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/sci/ - Science & Math

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6041268 No.6041268[DELETED]  [Reply] [Original]

Answering physics questions.

>> No.6041272

Can you know nothin?

>> No.6042412

I'm studying physics right now and have to write 3 exams in the next 3 weeks, I know nothing because I'm a lazy ass. Can I learn Analysis 1 in under a week? or should I use this week for another examn

>> No.6042417

What is isospin?

>> No.6042423

>>6041268
first:
Whats the best route to studying quantum einstien gravity? can I go straight from FQT and GR to it? (with some review of the renormalization group)?

second:
what are the applications of studying wave equations (Maxwell + Schrodinger) in fractional dimensions? ive done the derivation of equation in 1.58 dimensions and so on, and it seems to be used in material science but im not sure.

>> No.6042437

I guess hes not here anymore, cause it already has passe 14 hours

>> No.6042441

>>6042437
everyone on sci is here ~3.429

>> No.6042450

>>6042441
I'm here 31

>> No.6042456

>>6041268
Still here?

>> No.6042458

I'm a 22-year old felon who's failed college 4 years in a row.

I still want to be a physicist. I want to go back to school next fall, but haven't really made plans yet. I went to university for one year after high school, failed one semester and withdrew the next before being kicked out of the dorms for smoking pot. The next 3 years I attended a local community college, each year failing or withdrawing due to legal or personal issues. I've completed a handful of courses, but the failed/incomplete courses decimate my GPA beyond recovery.

What I want to know:

1. Is it even realistic to seek a career in physics with a felony (Terroristic Threats) and various misdemeanors (possession of pot, disturbing the peace, and willful reckless driving, among others) on my record?

2. If so, and given that I feel I've finally acquired the discipline and maturity to dedicate myself to academics, what steps do I take to go back to college? I'm primarily worried about my academic record, and if there's a way to "start over". I'm not interested in keeping any of the credits I've earned because my GPA is abysmal.

>> No.6042460

>>6041268
How the fuck do Josephson junctions work?

>> No.6042465
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6042465

How would you, in detail, describe the difference between the diffusion equation and the fokker-planck equation?

>> No.6042512

>>6042458

>Step 1: Don't ask the internet for advice.
>Step 2: ????
>Step 3: Profit

>> No.6042516

>>6042465
it's simple, we kill the batman

>> No.6043257

>>6041268
whats 1/3 in decimal

>> No.6043267

>>6043257
1.3

>> No.6043273

>>6042458
Mind telling us how the fuck did you get get a felony for terroristic threats?

This sounds entertaining..

>> No.6043279

how do you determine surface charge density for a sphere using gauss' method?

>> No.6043653

>>6042412
That really depends how quick you are at the stuff, and how experienced you are at self-learning. Sorry I can't give a better answer, but it's not really a physics question and depends entirely on your own skills and motivation.

>>6042417
Isospin isn't really a physical quantity, it's a dimensionless quantum number. It is used to wrap up the effects of changing the quark content between up and down, charm and strange, or top and bottom into a quantum number that basically says how many positively and negatively charged quarks make up that particle. So two similarly structure baryons (e.g. a proton and a neutron) can be described as two differently charged isospin states of the same particle. Isospin's use was motivated by the similarity in mass between protons and neutrons, and is somewhat deprecated now that we have a better understanding of quark dynamics.

>>6042423
Yes, if you study up on QFT (in which you will thoroughly cover renormalization) and GR, you'll be able to start reading introductory quantum gravity texts and some of the core papers on the subject. I'm not sure about fractional dimensions, never really looked into it.

>>6042458
The way employers and public services in the US treat felons is probably the most shameful thing about this country. Luckily for you, science is very much a meritocracy and you can get quite far without people judging you for anything other than your accomplishments. The best thing to do is start fresh at a community college that allows guaranteed admission to a nearby state school and make sure your grades are adequate to be brought on. From there, it's standard undergrad advice: do research in a lab, figure out how to self-learn, connect with professors, etc.

