# Physics Question



## RobynC (Jun 10, 2011)

This is a rather bizarre question, but what would happen if you basically had a Van de Graaf Generator inside a capsule with circulating mercury around it?

No, I'm not going to build something like that


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## Psychophant (Nov 29, 2013)

Not a physicist, but mercury is conductive, so I'm assuming the electrons would rearrange in the structure to neutralize the field.


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## HAL (May 10, 2014)

That is definitely a bizarre question.

Presumably you'd see lightning style arcs of electricity between the mercury and the head of the generator.

Either that or, well, nothing. Depends on how it's set up, I guess.


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## ae1905 (Jun 7, 2014)

RobynC said:


> This is a rather bizarre question, but what would happen if you basically had a Van de Graaf Generator inside a capsule with circulating mercury around_* it*_?
> 
> No, I'm not going to build something like that


is "it" the capsule or the generator?

what is the capsule made of?


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## RobynC (Jun 10, 2011)

1. The mercury would be circulating in the capsule surrounding the Van de Graaf Generator

2. Say the capsule was one of the following

a. Metal
b. An heavily insulating material
c. A moderately resisting material


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## ae1905 (Jun 7, 2014)

RobynC said:


> 1. The mercury would be circulating in the capsule surrounding the Van de Graaf Generator
> 
> 2. Say the capsule was one of the following
> 
> ...


the mercury would close the circuit between the poles of the generator so that the charges induced at one pole are conducted back down to the other...this electrical loop would induce a magnetic field that interacts with the circulating mercury to induce other electric currents which, in turn, would induce their own magnetic fields...the net effect _might _be self-reinforcing and create a stronger magnetic field...the earth's own magnetic field is generated in a similar way by the mutual interactions of electric currents and magnetics fields induced in the circulating molten metals in the outer core...the capsule material would only make a difference if the mercury circulated outside of it and not the generator


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## ae1905 (Jun 7, 2014)

RobynC said:


> This is a rather bizarre question, but what would happen if you basically had a Van de Graaf Generator inside a capsule with circulating mercury around it?
> 
> No, I'm not going to build something like that


what prompted you to think of this question?


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## RobynC (Jun 10, 2011)

@ae1905



> the mercury would close the circuit between the poles of the generator so that the charges induced at one pole are conducted back down to the other...this electrical loop would induce a magnetic field that interacts with the circulating mercury to induce other electric currents which, in turn, would induce their own magnetic fields...the net effect might be self-reinforcing and create a stronger magnetic field...


So it'd produce a powerful magnetic field that would feed back on itself and amplify itself to a point?



> the capsule material would only make a difference if the mercury circulated outside of it and not the generator


Do you mean outside the generator or outside the capsule?


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## ae1905 (Jun 7, 2014)

RobynC said:


> @_ae1905_
> 
> So it'd produce a powerful magnetic field that would feed back on itself and amplify itself to a point?


no, it'd be a weak magnetic field because the the current in the generator is weak...the generator is designed to create strong electric fields by slowly accumulating electric charges on one pole...by surrounding it with mercury you prevent charges from accumulating and strong electic fields from developing--ie, you defeat the purpose of the device...instead, you turn it into a widget that converts the mechanical energy of the belt into a small electric current that generates a weak magnetic field




> Do you mean outside the generator or outside the capsule?


the capsule...if the generator is surrounded by an electrically insulating material, such as air, it would work as designed and accumulate charges on one pole...as the resulting electric field grew stronger a point would be reached where the insulator breaks down and the accumulated charges discharge to the capsule...if the capsule was made of an insulating material it too would break down at the point of impact of the discharge...if it was made of metal, the discharge would simply flow into the capsule and out into the circulating mercury and return to ground...so the diff is damage to the capsule


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## HAL (May 10, 2014)

Honestly I think the description of this strange idea is too vague.

Draw a diagram.


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## Psychophant (Nov 29, 2013)

If it's just a conductive material that's grounded, then current would flow through it like a wire and produce a magnetic field (like any wire...), but that wouldn't feedback and amplify since the induced current would oppose the current in the material (there's no free energy.. inductors don't generate more current, though that would solve a lot of humanity's problems/break physics if they did).

I'm confused by the question now. If the mercury is conductive and touches the sphere, then it basically just makes the top of the generator bigger. If the mercury doesn't touch, then it just shields the field, and if it's grounded then.. well, what I said before.


