The h Bar

1:51 AM

@ChrisWhite Well that's pretty impressive

3:35 AM

@ACuriousMind true, although it is technically very easy to get rid of if the community decided to do so (i.e. there's a mod tool to remove tags)

At least there used to be. I guess they could have removed it and I wouldn't know, seeing as how I haven't tried to remove a tag in ages

Omen

good day

Physics Meta

11:23 AM

0

Q: Why is Physics SE so bad at sorting answers?

For instance take a look at this question: What technology can result from such expensive experiment as undertaken in CERN? Currently the top most answer has just 2 upvotes while a couple of 30-odd upvoted answers and even a 50 upvote answer is simply hidden away at the bottom. I'm finding this...

CoilKid

2:42 PM

Hey, anyone acive?
*Active
I have a question about the force exerted by an electromagnet (solenoid).
Basically, where do I get g from in F=(NI)^2 * mu_0 * A / 2g^2?
I know it is the distance between the electromagnet, but is it the shortest distance possible between the coils and the iron? Is it the farthest possible distance? Or is it the average distance away (middle of the iron)?

And to respond to the last comment on this, I did. But it has gotten no answers

I was thinking the electromagnet would act on the leading edge of the ferromagnetic rod, but the electromagnet produces a field which would act on the projectile as a whole (with different points along the projectile experiencing different amounts of force because of differing distance from the coils.)

soumyadeep

Sorry i do not know much about electromagnet.:(

CoilKid

If that is true, wouldn't I sum the forces to get the overall force acting on the projectile as a whole?

soumyadeep

May be you should post the question in the main site.

CoilKid

Someone said I should pose a question rather than ask in chat. I think I should say that I did. 49 views, no answers or comments.

Kyle Kanos

@CoilKid Presumably your coils are all equidistant from the iron bar....

CoilKid

2:49 PM

Yes, but that only acts on one "slice" if you will of the bar

The force will be different on the front of the bar then the back because they are different distances away from the coils

If the bar is 5mm long, and the distance between the front of the bar and coils is X, then I get two different distances to put in my equation: X, X+5mm

Can make a big difference considering the fore equation uses the square of the distance

The only thing I can think of is to calculate the force for every point along the bar and average.

I know that electromagnets are used extensively, and I have to assume that someone has already thought of this and the industry has a rule of thumb for this.

Kyle Kanos

Ummmm....I'm pretty sure the force calculation is assuming that the bar goes into the solenoid

CoilKid

Yup

That

That's the idea

Kyle Kanos

So you want to stick a bar into a solenoid sideways?

CoilKid

no?

Here.. one sec

Kyle Kanos

like, O=====O <- ||

or , O=====O <- =

CoilKid

2:54 PM

Like O===O =

Kyle Kanos

Then what's the problem?

g is the distance between o and =

Presumably the o is equidistant from the =

CoilKid

Yes. But I have to consider the length of =

Kyle Kanos

Surface-to-surface distance, if I'm not mistaken

CoilKid

Right?

Kyle Kanos

@CoilKid There's no L in the force equation....

CoilKid

2:55 PM

I have a picture, but I cant remember how to link them in chat

Kyle Kanos

Just drop the image URL into it

CoilKid

Hmm

Okay

5bbbafd3-a-62cb3a1a-s-sites.googlegroups.com/site/…

A in my equation will be acting on plane b

But the bar has a length

soumyadeep

@KyleKanos Sorry to disturb but i wanted to ask-is kirchoff's law only applicable in conservative force fields?

Kyle Kanos

@soumyadeep Only Kirchoff law's I know of are the circuital laws & there aren't any forces there, so I'm not sure what you mean

soumyadeep

I mean the battery establishes electric fields,right?

Kyle Kanos

3:00 PM

@soumyadeep Battery establishes current flow...

CoilKid

What I mean, is that for every point along the bar, the force will be different. 5bbbafd3-a-62cb3a1a-s-sites.googlegroups.com/site/…

For line 1, the force is 10N but a little farther down the length of the rod, the force is 6N and so on.

soumyadeep

@Kyle Kanos Yes but the current i.e the electrons flow due to a field established by the battery.

Kyle Kanos

@soumyadeep And that plays no role in the Kirchoff current/voltage laws....

CoilKid

Since an electromagnet produces a field, every point on the rod inside the magnetic field will have a force acting on it. Right? That must mean that it would work off of volume, not area. The equation uses area not volume so I'm not sure what to think.

