All Questions

8
votes
2answers
8k views

What is the difference between induced and motional emf?

At least from their names, it seems motional emf is induced, so what's the difference?
2
votes
1answer
3k views

Current from induced emf

If the induced emf in a circuit is negative, and current from this emf is the emf over the resistance, what happens to the negative sign in the induced emf when solving for the current? Surely there's ...
86
votes
6answers
6k views

What is known about the topological structure of spacetime?

General relativity says that spacetime is a Lorentzian 4-manifold $M$ whose metric satisfies Einstein's field equations. I have two questions: What topological restrictions do Einstein's equations ...
14
votes
7answers
839 views

What is an effective way to search for post-doc positions in physics? [closed]

I am a graduate student in experimental physics, currently writing up my dissertation and beginning to apply to post-doc positions. While it is very easy to find out about open positions in my ...
63
votes
7answers
10k views

Why is there no absolute maximum temperature?

If temperature makes particles vibrate faster, and movement is limited by the speed of light, then temperature must be limited as well I would assume. Why there is no limits?
2
votes
4answers
4k views

Are all objects in motion?

It seems like everything in the universe is in motion, at least relative to some other object. That leads me to believe that all objects are in motion. But how do we measure motion when we are ...
8
votes
6answers
9k views

Why doesn't air freeze?

I am in no way experienced in the Physics field so this question may seem a bit silly but i'd appreciate an answer :) Why doesn't air freeze?
5
votes
1answer
1k views

What is the range of validity of Fermi's Golden Rule?

It is well known that to calculate the probability of transition in the scattering processes, as a first approximation, we use the Fermi golden rule. This rule is obtained considering the initial ...
0
votes
2answers
2k views

Electron transitions in an infinite square well

Consider an electron in an infinite square well. The expectation values of momentum and angular momentum are all zero for energy eigenstates. An electron transition is accompanied by the emission or ...
7
votes
6answers
773 views

Interaction ranges in the Standard Model - Electrodynamics vs QCD

as you might know, the Standard Model of physics can be seen as a $U(1)\times SU(2)\times SU(3)$ gauge theory where each symmetry group accounts for different force fields. The behaviour for the ...
7
votes
5answers
9k views

Is there a travelling speed of for electric field? If yes, what is it?

Consider two circuits containing a battery, bulb, switch and conducting wires but of length 1 meter in one case and 1000KM in other. When switched on at the same time both the bulbs glow ...
8
votes
2answers
3k views

Does Tesla's photoelectric “solar cell” really work?

Tesla patented a device for gathering energy from light, using the photoelectric effect. (US 685,957 - Apparatus for the Utilization of Radiant Energy): Basically just a sheet of "highly polished or ...
3
votes
1answer
522 views

What is the pressure between two electric dipole sheets of finite extent?

I have recently become curious about modeling the repulsion of everyday objects in contact with one another. By repulsion I mean as you attempt to walk through a wall, the pain in your nose suddenly ...
1
vote
2answers
277 views

Isotope properties plotting tool?

I'm looking for something that will generate scatter plots comparing different properties of isotopes. Ideally I'd like some web page that lets me select axis and click go but a CSV file with lost of ...
4
votes
1answer
1k views

Phonons, rotons, and maxons

What are phonons, rotons, and maxons, and what does their dispersion curve have to do with superfluidity? I understand that they are quasiparticles, but I'm not entirely sure what that implies. Are ...
1
vote
1answer
165 views

How much can a laser's position be fine-tuned?

Suppose you wanted to do a time-resolved experiment with a molecular beam traveling at, say, 300 m/s involving a mobile excitation (pump) laser that scans across the length of the molecular beam and a ...
14
votes
5answers
9k views

Is crystal momentum really momentum?

Almost every solid state physics textbook says crystal momentum is not really physical momentum. For example, phonons always carry crystal momentum but they do not cause a translation of the sample at ...
35
votes
6answers
33k views

Ice skating, how does it really work?