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## ae1905 (Jun 7, 2014)

Yomiel said:


> If it's just a conductive material that's grounded, then current would flow through it like a wire and produce a magnetic field (like any wire...), but that wouldn't feedback and amplify since the induced current would oppose the current in the material (there's no free energy.. inductors don't generate more current, though that would solve a lot of humanity's problems/break physics if they did).
> 
> I'm confused by the question now. If the mercury is conductive and touches the sphere, then it basically just makes the top of the generator bigger. If the mercury doesn't touch, then it just shields the field, and if it's grounded then.. well, what I said before.



read the op again...the mercury is not stagnant, it's circulating around the generator...electric currents are induced in conductors moving in magnetic fields...these currents themselves give rise to their own magnetic fields...the _net _effect _may _be a stronger magnetic field...it depends on directions of flow of the circulating mercury....this, again, is the type of mutual interactions that happen in the earth's core that give rise to the earth's magnetic field

the energy sources in this lab example are the mechanical energy of the belt transporting charges from one pole to the other and the kinetic energy of the circulating mercury


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## Psychophant (Nov 29, 2013)

ae1905 said:


> read the op again...the mercury is not stagnant, it's circulating around the generator...electric currents are induced in conductors moving in magnetic fields...these currents themselves give rise to their own magnetic fields...the _net _effect _may _be a stronger magnetic field...this is the type of mutual interactions that happen in the earth's core that give rise to the earth's magnetic field
> 
> the energy sources in this lab example are the mechanical energy of the belt transporting charges from one pole to the other and the kinetic energy of the circulating mercury


Oh, ok. I wasn't clear on the details there.


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## ae1905 (Jun 7, 2014)

Yomiel said:


> Oh, ok. I wasn't clear on the details there.


that's my interpretation, but it's the one that makes this a question of any interest...otherwise, a stagnant conductor either shorts the gnerator so it can't accumulate charges and generate a strong electric field or, as you say, it enlarges the generator's effective head so that charges accumulate on the surface of the mercury..neither possibility is particularly interesting


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## Adikshith Ojha (Feb 4, 2014)

ae1905 said:


> the mercury would close the circuit between the poles of the generator so that the charges induced at one pole are conducted back down to the other...this electrical loop would induce a magnetic field that interacts with the circulating mercury to induce other electric currents which, in turn, would induce their own magnetic fields...the net effect _might _be self-reinforcing and create a stronger magnetic field...the earth's own magnetic field is generated in a similar way by the mutual interactions of electric currents and magnetics fields induced in the circulating molten metals in the outer core...the capsule material would only make a difference if the mercury circulated outside of it and not the generator


Isn't that how electromagnetic waves are created? Overlapping of varying electric and magnetic fields?


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## ae1905 (Jun 7, 2014)

Adikshith Ojha said:


> Isn't that how electromagnetic waves are created? Overlapping of varying electric and magnetic fields?


yes, if the fields are time varying...in practice, even in steady-state there will always be some small variations that will generate e-m radiation


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## RobynC (Jun 10, 2011)

So the effect is like a dynamo?


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## ae1905 (Jun 7, 2014)

a dynamo is designed to produce a current, your example isn't...but both have a conductor rotating around inside a magnetic field thereby inducing a current, so the "effect" is the same...the dynamo, however, also has a mechanical switch that rectifies the current to make it dc...your example has no such switch and the nature of the currents induced in the mercury depends on the flow of mercury


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## HAL (May 10, 2014)

I still think this whole premise is ridiculous, or at least poorly explained.


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## ae1905 (Jun 7, 2014)

RobynC said:


> So the effect is like a dynamo?





HAL said:


> I still think this whole premise is ridiculous, or at least poorly explained.


https://en.wikipedia.org/wiki/Dynamo_theory


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## HAL (May 10, 2014)

ae1905 said:


> https://en.wikipedia.org/wiki/Dynamo_theory


I don't see how a Van de Graaff generator, a poorly-described thing of circulating mercury on top, and a dynamo are related.

What's maintaining the circulation of this mercury? And regardless of whether it's circulating such that it can create its own magnetic field, how does the Van de Graaff generator contribute to this?