Am I supposed to use the distance to the leading edge?

Kyle Kanos

@CoilKid I presume then that you'd have to do an integral over the position of the magnetic field

CoilKid

3:05 PM

It doesn't use volume though:/

Area along the bar at every point is volume isn't it?

So that can't be right

Kyle Kanos

What can't be right?

And what doesn't use volume?

CoilKid

The force equation uses area, not volume

so I don't think I should do area at every point and average, or something like that

Kyle Kanos

g is measured in length, so A/g^2 is unitless

CoilKid

Yup

Units are arbitrary

Kyle Kanos

So if you use a volume, it'd be V/g^3

CoilKid

3:07 PM

Can't change the g though... That would mess something up, I think

soumyadeep

@Kyle Kanos Well the text book says that as the Field is conservative and the electrons coming out of the battery goes into it ,the work done is 0.So the sum of potential drop and rise is zero-which is the second law.

Kyle Kanos

@soumyadeep If that's the case, then it seems you have your answer. I've not had a circuits course in about a decade

CoilKid

Volume would be more accurate though I think. I'm just not sure if I can without messing up my equation

*if I can use it

soumyadeep

@KyleKanos What i am asking is whether the text book is correct.I am reading preliminary physics...

tpg2114

I've got a question that maybe can be answered here (if not, I'll head over to Academia.SE) -- is it okay to re-use entire chapters from a thesis proposal when writing the actual thesis? Is a proposal a stand-alone document that should not be self-plagarized, or is it just like a first draft of the thesis?

Kyle Kanos

3:13 PM

@tpg2114 I've copied a good portion of mine, but I'm not entirely sure

Is your committee going to remember it?

But I've gotta head out to class now

tpg2114

Yes, probably

CoilKid

@KyleKanos I believe what I'm getting at is a more fundamental question: The force equation I have now, using A and g^2 would only act on an object without depth. If I wanted to get the force for a three-dimensional object, I would use Volume and g^3?

soumyadeep

@KyleKanos Before leaving can you pls tell if the txtbk is right?

@CoilKid May be you can help too.

@Kyle Hello.

Kyle Kanos

4:06 PM

@CoilKid I think your interpretation is wrong. If you assume azimuthal symmetry (the case for cylinders), you can neglect a dimension

Physics Meta

0

Q: No marked answer when user found it usefull

I have seen several questions, already inactive, for which an answer was given which was useful to the user asking, but not marked as the answer. It was easy to see they were useful from the comments and flags from the user asking. But since they seem long time inactive, I wonder what happens wit...

support accepted-answer users

user54412

5:08 PM

So we already had physics.stackexchange.com/questions/136106/… and now there's physics.stackexchange.com/questions/136293/…

user54412

the existence of the second question makes me suspect the first was a cleverly disguised homework problem

user54412

too clever, really, because the answers are more realistic than the problem meant to be solved

3

user54412

5:55 PM

3

Q: Nuclear Blast in Space

It is possible to detonate a nuclear bomb in space, if possible then what will happen? In Armageddon movie we watched that, after discovering that an asteroid the size of Texas is going to impact Earth in less than a month, NASA recruits a team of deep core drillers to drill and blast a nuclear w...

nuclear

user54412

okay this is getting ridiculous

user54412

was there some big asteroid-hitting-earth movie screening somewhere?

user54412

was this shown to a class of HS students who were then instructed to ask the internet about it?

Kyle Kanos

Yeah...that is pretty ridiculous!

Danu

@ChrisWhite The 11 km/s really seals the deal on this one

Faheem Mitha

7:46 PM

I actually enjoyed Armageddon. Bruce Willis saves the world. And who watches Hollywood films expecting them to be good? Or (even less likely) scientifically accurate?

Actually, it's ironical how bad the science usually is, since the average HW movie could get advice from the scientist for the HW equivalent of petty cash.

user54412

11:24 PM

@FaheemMitha My quantum mechanics prof was consulted for I think Iron Man 2 - the one where he builds his own accelerator and synthesizes a new element. The science is just as ridiculous as ever. I think the consulting is just to get ideas they can run with, rather than to be realistic (which would never sell tickets).

user54412

And if you've heard of Interstellar its plot promises to be equally absurd, and there Kip Thorne is not only a consultant but executive producer.

zounds

got a silly beginner's question

say you have a 2d board with a bunch of pins placed randomly, and each pin has a rubber band attached on one end. the other end of the rubber bands are attached to a single small ball.

eh wait never mind

Ashley Davies

I'm reading an introductory Physics textbook for the fun of it, and I'm having proper bad trouble remembering formula derivations from page to page after not reading it for a while; is this a normal thing? If so, anyone have any tips? If not, would it be a bad idea to pursue Physics further?

