The h Bar - 2019-07-06

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mohamed azaiez

00:53

Hello guys i would like someone to help me clear some misconceptions about electricity

For example i don't get how potential difference of a battery add up in series there is an equilibrium formed between the electrode and the solution that can't be exceded and it is a property of metals for example in zinc it's 0.74 V why would it be more when you add up in series

JMac

@mohamedazaiez Because potential difference is just the difference that it can build up between one end and another. If you have the ability to lift something lets say 7 feet into the air. If you're standing at 7 ft up, and someone else with a 7 foot reach passes something up to you, you can raise it another 7 feet and get a 14 foot gravitational potential total with two people who can only raise 7 feet of potential alone. Potential is all relative

mohamed azaiez

Yep someone already awnsered like that but

I don't get how this idea applies to batteries in series

someone awnsered like you by saying you add up the potential of single cells

like 1.5 + 1.5 + 1.5 +1.5 = 6 But at the physical level how is this done

there can't be any more excess postive charges on the copper electrode or any more excess negative charges on the zinc electrode (nernst equation)

JMac

because the battery potential difference, like a person's reach, doesn't have any absolute reference; that's how potentials work. It doesn't know what "0 volts" is, as far as a battery is concerned, 0 volts is just the lower potential end of the battery. The "0 volts" in a circuit is generally just the arbitrary ground chosen based on the lowest potential the circuit can get to

mohamed azaiez

01:11

Thanks for the effort ,but that's the awnser in every post about this question i am starting to feel like no one knows what really happens x) i would say i have a starter to meduim backgroundd on this (columb's law , redox reactions , nernst equation) so you can use more complicated terms or if you can or if you know a link or a subject to tcheck for this i searched all across the internet without any good result /:

3 hours later…

PM 2Ring

03:47

@mohamedazaiez Voltage is a torsor.

Semiclassical

04:00

@PM2Ring yes, that's totally the kind of explanation that will help here :P

Novalium Company

04:21

If I want to immigrate to the US, can I just find a job? Like, I can clean floors, will the US acceptnme as a citizen member after I have gotten a cleaning job?

PM 2Ring

04:31

@Semiclassical Perhaps not, but he gives lots of examples of torsors, and seeing the general pattern might help.

mohamed azaiez

05:26

@PM2Ring

That didn't really help but thanks for trying

OneMug

05:42

@JohnRennie Updated! This has the intended intro paragraph: link CrowFly.pdf

John Rennie

@OneMug morning :-) I must admit I haven't read your document yet. I'll grab the updated version now.

Ryan Unger

I think it depends on if the crow is laden

and if it's a European or African crow

PM 2Ring

@mohamedazaiez Maybe it would help if we use a slightly more tangible example, like gravitational potential energy.

OneMug

@JohnRennie OK. @RyanUnger Neither, it's an American Crow!

John Rennie

@OneMug you've rediscovered the Tsiolkovsky rocket equation :-)

OneMug

05:51

@JohnRennie Knew it had to have a name, everything has already been discovered!

John Rennie

Tsiolkovsky rocket equation

The Tsiolkovsky rocket equation, classical rocket equation, or ideal rocket equation is a mathematical equation that describes the motion of vehicles that follow the basic principle of a rocket: a device that can apply acceleration to itself using thrust by expelling part of its mass with high velocity can thereby move due to the conservation of momentum. The equation relates the delta-v (the maximum change of velocity of the rocket if no other external forces act) with the effective exhaust velocity and the initial and final mass of a rocket, or other reaction engine. For any such maneuver (or...

OneMug

@JohnRennie So that's the rocket equation! Seems to work for birds too!

OneMug

06:06

@JohnRennie Well, they're almost the same, but I've used the "glide slope angle" in my derivation, a parameter that rockets don't usually have (unless they have wings). It's bed time here, I'll dig into it tomorrow.

John Rennie

In the Tsiolkovsky equation the rocket moves away from the source of gravity (e.g. the Earth) and the energy from the fuel goes into this change in PE.

person27

So every once in awhile I come back to the basic def. of relativity because I don't practice physics in any sense and forget stuff, and because I always feel like I'm missing some understanding on relativity.

John Rennie

In your case the bird/plane stays at the same height, but energy is lost due to friction, drag, etc. The glide angle captures all this so for the bird/plane staying at constant glide angle is the same as the spaceship staying at constant distance.

So the vertical distance between the position you'd be at ina glide and the constant height is analogous to the $\Delta v$ in the Tsiolkovsky equation.