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## ae1905 (Jun 7, 2014)

HAL said:


> I don't see how a Van de Graaff generator, a poorly-described thing of circulating mercury on top, and a dynamo are related.
> 
> What's maintaining the circulation of this mercury? And regardless of whether it's circulating such that it can create its own magnetic field, how does the Van de Graaff generator contribute to this?


obviously, a pump is circulating the mercury--it's the only inference that makes sense...the generator is immersed in the mercury encased in the capsule...the rest I've already explained above

edit: I take it you're still in school and are accustomed to getting questions where all the details are spelled for you but irl you will find this isn't always (or even often) the case....so it is necessary to infer what is probably the case incl, when necessary, making reasonable assumptions


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## HAL (May 10, 2014)

ae1905 said:


> obviously, a pump is circulating the mercury--it's the only inference that makes sense...the generator is immersed in the mercury and encased in the capsule...the rest I've already explained above
> 
> edit: I take it you're still in school and are accustomed to getting questions where all the details are spelled for you but irl you will find this isn't always (or even often) the case....so it is necessary to infer what is the case incl, when necessary, making reasonable assumptions


LOL

No I'm halfway through a theoretical physics degree..!!!

See, the way you just described it is totally different from how I was even remotely trying to imagine it, and I read and re-read quite a few of the posts on this thread to figure out what the whole idea was all about. At no point did I decide to place a generator inside a presumably donut-shaped capsule of mercury placed over a Van de Graaff generator (which I still can't figure out the purpose of?!).

It is still all utterly nonsensical to me. Then again RobynC tends to quite often come out with some rather funny questions on here, so I'll make allowances for the obscure and absurd nature of the whole premise.

If the mercury is spinning round you're gonna get one of these:










And if there's a Van der Graaff generator involved, well it depends on the direction the charge is drawn by the generator. In one direction, more charge will be added to the mercury so the current will be greater and resulting in a stronger B field. In the other direction, charge will be removed and the B field will reduce. If you close the Van der Graaff generator circuit (to get that wild spark) it'll temporarily restore the charge imbalance and the B field will be at a mid-point.

Happy now? :gentleman::gentleman::gentleman:


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## ae1905 (Jun 7, 2014)

HAL said:


> LOL
> 
> No I'm halfway through a theoretical physics degree..!!!
> 
> ...


why does the capsule have to be "donut shaped"?...and the capsule with the generator inside it isn't "placed over another generator"....the generator is placed inside the capsule--which can be any shape as long as the generator fits inside it--and mercury is poured in so the generator is fully immersed...I assume the generator is sealed so mercury doesn't get inside of it...the mercury is then circulated in some pattern inside the capsule--this depends on the arrangement of the pump and on the shape of the capsule...that's the schematic I envisioned


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## HAL (May 10, 2014)

ae1905 said:


> why does the capsule have to be "donut shaped"?...and the capsule with the generator inside it isn't "placed over another generator"....the generator is placed inside the capsule--which can be any shape as long as the generator fits inside it--and mercury is poured in so the generator is fully immersed...I assume the generator is sealed so mercury doesn't get inside of it...the mercury is then circulated in some pattern inside the capsule--this depends on the arrangement of the pump and on the shape of the capsule...that's the schematic I envisioned


And there we have it! Proof that this whole scenario is an absolute farce, too sparse in the details and still not an actual diagram from the one person who came up with the idea in the first place!

I suppose it wouldn't need to be donut shaped, but you can model it that way for ease of understanding. If the mercury is circulating then it'll be flowing in a 'donut' direction, no matter if it's a donut or a sphere or what. I hope we at least agree on it being taking a round shape, rather than being a box or something? And yes I really did just use 'donut' to denote direction... heh.

In my opinion it doesn't need to be so closely linked to dynamo theory because the flow of the mercury will be far less complex than that of the conventional and rotational currents of the hot magma inside large astronomical objects. I just see it as a uniform rotational flow of liquid metal, which is essentially nothing more than a fat loop of charged particles circulating, much like the current loop image I embedded above.