Okay, some textbooks I came across, and a homework assignment I had to do several years ago, suggested that the reason we can skate on ice is the peculiar $p(T)$-curve of the ice-water boundary. The ...
9
votes
4answers
4k views

Is it possible to accelerate air to supersonic speeds? What would it look like?

The speed of sound is the rate that disturbances in air propagate through it. Is it possible to have a wind that itself is moving at supersonic speeds relative to stationary winds around it? Or ...
3
votes
1answer
382 views

In photo-electric experiment, if the light is exposed to a metal plate for a long enough interval, does the plate become lighter?

I know from the book that electrons will be kicked out from the metal plate if the light of appropriate wavelength is exposed to the metal plate. My mental model says if we let the light expose the ...
19
votes
4answers
1k views

What sort of experiment would directly test time reversal invariance?

I guess the title says it all: how could/would you experimentally test whether our universe is truly time reversal invariant, without relying on the CPT theorem? What experiments have been proposed to ...
19
votes
10answers
7k views

Theoretical Physics - How to? [closed]

Although I doubt somewhat whether this question is really appropriate for this site, I hope it gets answered anyways. I guess, what I'm wondering is: How does one get to work as a theoretical ...
25
votes
13answers
124k views

Why does the (relativistic) mass of an object increase when its speed approaches that of light?

I'm reading Nano: The Essentials by T. Pradeep and I came upon this statement in the section explaining the basics of scanning electron microscopy. However, the equation breaks down when the ...
11
votes
1answer
5k views

The Euler-Lagrange equation in special relativity

How can I derive the Euler-Lagrange equations valid in the field of special relativity? Specifically, consider a scalar field.
41
votes
12answers
4k views

Mechanics around a rail tank wagon

Some time ago I came across a problem which might be of interest to the physics.se, I think. The problem sounds like a homework problem, but I think it is not trivial (i am still thinking about it): ...
6
votes
1answer
322 views

What kind of systems of black holes satisfy the laws of black hole thermodynamics?

I've come across black holes thermodynamics multiple times recently (both at this site and elsewhere) and some things started bugging me. For one thing, first law bothers me a little. It is a ...
2
votes
1answer
137 views

How long does it take a object captured by a star falling to the center?

If the captured object do not have tangential velocity, it's just the free-fall time. But when it has, it may take longer time to fall in, right ? The function should be $\ddot{r} = -GM/r^2 + (...
12
votes
3answers
21k views

Coulomb force in SI and cgs

Coulomb force in SI is $ F = \frac{Q1*Q2}{4\pi\varepsilon R^{2}} $ while in CGS $ F = \frac{Q1*Q2}{R^{2}} $ why is it? I mean doesn't it any make difference in dimension? since $ \varepsilon $ ...
7
votes
6answers
1k views

Why do liquids separate in space with no gravity?

I've seen videos of people in space (on ISS) who squeeze a bottle or something and liquid comes out, it then separates into smaller balls. Why is this surely it should stay pretty much together ...
3
votes
2answers
1k views

An Introduction to particle acceleration and particle accelerator

I am preparing to write my paper about particle acceleration recently, and i need some general knowledge to get my feet wet about it. May you please give me an introduction of what particle ...
18
votes
1answer
3k views

Turned to steel in the great magnetic field

This is obviously a "fun" question, but I'm sure it still has valid physics in it, so bear with me. How great of a magnetic field would you need to transmute other elements into iron/nickel, if that'...
4
votes
2answers
559 views

Low pressure, pulsating, series of water droplets

I want to create a stream of water that emits only a droplet of water, waits a few milliseconds, and then continues. The important thing is that I need to create a visible gap between drops. ...
24
votes
9answers
6k views

What's the difference between running up a hill and running up an inclined treadmill?

Clearly there will be differences like air resistance; I'm not interested in that. It seems like you're working against gravity when you're actually running in a way that you're not if you're on a ...
2
votes
1answer
312 views

What is required to create water droplets of a consistent size and shape over a distance of 3 to 10 feet? [closed]

What would I need to do to create water droplets of a consistent size and shape over a distance of 3 to 10 feet? Are there any special requirements for the nozzle? Can I use pure water or should I ...
2
votes
1answer
1k views

how to calculate gibbs free energy per unit mass, per unit volume, and per mole?