Like when I'm reading it I'm like "Oh, that makes sense. I can do that, it's easy." and then 10 minutes later it's just gone :c

user54412

@AshleyDavies If you enjoy physics, you should definitely pursue it! Don't let one bad experience with a book get in the way.

user54412

Actually, there are two points: (1) not every book is good for everyone, and (2) not every way of reading a textbook is a good idea

Ashley Davies

11:40 PM

It doesn't really show many steps in derivations (I think this might be a standard undergrad thing, I'm used to high school textbooks), which is helpful since I can remember them better having focussed on finding out how they derived it, but I'm a bit worried if I can't keep to memory a couple of basic mechanics equation derivations (I can remember the equations fine from long months of applying them) then it's going to be a huge pain when there's a whole lot more is all.

user54412

For the first point, I really recommend trying a bunch of different books. No need to read one cover to cover if it's not working for you. If you want suggestions for good intro books on particular topics you can ask here in chat and we might have suggestions.

user54412

This gets to my second point - I think you might be too focused on the details of the derivation.

user54412

A lot of students starting out in physics get caught up in the formulas, probably because intro textbooks are written in the "here's a formula, now plug in some numbers" style

user54412

But physics is really about concepts

Ashley Davies

Yeah, it's really hard to adjust to switching from remembering hundreds of formulas for something to figuring them out on the spot

user54412

11:43 PM

As you're reading the derivation, ask if the steps make physical sense, not just if the symbol manipulation looks good. And at the end ask yourself if the result agrees with your intuition.

user54412

At first not everything will make sense, but as you've been exposed to more and more, certain patterns and useful techniques will emerge and stick with you.

Ashley Davies

So you think it's worth sticking with it then?

user54412

definitely

user54412

also, remember there are multiple levels to knowing a formula

user54412

the most basic is knowing what variables are involved, i.e. what quantities does the physics depend on and what don't matter

user54412

11:46 PM

then you can ask how the thing scales with those quantities: does temperature increase or decrease with pressure at constant density? does potential energy fall off like a power of radius or like a logarithm?

user54412

the last level is knowing the exact numbers, and I'd argue this is the least important

Ashley Davies

That's quite good advice; thanks! :)

user54412

Also, since your profile says you're in high school, I'll point out that pretty much every subject in high school is very different from what's taught in college.

zounds

is lagrangian mechanics something that was just observed to be true empirically or is it something that can be proved from newtonian mechanics with force laws?

user54412

Mathematics is nothing like doing trig problems, physics is not just blindly applying formulas, computer science is not really about coding

5

Ashley Davies

11:48 PM

Yeah, I'm reading ahead to try to see what I want to study for definite

It's kinda frustrating that our A-Levels (AFAIK, AP classes equivalents) are pretty much just using like 50 given formulas and subbing them into eachother and memorising exam board's definitions

user54412

good idea, but I also recommend not limiting yourself until you've sampled the subject in person once at university - attend some lectures, talk with older students, get a feel for what things are really like

Ashley Davies

Yeah, I really like the way the US college system does that; over here we pick our major in senior year of HS and pretty much have to stick with it (Although some places let you switch between similar fields if you do it early enough)

user54412

@zounds It is perfectly equivalent to Newtonian mechanics (given that you're dealing with a certain class of appropriate problems - one keyword is "holonomic constraints"), as well as to a third formulation, Hamiltonian Mechanics

user54412

any basic analytic mechanics textbook should have a proof

user54412

@AshleyDavies Ah yes, I forgot how the UK does that. Well, at the very least try reading through more than just one textbook. And since you're trying to get a feel for the subject, don't read it to memorize.

Ashley Davies

11:54 PM

Yeah; it's a bit of a shame :( I'm applying to some of the US colleges that offer international financial aid but it's too competitive to get my hopes up much I guess

Are there any intro textbooks you'd recommend that aren't highly expensive?

user54412

Alas, I never actually had any intro physics book that I liked... which isn't to say there isn't one.

user54412

As for expense, I feel like in this day and age that ideally shouldn't be a problem

user54412

Perhaps a nearby university library?

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