The big difference between the rocket and the bird being the lack of aerodynamic drag for a rocket.

person27

06:21

Spacetime is a nice unified model for math and advanced concepts I'm sure. It's not an absolute measure; {x, y, z, t} is always from a frame of reference describing another, but why is time considered a 4th dimension when x, y, z can only be known if t is given, and their derivatives are time-dependent?

Essentially asking why time is appropriate as a dimension if there are ties between it and the other dimensions, since x, y, z are not dependent between each other.

John Rennie

@person27 because in relativity a Lorentz transformation mixes up the spatial and time dimensions.

OneMug

@JohnRennie Still it's most curious that the same form of equation is derived starting from different considerations/constraints. I didn't use the velocity or delta velocity anywhere in my derivation! In fact, every place I mentioned velocity, it was held constant so that delta v was ZERO!

John Rennie

Suppose you consider the $x$ and $y$ axes in a plane. Rotations mix up these two axes. For example suppose my axes are rotated 90° compared to yours. Then what I can $x$ you will consider to be a mixture of $x$ and $y$.

person27

I heard about the Lorentz transformation, but thought it might not apply. Well you're right, I should read it all first and come back if I still have a question about it. Thanks

John Rennie

That means the $x$ and $y$ axes are observer dependent and can't be treated as completely separate quantities.

In relativity we get a similar effect. Lorentz ransformations are in effect hyperbolic rotations and they mix up the $x$ and $t$ axes.

That means the $x$ and $t$ axes are also observer dependent and can't be treated as completely separate quantities.

All four axes, $x$, $y$, $z$ and $t$ have to be taken together as forming a 4D manifold.

OneMug

06:30

The fact that I did not have to introduce a coordinate system was not lost on me. I had the fortunate fact that lift and drag form a natural orthogonal system that could be (and was) used.

person27

06:44

Knowing it's convenient to have the spacetime model, I think the answer to my question is just no, it doesn't matter that there are intrinsic relationships between the dimensions -- they can still be dimensions.

John Rennie

@person27 there are always relationships between the dimensions.

Suppose I set up my $x$ and $y$ axes and I note your trajectory lies along the line $x=y$ in my coordinates.

Then for your trajectory I have $dy/dx = 1$

This is no different from saying, for example, $dy/dt = v$.

It's just that time seems different because we are always moving in time whether we want to or not while we can be stationary in space.

person27

What trips me up is that you need t = {constant} to compute x, y, z

That is, it has to be known (I hope I'm not wrong on this at least)

John Rennie

@person27 You mean because you are expressing position as a function of time? So we have $x(t)$, $y(t)$ and $z(t)$?

person27

I don't believe this is true of the other three dimensions, regardless how you may view them with transformed coords or whatnot. I'm probably babbling at this point

Isn't it a requirement due to relativity, that x, y, z can only be known at some point in time?

John Rennie

No. The four numbers $(t,x,y,z)$ label a single point in spacetime.

And a trajectory, i.e. a line in spacetime, is a set of spacetime points $(t,x,y,z)$.

So on the line it's certainly true that at any time $t$ the equation of the line gives $x,y,z$ corresponding to that time. But you could just as well choose some value of $x$ then the equation of the line gives $t,y,z$ for that value of $x$.

If you're interested I go into this here:

69

Q: What is time, does it flow, and if so what defines its direction?

This is an attempt to gather together the various questions about time that have been asked on this site and provide a single set of hopefully authoritative answers. Specifically we attempt to address issues such as: What do physicists mean by time? How does time flow? Why is there an arrow of ...

thermodynamics special-relativity time entropy arrow-of-time

person27

07:14

That's a fantastic answer John, really cleared it up for me. So time does deserve to be a first-class citizen. I do have one question on the paragraph in that answer about entropy and perception of time. It's a little vague to me, perhaps because I'm also not versed in thermodynamics, why the whole universe would seem to be increasing in time from our perspective.

John Rennie

@person27 the last section is vague because no-one really knows why time flows, or at least appears to flow.

I suspect most physicists believe that time doesn't really flow and it's just the way that human brains work that gives us the perception that time is flowing.

