of the battery.

Changing magnetic also produces emf.

it is almost always measured in volts in both metric and imperial systems.

Shootforthemoon

@YuvrajSingh... Unfortunately that I already know, maybe I formulated the question poorly, as English is not my mothertongue.

@YuvrajSingh... that's true, emf is a work per unit charge

Yuvraj Singh...

7:56 AM

@Shootforthemoon same here.

But that work in, t simple as f. Ds

@Shootforthemoon rennie sir can help you.

Shootforthemoon

Thanks @Yuvraj

@John do you know any reference or source I could use to address the topic under 360 degrees?

John Rennie

@Shootforthemoon I don't understand what you are asking.

If an EMF is nonconservative then either energy is being supplied or consumed by sme external force, or we have a violation of energy conservation.

With induction we get non-conservation if you look only at the circuit because the external magnetic field does work on the electrons in the circuit.

In a circuit with a battery there is no external agent acting, so if the EMF was non-conservative we'd have a violation of conservation of energy.

Yuvraj Singh...

But sir that violation of energy is not define?

This one sir.

Shootforthemoon

@JohnRennie ok, in the question I make a strange example and say if there was gravity acting on a single side of the circuit say to move electrons downwards towards the cathode (protons fixed), would gravity be nonconservative?

John Rennie

8:12 AM

@Shootforthemoon that's not a physically realistic example. You can't just say gravity only acts on half the circuit.

Shootforthemoon

ok, but emf only acts on half the circuit, no?

John Rennie

If you create physically unrealistic situations you're going to get physically unrealistic rsults.

@Shootforthemoon In a battery the electrons start at the cathode with a higher energy than they have at the anode. That higher energy is supplied by the Gibbs free energy of the reaction in the battery. As the travel round the circuit they gradually lose that initial energy.

Shootforthemoon

That's right, I see that emf provides electrons with more potential energy, but this is done along a single side of the circuit, not on the whole path. If the emf were present on the whole circuit then it would be ok to evaluate the work done by the battery as a closed integral

I don't see instead why we can do such procedure

@John Consider gravity in general, not on a circuit, we can create gravity at a single region in space. if our path is closed but goes outside of that region where gravity is, the work done by gravity may be nonzero.

John Rennie

@Shootforthemoon there are two ways you can look at this. Strictly speaking in a circuit the electrons don't move in a look. Take for example a circuit with a zinc-copper battery. At the anode the reaction Zn -> Zn2+ + 2e generates electrons. Those electrons travel round the circuit and end up at the cathode where the reaction Cu2+ + 2e -> Cu consumes the electrons.

So if you consider a single electron it doesn't complete a loop. So integrating round a loop doesn't describe what actually happens to an electron.

Shootforthemoon

Aha, thanks

John Rennie

8:26 AM

What we'd actually expect is that the integral of the EMF from the anode to cathode is equal to the free energy difference between the electrons in the zinc anode and the electrons in the copper cathode. Which of course it is.

But ...

We tend to ignore this detail and treat the circuit as electrons flowing through the battery.

Shootforthemoon

But so what does integrating mean in this context?

John Rennie

In that case the battery is treated as acting like a pump, and in that model there is an EMF acting inside the battery to pump the eletrons from the cathode to anode through the battery.

@Shootforthemoon potential is the integral of EMF.dx

Or equivalently E = -dV/dx

Shootforthemoon

When we define potential as the work done per unit charge we consider however that each electron completes a loop after an undefined period of time

no?

John Rennie

That's the point I was making above. Rather than worry about what exactly happens in a battery we just assume the electrons flow through it i.e. that they do move in a loop. This isn't actually what happens, but it works fine for doing circuit calculations.

In that case we have to assume there is a potential gradient inside the battery.

There is a sense in which this does exist, because it's the difference in the free energies of the electrons in the zinc and copper (or whatever the battery is made from).

Shootforthemoon

@JohnRennie Ah, so we could demonstrate that the two situations are equivalent via such gradient, right?

John Rennie

8:33 AM

Yes.

@Shootforthemoon Have you studied electrochemistry?

Shootforthemoon

@JohnRennie Thanks very much! I would like to know the proof, but your words already reassured me a bit

@JohnRennie General electrochemistry

John Rennie

I ask because the fundamental equation is the Nernst equation and it works precisely by equating the free energy of the reaction in the battery to the EMF developed across the battery.

Shootforthemoon

We've mentioned it, but never defined it in such way, so I'll take note of that, thanks!

And instead, is motional emf delocalized on a circuit?