>> No.6043655

>>6041268
What's a good introduction to Term Symbols?
Fuck, I'll look on youtube.

>> No.6043667

>>6042460
They work because of tunneling, which is a quantum mechanical effect allowing particles to enter classically forbidden regions. It's interesting because it effectively allows superconduction through non-superconducting media. The fact that you are asking about Josephson junctions makes me suspect you already are familiar with tunneling; if I am mistaken let me know.

>>6042465
Physically, Fokker-Planck describes the time evolution of the PDF of the velocity of a particle undergoing Brownian motion. The diffusion equation describes the same, except for position instead of velocity. Mathematically, Fokker-Planck includes a term for drift in addition to diffusion.

>>6043257
0.3 repeating.

>>6043279
What are you given? The sphere's charge? The sphere's radius? The field at a point outside the sphere? The field just outside the surface of the sphere?

>> No.6043669

>>6043653
>>6043667
Sorry, these were me, forgot my trip.

>>6043655
Why would you ever want to learn term symbols? That's one of those things that you just pick up if you are a physical chemist or experimental particle physicist or similar and have to use those every day. It's not something you need to take a class on, really.

>> No.6043762

>>6041268
Why does the z-projection of the angular momentum commute with the Hamiltonian but the other projections do not?

>> No.6044654

Why don't I have any problems with mechanics whatsoever, but as soon as rotations come into play I am completely lost? I can imagine angular velocity, acceleration and moment of inertia, but everything else is just eldritch space magic to me.

>> No.6044666

What is an amplituhedron?

>> No.6044770

>>6042465
>Physically, Fokker-Planck describes the time evolution of the PDF of the velocity of a particle undergoing Brownian motion. The diffusion equation describes the same, except for position instead of velocity. Mathematically, Fokker-Planck includes a term for drift in addition to diffusion.

That's not detail, that's equivalent to a line from Wikipedia. I was more thinking of examples for the diffusion coefficient, Einstein relation, Wiener processes.
But thanks anyway.

>> No.6044817
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6044817

just a random thermodynamics question i was thinking about the other day.


Would stirring sugar into hot coffee cool the coffee or heat it up?

the enthalpy of the sugar mixing with the coffee being negligible. just focused on the energy from stirring. I guess the metal spoon absorbing heat will be negligible also

>does the energy from stirring coffee heat or cool the coffee

room temp being like 25C @ 1atm

>> No.6044838

>>6044817
stirring is heating but even more negligable than spoon heat absorption

>> No.6044882

Neglecting everything else, stirring will heat the liquid (You're adding energy). Here are as many of the things you neglected that I can think of, and how they would effect the heat transfer (if everything else is neglected):

Heat absorption by spoon: Would cool the coffee
Enthalpy of mixing from sugar: Would heat the coffee
Increased rate of convection from stirring motion: Would cool the coffee.
Increased rate of conduction through the cup walls due to stirring: Would cool the coffee

I think that in practice the coffee will cool (The convection and conduction terms being dominant).

>> No.6044894

>>6044882

yeah my main focus was the energy from the stirring + absorption of heat through the metal spoon compared to the convection from stirring.

wasn't sure which would dominate each other

>> No.6044913
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6044913

there's a rope (of length l) on two inclined surfaces, both at the same angle. the coefficient of friction is 1; whats the optimum angle which results in the longest length of the rope not in contact with the surface?
how would you start solving this?

>> No.6044934

I have a pleb-tier QM question. What exactly is a wave function? It seems it doesn't have a real meaning until you square it to give you a probability. My professor says it contains everything you need to know about the particle, but is there more to it? Does it represent anything by itself?

>> No.6044973

>>6044913
Is the answer known? I got 67.5 degrees but I'm not sure if this is correct.

>> No.6044986

>>6044973
its more about the method not the answer m8
what was your method to solve it?

>> No.6045003

The answer to a question in my physics text was some amount of potential energy, but it was negative. Can you really have negative energy, or negative potential energy?

>> No.6045011

>>6045003
Gravitational potential energy is negative by convention.