The idea that time doesn't flow is known as eternalism or as the block universe:

Eternalism (philosophy of time)

Eternalism is a philosophical approach to the ontological nature of time, which takes the view that all existence in time is equally real, as opposed to presentism or the growing block universe theory of time, in which at least the future is not the same as any other time. Some forms of eternalism give time a similar ontology to that of space, as a dimension, with different times being as real as different places, and future events are "already there" in the same sense other places are already there, and that there is no objective flow of time. It is sometimes referred to as the "block time" or...

person27

That statement that time isn't flowing and only seems to be from our perspective makes it unclear why I can walk through an open door, since it would imply I'm walking through a door that's both closed and opened (plenty of times, depending on use)

John Rennie

If we treat you as a point then you have a worldline that starts at the time and place of your birth and ends at the time and birth of your death. The world line passes through all points in spacetime that you have occupied.

person27

I did notice your careful distinction of consciousness -- I should've known I'd be getting a philosophy link at some point. I understand what you said, but unless you're writing more, that doesn't explain it yet. [Now reading this link]

John Rennie

Similarly take some point in the door and draw its worldline. That worldline starts at the time and place the door was made and ends at the time and place the door is destroyed.

person27

07:21

Yeah I'd never intersect the door because I'd use a better model and record the positions of the parts of the door -- I'd get really close when I touch the door, but I wouldn't intersect if my model is sufficiently detailed

John Rennie

For you to walk through the door, as opposed to walking into the door we just need those two world lines not to interest i.e. you and the door never occupy the space point in spacetime.

person27

Yeah, what I wrote above.

Oh geez, eternalism is an explanation for the eternal universe or eternal deduction from when I took a course on philosophy. Neat, now I can wrap my head around something being eternal because for some reason I didn't think about that aspect of spacetime until now

Ok. I think I finally have all these questions about relativity at the conceptual level answered. I'm sure there are other realizations I'll make later

Thanks

John Rennie

You're welcome :-)

Secret

07:43

user image

:50928391

Basically for each redox reaction in equlibrium, the ratio between the activities of the reductant and oxidant dictates how much the Gibbs energy differs between both ends

It is true that for each reaction (say Zn/Zn2+ and Cu/Cu2+ pairs), the standard potentials for the chemical species were fixed. However the chemical reaction will lead to an imbalance in charges at both ends, so the activity of the oxidant at one end is higher than those in the other end. Now if you disturb this equlibrium by putting another Zn/Zn2+ and Cu/Cu2+ pairs at both ends, it is clear that the concentrations of the oxidants and reductants are already very different from those without

any redox reaction present, and thus on the positive end, you started with lots of positive charge carrying species which is polarised further by the electric potential set up by the Zn/Zn2+ and Cu/Cu2+ pairs and similarly for the -ve side. Thus the effect overall is additive and this lead to an overall increase in the potential difference across the series

Mathematically, one can demonstrate that by adding multiple nernst equations together. Note how ln (xyz...)=ln(x)+ln(y)+ln(z)+... and that gibbs energy are additive, thus you end up with each individual difference in the activities at the anode and cathode, adds up together to a huge potential difference

Captain Bohemian

08:07

@vzn do you live in New York? I don't know those terms, like concrete jungle, metropolis, is specifically used to refer to New York. I mainly use it to refer to my suffocating ennui-ridden home city. I don't know what New York is like, but in the home city and Beijing, there are places in a commercial building called New York-New York; I don't know if that's what New York is like.

user image

the landmark in this concrete jungle where New York-New York is around.

in this commercial building everything is exorbitant.

Emilio Pisanty

09:19

@JohnRennie the technical term is 'uninteresting', then?

skillpatrol

nonintersecting

Jake Rose

@NovaliumCompany American immigration policy is a beast of its own. Getting working visas isn’t as simple as just cleaning floors. You’re more likely to get one if you’re a skilled worker, or have a useful degree of some sort.

skillpatrol

also, country of origin is playing a larger role with the trump admin

Jake Rose

10:26

Mhmm, not sure what the opinion is towards Bulgaria

Right now even id probably struggle

skillpatrol

unfortunately, the trade wars are going to make it worse :(

Emilio Pisanty

10:44

Bam, 25k in offered bounties

2

skillpatrol

niiice

Emilio Pisanty

Meanwhile @JohnRennie continues to procrastinate =P

skillpatrol

procrastination is the mother of conservation :P

John Rennie

11:14

@EmilioPisanty ah, yes :-)

One of the few benefits of being a mod would be the ability to edit my chat posts after two minutes :-)

skillpatrol

@JohnRennie are you planning on watching any of the world cup of rugby?

