# All Questions

148 views

### What is it that undulates in a particle?

When there is a wave, something is undulating. In the example of a rope, the rope is what undulates. In the case of a ripple on a pond, the water is undulating, and when a sound wave propagates, the ...
1k views

### Degree of ionization and Saha equation

Say you want to calculate degree of ionization for different gases in atmosphere of a star with abundances similar to those in Sun (let's assume you only have hydrogen, helium and sodium) over the ...
1k views

### Maximum limit on the number of paper-folds possible after tearing into halves [duplicate]

Possible Duplicate: Why can't a piece of paper (of non-zero thickness) be folded more than "n" times? On skeptics.stackexchange, there is a question on the maximum limit on ...
24k views

### Why does the gas get cold when I spray it?

When you spray gas from a compressed spray, the gas gets very cold, even though, the compressed spray is in the room temperature. I think, when it goes from high pressure to lower one, it gets cold, ...
5k views

### Why exactly does current carrying two current wires attract/repel?

When to parallel wires carrying currents in same direction I1 & I2. http://www.youtube.com/watch?v=43AeuDvWc0k this video demonstrates that effect. My question is, why exactly does this happen? ...
2k views

### Ricci scalar for a diagonal metric tensor

I was wondering if there is a general formula for calculating Ricci scalar for any diagonal $n\times n$ metric tensor?
212 views

### How dangerous is firing a bullet straight up in to the air? [closed]

Question's title says it all how dangerous are the bullets which are fired up straight in to the air? I often find myself in this discussion where there are people who argues that it is harmless and ...
254 views

### under what conditions happen the anti-Zeno effect?

As you might know, the Zeno effect is intuitively expressed as what happens when a system is measured in intervals smaller than the half life of the state it is currently on. As a consequence, the ...
428 views

### When is many-body perturbation theory valid?

I'm calculating expectation values (thermal, time-independent) using many-body perturbation theory, but I'm unsure how to work out what values the parameter I'm expanding the perturbation series in ...
350 views

### in SUSY, does WW scattering unitarisation needs the higgs boson?

One of the arguments of LHC "win-win situation" is that the scattering of W particles needs to include new terms to preserve unitatity begond 500 GeV or so. In the SM, this is realized by the higgs ...
542 views

### Simultaneously commuting set

How does one determine the members of an simultaneously commuting set (of operators)? For example, I have read that for orbital angular momentum, the set is {$H,L^2,L_z$}. How does one know that these ...
71 views

### stress work of uniformly deforming continuum

I have a volume which is deforming (using explicit time-integration scheme) uniformly with velocity gradient $L$ and stress tensor $\sigma$. I would like to determine work done by the volume ...
72 views

### Why is the measured distance to the Triangulum Galaxy much more uncertain than to the Andromeda Galaxy?

Why is the measured distance uncertainty for M33 (835 ± 105 kiloparsec) six times bigger than the measured distance uncertainty for M31 (778 ± 17 kiloparsec)? They are approximately at the same ...
3k views

### What's wrong with this derivation that $i\hbar = 0$?

Let $\hat{x} = x$ and $\hat{p} = -i \hbar \frac {\partial} {\partial x}$ be the position and momentum operators, respectively, and $|\psi_p\rangle$ be the eigenfunction of $\hat{p}$ and therefore ...
1k views

### What is the effective spring rate of a magnetic spring

Consider a magnetic spring as seen on this YouTube video, but ignore gravity. If I wanted to calculate the effective spring rate (Force vs. Deflection) curve for the top magnet, how would I go by ...
128 views

### What to show a beginner [closed]

At my new job, it's soon going to be my turn for doing night/graveyard shifts for a fair amount of weeks. Perfect excuse to buy a decent beginners telescope to replace the 4.5" 15 y/o Meade that ...
94 views

### References for the source and application of bonding-antibonding splitting on electronic structure?

I am currently doing research on semiconductor materials, so I need a very strong background in band theory to understand the literature. I am currently trying to understand the relationship between ...
418 views

### Pollen Particle Attracted to TV due to Which Force?

A pollen particle has no charge so I cannot understand how the Lorentz force $\bar{F} = q \bar{E} + q(\bar{v} \times \bar{B} )$ could explain the event. I speculated that it is because of the electric ...
1k views

### Can smoke stay still in the air?

Can a small amount of smoke be dense enough to stay in the air keeping its shape for a minute or so? Or does it always dissipate quickly? If not smoke, can anything else stay in the air for a minute ...
11k views

### What happens to the absorbed light energy?

When light comes across with a solid material, some of it is reflected, some of it passes through and some of it is absorbed. I understand the reflection and passing through, but I don't understand ...
486 views

### Curvature of a cable supporting a suspension bridge in the regime where the mass of the cable is non-negligible

Assume I have a chain of uniform mass, $M_c$, suspended between two points such that it will follow the trajectory of a catenary curve when only supporting its own weight. Let's also say that this ...
60 views

### Assuming an observer is 50 light years away, in the plane of the solar system and observing earth, what is the light flux of earth he would see?