>> No.6045105

>>6043762
It only commutes with the hamiltonian if the z eigenstates are also eigenstates of the hamiltonian- e.g., if there is a magnetic field in the z-direction.

>>6044666
Some arcane particle theory artifact. I'm not a particle theorist, and this a pretty niche subject.

>>6044770
Sorry, I am answering as many as I can. I like to start general, because I can't tell at what level the asker will be able to comprehend a given reply. Do you have specific questions? I can't really write a paper for you on the subject here.

>>6044817
Stirring greatly speeds up evaporation, so yes, it accelerates cooling a lot if there is any significant difference between the liquid and ambient temperatures. This is well-established, try it yourself with a mug and a thermometer.

>>6044913
If the coefficient of friction is one and the rope is perfectly flexible, it will always slide in until the entire rope is in contact with the slopes and making a right angle.

>>6044934
A wave function IS the particle. It contains all the information about the state of the particle. As you say, by taking its inner product with some other state, you find the probability that a measurement of the system will yield that state.

>>6045003
Negative potential energy makes perfect sense- in fact, only differences in potential energy matter. If you have a hill going from 0 to 10 J, that's exactly the same as a hill going from -1000000 to -999990 J.

>> No.6045130

>>6045105
Wait, never mind re: the rope problem. Let me puzzle on it for a bit and I'll get back to you.

>> No.6045142

>>6041268
>Answering physics questions.
Lets say we have a neutron star that's a few kilograms short of becoming a black hole.
If we start to spin it, would its rotational energy(converted into mass via E=mc^2) then suddenly turn it into a black hole?

>> No.6045167

>>6045130
>>6044913
So the problem is that the coefficient of friction depends on the area of contact. What you really need to define is a frictional coefficient per unit length, which I'll call <span class="math">\mu[/spoiler].

A hanging rope takes the shape of the function <span class="math">\cosh(x)[/spoiler]. The angle of the rope at any point along the curve is <span class="math"> \theta = \tan^{-1} (\frac{dy}{dx}) = \tan^{-1}(\sinh(x))[/spoiler]. We can tell where that hits the slopes by setting the angles equal, which gives us <span class="math">x_0 = \pm \sinh^{-1}(tan(\theta))[/spoiler]. Now we need to figure out how long the hanging piece of rope is. Our total hanging length, <span class="math">L_h[/spoiler], can be calculated by integrating its length elements along its path:

<span class="math"> L_h = \int_{-x_0}^{+x_0} \sqrt{1 + (\frac{dy}{dx})^2} dx [/spoiler]

To be continued...

>>6045142
Yes, it would. You've increased the energy density, and therefore the curvature of spacetime, within the event horizon of the object, so it could very well become a black hole.

>> No.6045170

>>6041268

Why are all these subatomic particles separated and categorized and act so differently? Why don't we say they are all the same thing, in different sizes and strengths? Why doesn't Occam's Razor apply to science? Why is it all one big convoluted, complex mess? Why does it have to be so complicated that only the ultra-smart can "figure it all out"? Is the reason why we persist in believing it is so complicated because it makes one feel like they're a part of an exclusive club if they attain a deeper understanding of it?

Thanks.

>> No.6045173

>>6045167
>>6044913
We can go ahead and calculate the tension on the rope now using a simple force-body diagram, which turns out to be, at the ends, <span class="math"> T = \frac{L_h}{L} \frac{m}{2 \sin \theta}[/spoiler]. We need to compare this to the amount of frictional force, and make sure that the rope is stable here, putting a hard limit on where we can look for maxima of <span class="math">L_h[/spoiler]. First, we multiply the remaining length, <span class="math">\frac{L - L_h}{ 2}[/spoiler], by <span class="math">\mu[/spoiler] to get the total coefficient of friction on the rope. Next, we calculate the normal force using the mass of this piece of rope, <span class="math">\frac{L - L_h}{ 2 L} \frac{m}{L}[/spoiler]. Our result is

<span class="math"> F_N = \frac{L - L_h}{ 2} \frac{m}{L} g \cos \theta[/spoiler]

<span class="math"> F_f = \frac{m g \cos \theta \; (L - L_h)^2} {2 L} [/spoiler]

ctd...