I've ran into conflicting information on how to calculate the Gibbs free energy of fuels during combustion per unit mass, volume and mole. A sample solution for hydrogen would be really appreciated!
2
votes
3answers
7k views

Velocity and acceleration question

1.The magnitude of the total force acting on a ball rolling without slipping down a ramp is greater than the magnitude of the total force acting on the same ball if it slides down the ramp ...
2
votes
3answers
23k views

Total number of subatomic particles in the universe. Are they finite ? assuming any of GR or QM or even ST

Total number of subatomic particles in the universe. Are they finite ? assuming any of GR or QM or even ST.
3
votes
3answers
561 views

Is the curvature of space-time a smooth function everywhere ? (except at black holes)

Is the curvature of space-time a smooth function everywhere (except at black holes) in view of General relativity. By 'smooth' it is meant that it possesses derivatives of all order at a given point.
9
votes
8answers
98k views

Why won't my door close in the winter?

Please, take this question seriously, because this is a real problem to me. I have a door in my flat. A closet door, to be specific. And there is a problem with it. In the summer everything is ok, ...
2
votes
3answers
312 views

A Question on Singularity

I am not aware of GR, but due to curiosity i have a question in my mind. Please let me know if it is inappropriate to ask here. My question is about singularity. I am under the assumption that ...
1
vote
1answer
200 views

Fluid to particles under newtonian gravity

How to start with a perfect fluid concept and reach (by approximations through certain mathematically well defined assumptions) to the concept of particle ? Here newtonian gravitation is being assumed....
4
votes
2answers
13k views

Spectral Line Width and Uncertainty principle

so I've been at this for about 3 - 4 hours now. It is an homework assignment (well part of a question which i've already completed). We did not learn this in class. All work is shown below. An ...
4
votes
2answers
305 views

Does the recent re-count of stars in elliptical gallaxies affect our understanding of the universal mass balance?

I've seen several popular reports of a new count of low-mass stars in elliptical galaxies (here's one). Edit: Pursuant to several correct comments I've changed the title to agree with the actual ...
40
votes
10answers
10k views

Applications of Algebraic Topology to physics

I have always wondered about applications of Algebraic Topology to Physics, seeing as am I studying algebraic topology and physics is cool and pretty. My initial thoughts would be that since most ...
41
votes
8answers
9k views

Classical mechanics without coordinates book

I am a graduate student in mathematics who would like to learn some classical mechanics. However, there is one caveat: I am not interested in the standard coordinate approach. I can't help but think ...
13
votes
2answers
330 views

Can a web community write papers? [closed]

the internet has changed science drastically. Not only in terms of distributing knowledge e.g. via online encyclopedias as wikipedia and freely available sources of publications as arXiv but also as a ...
7
votes
1answer
1k views

“Natural units” of mass

Gravitational attraction is given by $\frac{GMm}{r^2}$ while attraction due to electric charge is given by $\frac{q_1 q_2}{r^2}$. Why does gravity need a constant while electric charge doesn't? ...
5
votes
2answers
2k views

What if physical constants were increased or decreased? [closed]

(Probably related to this one, and probably should be CW.) A very long time ago, I had the good fortune to read George Gamow's excellent series of Mr. Tompkins books. That introduced me to the idea ...
22
votes
3answers
7k views

Deriving the speed of the propagation of a change in the Electromagnetic Field from Maxwell's Equations

I've been told that, from Maxwell's equations, one can find that the propagation of change in the Electromagnetic Field travels at a speed $\frac{1}{\sqrt{\mu_0 \epsilon_0}}$ (the values of which can ...
5
votes
2answers
2k views

How to determine the (n,m) dimensions of a carbon nanotube?

I've been reading about nanotubes lately, and I keep seeing the $ (n,m) $ notation. How does this describe a nanotube's structure? How do I determine which is $n$ and which is $m$ ? I'm familiar with ...

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