PM 2Ring

11:32

Does this count as non-mainstream physics.stackexchange.com/questions/490072/… ? Or is it just a wobbly tower of misunderstanding and confusion?

skillpatrol

it appears that Calculus Humor wasn't so funny after all

Novalium Company

11:58

I'll just go to Mr. Trump and ask him if I can join his little country. I'm sure he wouldn't mind.

skillpatrol

sure, you can work in one of his casinos :P

Novalium Company

:D i'll collect the jackpots

skillpatrol

oops, i think you have to be 21

Novalium Company

:(

skillpatrol

no big deal, by then you should have your BSc

Novalium Company

12:09

BSc?

skillpatrol

Bachelor of Science

A Bachelor of Science (Latin Baccalaureus Scientiae, B.S., BS, B.Sc., BSc, or B.Sc; or, less commonly, S.B., SB, or Sc.B., from the equivalent Latin Scientiae Baccalaureus) is an undergraduate academic degree awarded for completed courses that generally last three to five years, or a person holding such a degree.The first university to admit a student to the degree of Bachelor of Science was the University of London in 1860. Prior to this, science subjects were included in the BA bracket, notably in the cases of mathematics, physics, physiology and botany.Whether a student of a particular subject...

Novalium Company

If you dont know already, I'm very negatively tuned towards universes and colleges, and generally the schooling system.

I am NOT going to college.

skillpatrol

:(

Novalium Company

:)

So i'll look for another way to the USA except working in Trumps casinos and colleges

PM 2Ring

12:31

@NovaliumCompany When you find it, a lot of Mexicans will be interested in learning your method. ;)

skillpatrol

-__-

please, people of color are suffering the most under trump

both NBA finalists refused to visit the white house

PM 2Ring

I'm not sure how that relates to the US immigration policy.

skillpatrol

2 hours ago, by skillpatrol

unfortunately, the trade wars are going to make it worse :(

that^ applies to the US immigration policy

Novalium Company

@PM2Ring I'll make sure to inform them for the little fee of a few thousand dollars.

AccidentalFourierTransform

13:17

user image

2

someone was salty for some reason

anyway, will the dv on math.of be reversed as well?

or is the serial dv script site specific?

bc if so, a single dv is not going to trigger it

inb4: Stack Exchange is Cancer is getting a suspension

I guess mods already know about this anyway

skillpatrol

wow

AccidentalFourierTransform

@EmilioPisanty data.stackexchange.com/physics/query/49794/… :D

skillpatrol

niiice

Jake Rose

@NovaliumCompany any ideas what you’ll be doing instead?

Emilio Pisanty

@AccidentalFourierTransform lolz

@PM2Ring looks like the latter to me

@PM2Ring that's rather past the line.

skillpatrol

13:45

"neutrality" is a word you rarely hear during trade or any kind of war

When the two biggest economies in the world have a trade war; everybody suffers.

cool tweet

PM 2Ring

14:21

@EmilioPisanty Sorry, I didn't intend to offend anyone. I was just trying to point out to Novalium Company that the US immigration policy is rather strict, especially towards people without higher education.

X 47 48 - IR

14:55

I've read the current makes the magnetic field which causes motors to spin. But why some powerful motors need high voltage instead of current? What does high voltage do with RPM?

Jake Rose

@X4748-IR I don’t know what you are specifically talking about. But you use high voltage to give a high power output

@X4748-IR I don’t know what you are specifically talking about. But you use high voltage to give a high power output

dmckee

@X4748-IR THe power of the motor comes at the expense of power in the circuit, which is $P = IV = \frac{1}{2} I^2 R = \frac{1}{2} V^2/R$.

I included the variation to make it clear that you are going to have to engineer the system with a resistance in mind.

The resistance comes from the wires making up the coil, and while you have some choice in designing that once you have it, your have it.

Want more power? You have to increase either the current or the voltage. But the coil is a plain ohmic resistor, so they go up or down together.

To break that relation ship you re-design and re-build.

skillpatrol

15:14

\o @danielunderwood

danielunderwood

o/

skillpatrol

how are you?

danielunderwood

Pretty great considering it's the middle of a 4-day weekend!

skillpatrol

coolio

1010011010

15:33

Hi, I found an article in which the notation "det_n(.)" is used

As in, subscript n

What does this mean?

I've never seen this notation before

Does it mean that I'm considering an $n\times n$ matrix?

user image

JMac

16:16

I need to stop looking at the suggested edits queue... it just frustrates me. Just saw a proposed edit from a very common "mass editor" where they changed "though" to "through" when the poster clearly meant "though"; and then changed a bunch of british spellings to american ones, as if that fixed anything

bolbteppa

16:38

$R_{a[bcd]} = 0$ is true, is $R^a \,_{[bcd]} = 0$ true?