>> No.6045175

>>6045167
>Yes, it would.
Does this apply if we accelerate it linearly too?
If yes, wouldn't this mean that there's a true zero-velocity vector in the universe?

Another scenario also: If the object is toroidal and we fire an ultra-intensive laser pulse through the hole(missing the actual toroid) could this also be a catalytic even even though the photons are just passing by? And if so, how could this happen? Wouldn't the light essentially outrun all the force carriers?

>> No.6045179

Could you explain in layman's terms why the wake of anything is independent from mass and speed asides from the speed it takes to get a wake? Also why is it ~19degrees?

>> No.6045180

>>6045173
>>6044913

So now we just maximize L_h (the integral we gave earlier) with respect to theta, which is a highly nonlinear function considering both the limits of integration and the integrand depend on theta, so it should be done computationally. We have a hard limit which can be checked, ensuring that

<span class="math">\cos \theta \sin \theta \geq \frac{L_h}{(L-L_h)^2}[/spoiler].

>> No.6045183

>>6045179

Because it's the best they could do.

>> No.6045190

>>6045170
The degree of simplicity that we've been able to attain in physical theory is actually pretty remarkable. Occams razor completely applies, and scientists regularly discard ideas because they are too complicated or not able to be clearly stated.

>>6045175
No, it just means relativity is right. The location/existence of an event horizon depends on your frame of reference.

Yes, a laser pulse through a toroid could possibly cause a singularity to be created. No, it wouldn't create one if the photons were able to escape.

>>6045179
I don't understand this question. Maybe give an example or draw a picture?

>> No.6045197

>>6045190
http://www.. sendspace. com/file/ubizvv

>> No.6045216

>>6045190
>The location/existence of an event horizon depends on your frame of reference.
So It would appear to have an event horizon when approaching something, but if you accelerate up to speed it actually is just a neutron star zipping by?

>>6045190
>Yes, a laser pulse through a toroid could possibly cause a singularity to be created.
So, ignoring the engineering behind it, a laser pulse(or several focused ones) in freespace could create a black hole too?
Assuming a single beam pulse, would the black hole travel at lightspeed until it have mopped up some mass and slowed down, or would it decelerate?

>> No.6045222

Why the fuck does moving faster make you move faster forward in time compared to everyone else.


I've tried countless times to wrap my head around it. It's the most frustrating thing to know what the concept is, but not why. I've watched videos, read articles, but nothing has helped.

>> No.6045236

>>6045222
>Why the fuck does moving faster make you move faster forward in time compared to everyone else.
frequent location updates and collision detection means there's less clock cycles availible for updating your internal state.

>> No.6045238

>>6045222
Lorentz contraction of the outside world

>> No.6045240
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6045240

>>6045236

>> No.6045247

>>6041268
Is it possible to travel so close to C that you pass through other massive objects due to your much shorter wavelength?

>> No.6046944

>>6045216
Other way around. If something is accelerating away from you, there is a distance from it after which any light leaving it could not reach you.

>>6045216
The black hole would have the momentum (both angular and linear) of the photons that created it. Since the black hole has mass, it would not be moving at light speed.

>>6045222
It certainly is a hard thing to understand. Moving faster actually makes you move forward in time SLOWER than in a stationary reference frame, not faster. The basic reason is that the laws of physics, including optics and electromagnetism, have to work the same no matter how fast you are going- otherwise the universe would have some fixed rest frame, which it does not. This requirement means the speed of light always has to be the same, because if it weren't, you could tell how fast you were moving by looking at how fast light moved, which you can't. With this in mind, you might be able to understand http://en.wikipedia.org/wiki/Time_dilation#Simple_inference_of_time_dilation_due_to_relative_velocity..

>>6045247
This is not a terribly large effect until you start getting to extremely high energies, but wavelength shifts do affect the interaction rates of highly relativistic particles. So sort of, for matter in general, but at such high energies any collisions that do occur would be hugely energetic and instantly fatal.