As in, from what the planet emits and re-radiates out into space (this is going to be important if we are to image any Earth-like planet)
2k views

### How do we determine what distant planets, stars etc are made of?

I remember this being covered somewhat back in school and I have casually read about it. I know it involves inferring from spectral analysis what physical properties an object may have right? Though ...
2k views

### What is the origin of the Dirac delta term in the dipole electric field?

I am a bit lost how one has deduced the formula for electric field with electric dipole because of some inconsistency between different sources. The Wikipedia article contains a delta function in the ...
913 views

### If a Kerr-Newman black hole is like a charged, spinning, heavy magnet, what kind of magnet is it like?

I was reading up on De Sitter spaces, which states that the gravitational effects from a black hole is indistinguishable from any other spherically symmetric mass distribution. This makes a lot of ...
41k views

### What is the physical significance of dot & cross product of vectors? Why is division not defined for vectors?

I get the physical significance of vector addition & subtraction. But I don't understand what do dot & cross products mean? More specifically, why is it that dot product of vectors ...
148 views

### Confusion with the torque

Consider an imaginary vertical plane. Now say, a body is falling freely (under earth's attraction). If you consider any axis that is perpendicular to that plane. We get a non-zero value for torque. ...
162 views

### Why do we fall when we ride bicycle slowly but we don't when riding it fastly [duplicate]

Possible Duplicate: Why doesn't the bike fall if going with a high speed? Why is it that, when we ride bicycle at good speed, it is perfectly balanced and we don't fall. But we lose ...
2k views

### Using Ampere's circuital law for an infinitely long wire & wire of given length

According to Ampere's Ciruital Law: Now consider two straight wires, each carrying current I, one of infinite length and another of finite length ...
19k views

### What are the prerequisites to studying general relativity? [closed]

This question recently appeared on Slashdot: Slashdot posts a fair number of physics stories. Many of us, myself included, don't have the background to understand them. So I'd like to ask the ...
253 views

### Quantum Fluctuations as a model for the Big Bang?

I have quite often heard (and even used) the idea that quantum fluctuations are a way to explain the whole "something from nothing" intuitive leap. I am about to give a talk at a local school on ...
112 views

### Mangnetic Flux summing up like Kirhoff?

You have a coil over an iron hearth. There is a current in coil which creates the flux $\phi_{1}$. The flux then distributes over the wider area in the iron (using wrong word?): $\phi_{2}$ the flux ...
2k views

### Please explain the physics of a Cloud Chamber

A friend of mine was telling me about building a cloud chamber while he was in graduate school. As I understand it, this allows you to "see" interactions caused by high energy particles going through ...
2k views

### Magnetic field in the Centre of Circular loop wire with Current?

By biot-savart: $$\bar{H} = \frac{I}{4\pi} \oint \frac{d\bar{l} \times \bar{r}}{r^{3}}$$ so $$\bar{H} = \frac{I}{2a} \hat{n}$$ Please, explain the last implication. I cannot find such integral to ...
532 views

### Can a black hole be formed by radiation?

I'm trying to find out if black holes could be created by focusing enough light into a small enough volume. So far I have found (any or all may be incorrect): Maxwell's equations are linear, ...
8k views

### Comparing scales of atomic level objects to scales of everyday size objects

I am trying to come up with everyday size objects comparisions of atomic scales items, e.g. if a proton probability cloud was of size basketball how far would the next atoms to it be? reason being is ...
8k views

### How does gravitational lensing account for Einstein's Cross?

Einstein's Cross has been attributed to gravitational lensing. However, most examples of gravitational lensing are crescents known as Einstein's rings. I can easily understand the rings and crescents, ...
1k views

### Is the wind's force on a stationary object proportional to $v^2$?

I am on a boat docked at Cape Charles, VA, about 30 or 40 miles from the center of Hurricane Irene. This understandably got me thinking about the force of wind on the boat. Since air friction is ...
4k views

### Can superconducting magnets fly (or repel the earth's core)?

If a superconducting magnet and appropriate power supply had just enough $I\cdot s$ (current $\cdot$ length) so that when it was perpendicular to the earth's magnetic field, the force of the ...
794 views

### Plotting a wave function that represents a particle

The problem is this: A particle is represented by the wave function $\psi = e^{-(x-x_{0})^2/2\alpha}\sin kx$. Plot the wave function $\psi$ and the probability distribution $|\psi(x)|^2$. This ...
3k views

### Is there a stable numerical algorithm for FWHM that isn't 2.35*sigma?

This is a question that should have a simple answer, but which I can find no proper discussion of in the literature or on the internet. I start from the assumption that I have a noisy numerical ...
917 views

### Lean angle of a turning bicycle

I'm asked to derive a relationship for the leaning angle of a bicycle with the following specs: Center of gravity for bike and rider is a distance $L$ above the ground when vertical, and the total ...
486 views

### “Seeing” the solution

I often hear people say that so-and-so [some famous physicist] can see the solution without calculation. What exactly does this mean? Is it an intuition? But how does one have intuition when it comes ...
365 views

### Schematic expression of the Schrodinger equation

it would be great if someone could help me understand the following quote regarding wavefunctions :) "$$\psi(x)=\sum_n C_nu_n(x)+\int dE C(E)u_E(x)$$ The expression is schematic because we have ...
693 views

### Estimation of accuracy of measurements?

When stating an estimation of the accuracy of my measurements (of electric current, in this case) would I state that it is equal to the residual error, or the limit of reading? (in this case, ±0.2 or ...
1k views

### Is this Landau's other critical phenomena mistake?

There was an old argument by Landau that while the liquid gas transition can have a critical point, the solid-liquid transition cannot. This argument says that the solid breaks translational symmetry, ...
8k views

### Why is the magnetic field inside a solenoid constant?

Why is the magnetic field along the length of a solenoid constant? (preferably in relatively simple terms) Thanks!
439 views

### Air vs Glass refraction coefficient

I have a rather strange question, let's say I can play with the temperature and atmospheric pressure conditions in a sealed room. What will I need to do to make the air's refraction coefficient ...