ACuriousMind

@bolbteppa Uh, how could just raising/lowering an index ever change a zero value to a non-zero value?

bolbteppa

Yeah

It's just never stated the latter way which looks better to my eyes and I never checked it and maybe there's some chicanery with the fact $g$ doesn't commute with $\partial$ but nah

Sir Cumference

17:00

@ACuriousMind Do mods see retracted flags?

ACuriousMind

@SirCumference I think we can see them in the timeline of the flagged post, but they don't show up in the usual flag places - what would be the point of retracting if they did?

Novalium Company

@JakeRose Yes, I have. Dangerous, risky ideas with a high failure rate, but it's what I've always wanted to do. I'm not going to get into to topic of what it is and start a long discussion with you telling me how wrong I am.

I mean, I have a few ideas, some risky, some not, but I prefer to keep them to myself. None of them includes college.

Sir Cumference

17:37

@NovaliumCompany Well, all I can say is just have a plan B

Chasing your dreams is great, but only if you have a backup

Though tbh I'd expect you'd enjoy college a lot, since you seem to like learning. It's also quite different from high school

bolbteppa

arXiv Vanity renders academic papers from arXiv as responsive web pages so you don’t have to squint at a PDF hmm

Novalium Company

18:00

@SirCumference I've done my research on what college is like and my opinion is pretty hard stoned. I doubt anyone can change my mind but I'm keeping an open mind.

@SirCumference What do you mean a backup plan? I'm not scared of failure.

ACuriousMind

@bolbteppa I don't get it. When does one "squint" at a PDF? IF the font is too small for you, you can just zoom in!

dmckee

@ACuriousMind Old people—like me—tend to use PDFs as a way of killing trees. Then if the typeface is too small and you forgot your reading glasses (or don't wear them due to vanity or simple failure to go get a prescription for progressives) you squint.

At least being near-signed and can bring the page close.

Sir Cumference

@NovaliumCompany There's no need to fear failure (i.e. you don't need to give up because of the risk), but there's no reason to accept it either when you can take steps to avoid it. My advice is to have a plan for where to go in case things don't work out, even if you are dead certain right now on what you want to do.

Determination is good, but putting your eggs into a single basket can be careless. :/

Novalium Company

18:30

@SirCumference Yep, I understand and agree and I said I have a few options, some risky, some not. So I have backup plans.

Ryan Unger

just go to college

it's not that hard

Novalium Company

19:09

lol no

@RyanUnger For me, you telling me to go to college is like me telling you not to go.

Ryan Unger

what?

1 hour later…

Sir Cumference

20:20

@AccidentalFourierTransform Oh hey, you're back!

Novalium Company

21:03

@RyanUnger I am confusion

@RyanUnger It's actually hard for someone who doesn't like college. (for many reasons, dont ask)

Ryan Unger

we've had this conversation before and I was just as perplexed then

you seem to have very strange ideas about how the world works

Novalium Company

ok

1 hour later…

dmckee

22:09

In carreer as a professor I met many people in the thirties and forties who were either coming back after struggling with college in in their youth or attending for the first time.

Most of them faced more challenges then the "traditional" students, but almost without exception they were doing what it took to succeed.

So you can put it off, but be aware of what it will cost you in convenience (once you have a family there will be more demands on your time), and be sure to start soon enough for your goals: it can take longer than the traditional four (or five) years.

I tell high scholers who are wondering if they should go to college, that even if they don't want to "go to college", they should start on a one-class-per-semester basis.

Because that is the ten-to-twelve-year-plan which is a similar scale to when they will need either the credential to join management or some business training to start their own business as a skilled tradesman or contractor.

Ryan Unger

@dmckee The person I was talking to has proposed a plan in which he mops floors at a national lab or research corportation and absorbs knowledge that way.

I'm not sure if he's actually serious.

dmckee

I hope not. That way leads to a long and secure career of cleaning things.

Jake Rose

@NovaliumCompany you’re in a chat room with some very intelligent people (I’m not saying I’m one of them, but I do believe I’m above average). You yourself said your ideas are unlikely to succeed so I doubt I’d tell you you were wrong. And despite what a lot of motivational speakers say (e.g. don’t let people change your mind) listening to other people’s opinions on why you might be wrong, and perhaps heeding advice on other options you have might be useful

Your opinions should very rarely be set in stone. They should be something you carry with you, but can trade for a better one when it comes a long

You’re also in a room filled with people who have gone to university. And none of us would try to convince you to go if it didn’t seem like it would be a beneficial path. We’re not university enrolment teams ;)

@RyanUnger have you never seen good will hunting? It’s how all the geniuses are made these days.

2 hours later…

Ryan Unger

23:58

@JakeRose I haven't

I know generally what it's about

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