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## ae1905 (Jun 7, 2014)

HAL said:


> And there we have it! Proof that this whole scenario is an absolute farce, too sparse in the details and still not an actual diagram from the one person who came up with the idea in the first place!
> 
> I suppose it wouldn't need to be donut shaped, but you can model it that way for ease of understanding. If the mercury is circulating then it'll be flowing in a 'donut' direction, no matter if it's a donut or a sphere or what. I hope we at least agree on it being taking a round shape, rather than being a box or something? And yes I really did just use 'donut' to denote direction... heh.
> 
> In my opinion it doesn't need to be so closely linked to dynamo theory because the flow of the mercury will be far less complex than that of the conventional and rotational currents of the hot magma inside large astronomical objects. I just see it as a uniform rotational flow of liquid metal, which is essentially nothing more than a fat loop of charged particles circulating, much like the current loop image I embedded above.


a single donut-shaped loop of flowing mercury is a simple arrangement...more complex ones are possible by changing the flow of mercury or the shape of the capsule...in any event, the mercury will interact with the magnetic field generated by the van de graaf machine and induce other magnetic fields that will interact in some way...the principle here of electromagnetic induction is the same as the one at work in man-made dynamos and in the earth's geodynamo...this is what gives this example its interest


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## HAL (May 10, 2014)

ae1905 said:


> a single donut-shaped loop of flowing mercury is a simple arrangement...


Indeed :th_wink:












> more complex ones are possible by changing the flow of mercury or the shape of the capsule...in any event, the mercury will interact with the magnetic field generated by the van de graaf machine and induce other magnetic fields that will interact in some way...the principle here of electromagnetic induction is the same as the one at work in man-made dynamos and in the earth's geodynamo...this is what gives this example its interest


A Van de Graaff generator doesn't make a B field. Charge is pushed to one end of the system and stays there.

The only possible B field is from the moving charges in the circulating mercury.

Do you study physics too then?


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## ae1905 (Jun 7, 2014)

HAL said:


> Indeed :th_wink:
> 
> 
> 
> ...


the generator is _immersed _in the mercury, so charges transported to the head don't accumulate there but are conducted back to ground (whence the charges were pulled in the first place)...it's this electrical circuit that produces the generator's magnetic field


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## HAL (May 10, 2014)

ae1905 said:


> the generator is _immersed _in the mercury, so charges transported to the head don't accumulate there but are conducted back to ground (whence the charges were pulled in the first place)...it's this electrical circuit that produces the generator's magnetic field


The entire Van de Graaff generator - both positive and negative ends - is submerged in mercury..????


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## Nightmaker81 (Aug 17, 2013)

HAL said:


> LOL
> 
> *No I'm halfway through a theoretical physics degree..!!!*
> 
> ...


Find me the answer to integral of of sin(x)/x from the bounds of negative infinity to infinity using complex analysis(this is the way this integral was originally derived) . Also tell me what Maclaurin series you used and why you used a Maclaurin series and also what did you get from the series to use in something else to find the integral. 

This is in relation to the fact that @_ae1905_ information in this thread has been the most accurate and you're being condescending for no reason. If you truly have some theoretical physics knowledge I'm sure solving a simple complex analysis problem wouldn't be a problem

Edit: One last subtly just to truly show me you know what you're talking. Why are we using a Maclaurin Series instead of a Taylor series? They're very similar series but there is a reason we're using it. And don't tell be "a=0". Tell me why a=0 serves as a limitation


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## HAL (May 10, 2014)

Nightmaker81 said:


> Find me the answer to integral of of sin(x)/x from the bounds of negative infinity to infinity using complex analysis(this is the way this integral was originally derived) . Also tell me what Maclaurin series you used and why you used a Maclaurin series and also what did you get from the series to use in something else to find the integral.
> 
> This is in relation to the fact that @_ae1905_ information in this thread has been the most accurate and you're being condescending for no reason. If you truly have some theoretical physics knowledge I'm sure solving a simple complex analysis problem wouldn't be a problem
> 
> Edit: One last subtly just to truly show me you know what you're talking. Why are we using a Maclaurin Series instead of a Taylor series? They're very similar series but there is a reason we're using it. And don't tell be "a=0". Tell me why a=0 serves as a limitation


!!!!!!!!!!!!!!!!!!!

I was being 100% genuine, I think the entire concept being outlined here is farcical and poorly explained in the OP. I can't help taking some offence when someone decides to make a lowest common denominator assumption and say I must be in high school because I'm not 'thinking for myself' properly - that was the first and only condescending comment I see here, and it was directed towards me..!