John Rennie

To summarise I think the confusion is that in real circuits with a battery electrons don't flow in a loop so integrating the EMF round the loop is inappropriate. In our model circuit electrons do flow round the loop (through the battery) and in that case our model circuit does have a potential gradient inside the battery.

Shootforthemoon

Thanks very much! @John

John Rennie

8:40 AM

@Shootforthemoon do you want me to try and put all this in an answer to your question?

Shootforthemoon

@JohnRennie That would be great! if you have time, of course.

Also if there's some reference you would suggest me for that, I'll follow your advice

user129412

I have a silly question... What happens if I cover a loop of area A with a thin (thicknes << penetration depth) superconducting film, and apply a perpendicular magnetic field? Will I thread a flux through that loop?

John Rennie

@Shootforthemoon actually, rereading your question you're asking something slightly different to what I thought.

Shootforthemoon

9:44 AM

@JohnRennie Ahh, I felt the discussion of before was related but somehow I missed some relationships. Thought it was just me lol

@JohnRennie But still would you suggest something for that?

John Rennie

@Shootforthemoon rereading your question it looks as if you are saying that the electrons have no force exerted on them once they leave the battery. And if there's no force there cannot be any EMF. Is that correct?

Shootforthemoon

10:08 AM

@John Ohh, I see, so that force from the battery continuously works also on the outer electrons (the ones on the wire)?

John Rennie

The electrons in the wire behave much like a gas. In fact there is a model for electrons in metals called the free electron gas.

Shootforthemoon

so isn't it that once a separation of charges have been created and continuously kept by moving electrons at the cathode what happens outside is a result of coulomb's interactions between charges?

John Rennie

In this model what the battery does is compress the gas so the "pressure" is higher at the anode than at the cathode.

So along the wires between the anode and cathode we get a pressure gradient.

The electrons in the wire feel a force because the pressure is higher on one side of them than on the other, and they move towards the lower pressure i.e. towards the cathode.

Shootforthemoon

ohh, wow i see

and that would also explain why an induced emf is eventually delocalized on the circuit?

1010011010

12:19 PM

Howdy, I'm studying Matsubara frequencies again

I have the case where I do have poles on the imaginary axis

Shifting my integration parameter by a small $\varepsilon$ shifts the poles of my weighting function and the pre-existing poles in opposite directions

I'm just a bit confused what the integration path is

Guru Vishnu

1:42 PM

Hi. Some time ago, I saw a question about the possibility of refractive index of red being larger than that of violet such that the spectrum gets reversed in its order. I tried different queries - `spectrum reversed`,`refractive index red`, etc., but couldn't get close to the question I'm looking for. If possible, can someone help me find that question?
Thanks!

Jim

2:09 PM

@GuruVishnu can you give any more specifics because a cursory google search comes up with nothing

Guru Vishnu

@Jim I actually made a search using the search box on the main site and not Google.

I think in addition to the queries I used, you might need to use answers:1 to eliminate duplicate tuples.

Jim

I figured that, but the site's search engine is not nearly as good as google. So I use that when I need to find something specific

Novalium Company

To make video games more relatable and immersive, let's put the player's face as the main character. In that way, the player will feel like he is literally in the game.

Guru Vishnu

Thanks for letting me know that. Do you use site:physics.stackexchange.com while searching on google for searching questions within this site?

Novalium Company

Let's make a local social network. An app that uses GPS tracking to allow other users nearby to rate your look, comment on you and so on... it's an anonymous way for people to know how others feel about them in real life.

Let's make a contact lens mouse. It tracks where you look and moves the cursor there.

Guru Vishnu

2:19 PM

@Jim, I'm looking whether is it possible for the refractive index of red colour in a medium is more than that of violet colour, so that the order in which the colours in the spectrum appear get reversed.

I'm sorry if I'm disturbing you.

Jim

@GuruVishnu I'm not a materials scientist, but generally, the answer is no. That said, some materials have been engineered with various "magical" properties. I wouldn't be terribly surprised if it were possible to create such a material. But my best guess is that one cannot

Guru Vishnu

@Jim Thank you for your help. I was also thinking of the same thing but was unsure of the other possibility.

Guru Vishnu

2:38 PM

@Jim: Just now, I came to know (from your profile) that you did your undergrad in rocket science (aerospace engineering?). I'm also interested to do a career in a space agency. If possible, could you provide any guidance?

To be more specific - which is the best choice amongst - aerospace, electrical, computer science, etc..? I find all of these fields to be related to space sector?

PM 2Ring

@GuruVishnu That's called anomalous dispersion. You might find something useful to you in these questions: physics.stackexchange.com/search?q=%5Bdispersion%5D+anomalous+

Novalium Company

Can we automate automation?