Sorry, I haven't done complex analysis yet. Actually I admit my university seems quite shit in terms of the maths-related content on my course. I moved over from another (apparently 'worse') university this year, and my previous university was vastly more maths-oriented. But that was my first year so we'd only done the starting blocks for 'normal physics', up to vector calculus and things like Fourier Analysis. Anyway I don't see how that relates to my understanding of this poorly explained Van de Graaff generator in mercury scenario...?

I mean seriously... What you've just said here is, "_You absolutely cannot have any idea about electrostatics or induced magnetic fields, unless you can tell me why a Maclaurin series is used instead of a Taylor series in the original integration of sinx/x from minus infinity to infinity..!_" WTF..??? How about I ask you to prove your knowledge of... gyroscopic motion... by demonstrating you have sufficient coding skills in C++. That's about the same level of relevance, I think!


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## Nightmaker81 (Aug 17, 2013)

HAL said:


> !!!!!!!!!!!!!!!!!!!
> 
> I was being 100% genuine, I think the entire concept being outlined here is farcical and poorly explained in the OP. I can't help taking some offence when someone decides to make a lowest common denominator assumption and say I must be in high school because I'm not 'thinking for myself' properly - that was the first and only condescending comment I see here, and it was directed towards me..!
> 
> ...


It was more about the fact you said you are halfway towards a theoretical physics degree. Graduate level classes in electricity and magnetism and quantum mechanics require complex analysis....there isn't anyway around that. You need to know complex analysis to do that kind of physics 

And in terms of what is ae1905 is saying, he's right about physics. Physics research specifically is all about trying to figure out something you don't know and you don't have enough parameters. I'm impressed by ae1905's problem solving because in a physics lab you fail a lot. You pretty much don't know what is going on and the objective is trying to fill in the details with your intuition. Yeah this problem isn't the clearest, but it's still possible to understand once you get down to the nitty gritty.

And finally ae1905's knowledge on this has been pretty accurate. I would disagree with a few small things here and there, but everything he's said is definitely on point including the dynamo part. He obviously knows his physics and his E and M and I don't think saying there isn't enough information is enough to discredit him since once you're at the end of modern physics you're not going to any information and it's purely trying to problem solve based on intuition and doing things over and over again.


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## HAL (May 10, 2014)

Nightmaker81 said:


> And finally ae1905's knowledge on this has been pretty accurate. I would disagree with a few small things here and there, but everything he's said is definitely on point including the dynamo part. He obviously knows his physics and his E and M and I don't think saying there isn't enough information is enough to discredit him since once you're at the end of modern physics you're not going to any information and it's purely trying to problem solve based on intuition and doing things over and over again.


I genuinely wasn't ever trying to discrdit @ae1905 - I was simply highlighting how we'd both interpreted the 'problem' in quite different ways, hence proving that the details were too thin to be going off. I wasn't saying the dynamo suggestion was wrong at all, just that my own personal visualisation was more like that of a single-loop of moving charge. As I said, I just thought it funny when I was called out as a high school student..!

There's a chance I have done complex analysis, but without it being specifically named as such. A lot of the material I've covered in my course has just been thrown into the wider context of a given module. The modules I've done were all simply called "Maths 1", Maths 2", etc, where loads of stuff was thrown in at once.

In my defence, I was pretty hot on all this stuff in my first year. I was one of the go-to guys for course mates who weren't sure about things. We covered up to what is essentially "Calc IV" by American standards, I know that much. I did pretty good in my exams, too. Then I had a year out on a scholarship learning Chinese in China, so I put the physics on hold and now it's been a verrrry long time since I did any maths or calculus or anything at all. I'm just getting back into the swing of things now, having just returned home and restarted the physics only 4 weeks ago. Problem is we're onto new stuff now (Group Theory and Vector spaces, primarily) so I need to refresh the calculus stuff in my own time. I admit I googled "sinx/x complex analysis" and it put the fear of god into me. I have a lot of re-learning to do... I quite literally swapped my physics knowledge for Chinese knowledge. Now I need to swap it all back again.

Anyway, I still hereby declare a Van de Graaff generator immersed in a rotating, oddly-defined blob of mercury is... very, very silly!


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## RobynC (Jun 10, 2011)

From what I remember you got a right hand rule in effect so it doesn't matter whatever the shape is, the direction of current produces a magnetic field perpendicular to the flow


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