Guru Vishnu

@PM2Ring Thank you. I didn't know that keyword. That query gives a bunch of stuff to read about. It would also help me learn more from google :-)

Johan Liebert

Avoid pen aper scree shot of the answers, try write it, and for more information use meta! — Yuvraj Singh... 4 hours ago

Novalium Company

It's the decade of putting AI everywhere people can find. Why not make a machine that will randomly automate things... donno?

Jim

2:47 PM

@GuruVishnu If those are your only options, aerospace is the one to go with. You will have to do a master's degree to specify your studies in space systems, but that's standard for everyone. I was lucky enough to have the option to do an undergrad in space engineering (none of the aero nonsense, just space stuff), which meant that I technically didn't need a master's degree, although most of my colleagues still went for one

PM 2Ring

@YuvrajSingh... Also, what does "pen aper" mean?

Pᴀᴜʟsᴛᴇʀ2

@ACuriousMind @Chris @DavidZ @rob @Qmechanic - Would you all be interested in the following:

0

Q: What is the angular velocity limit and maximum bank angle of a standard road curve?

I'm trying to declare some simple test conditions for a loaded vehicle where the test will output the maximum limits for that state of the vehicle. In order to calculate the maximum safe velocity, the following parameters must be taken into account: Height of present center of gravity (CG) of...

PM 2Ring

@JohanLiebert I suppose that Yuvraj means that Jujhaar can learn on Physics meta why we don't like screen shots of text & equations.

Jim

@GuruVishnu space engineering, or aerospace if that's what's available, will give you plenty of training in electrical and computer systems. It's the most relevant for working in a space agency, although the other options can provide a path to such a career.

PM 2Ring

Jan 11 at 15:04, by ACuriousMind

> Don't advertise your recent questions. If you just posted something on the main site, give it some time, and don't tell people to go there and look at it. Particularly by pinging people. That's rude. Those who can answer are already watching the queue on the main site!

Guru Vishnu

2:51 PM

@Jim Thank you for you reply. There are still a lot of options which I need to search after my exams. Here there are no courses like "rocket science". Only things are "aerospace" and "avionics" where the latter deals with the electronics part.

Especially, some people say aerospace is slightly modified version of mechanical engineering.

Pᴀᴜʟsᴛᴇʀ2

@PM2Ring - Not sure what that has to do with the price of tea in China?

Jim

@GuruVishnu That's common most places. "Space engineering" is still a new field and most areas do not offer undergraduate programs in it. However, a master's degree will let aerospace students specialize in space systems

Guru Vishnu

@Jim Thank you very much for your time and reply. Are you working in the Canadian space agency? I like the Canadarm's function on the ISS :-)

Johan Liebert

@PM2Ring yes you are. Initially I misinterpreted it but as soon as I realised this I removed question.

Pᴀᴜʟsᴛᴇʀ2

@PM2Ring - Not my question I'm asking about. I'm asking if Physics would be interested in a question which doesn't belong on Mechanics.SE. As a Moderator of Mechanics, I try to give common courtesy before arbitrarily moving questions.

Jim

2:54 PM

@GuruVishnu No, after my undergrad, I decided to leave the space industry and go into physics. I still have strong ties to the industry and many of my former peers and close friends are working for the CSA or in the space industry, but I work at a university.

PM 2Ring

@Pᴀᴜʟsᴛᴇʀ2 I realise that it's not your recent question, but it's still a very new question. ACuriousMind is a moderator & an owner of this room. I was just posting that link to let you know the room policy on linking recent questions.

Pᴀᴜʟsᴛᴇʀ2

@PM2Ring - You still do not understand.

Jim

Incidentally, the Canadarm was a contribution by private company MDA, not the actual Canadian Space Agency itself. In Canada, most of our space industry is privatized with the CSA only managing the front-end/astronauts

PM 2Ring

@Pᴀᴜʟsᴛᴇʀ2 Fair enough. I'm on my phone, so I didn't realise that it wasn't a Physics site question. Sorry about that.

Pᴀᴜʟsᴛᴇʀ2

@PM2Ring - No worries.

PM 2Ring

2:56 PM

Phew! :)

Guru Vishnu

@Jim That's great! If possible, I'd like to discuss some more after all my exams with you. Thank you :)

@Jim I didn't know that. But the mechanism with which it grabs resupply vessels for berthing is brilliant!

Yuvraj Singh...

3:20 PM

@PM2Ring I just said that it is better if we use math Jax and other options on site rather than posting screen shot!

@JohanLiebert does that make sense posting my comment here because I wrote that comment for the OP and he understand that we'll!

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