# All Questions

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### Derivation of Maxwell stress tensor from EM Lagrangian

From Noether's theorem applied to fields we can get the general expression for the stress-energy-momentum tensor for some fields: T^{\mu}_{\;\nu} = \sum_{i} (\frac{\partial \mathcal{L}}{\partial ...
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### Superpositioning of fire

I once recognized that when you light two candles and you move one of the candle towards the other, you will see that the total fire height (lets call it $f_3$) is higher than the sum of the single ...
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### fugacity of gases and gaseous mixtures

Is fugacity defined only for gases and gaseous solution.? In many books it is given that fugacity is "effective pressure", what does it mean? If it is applicable to liquids and liquid solutions then ...
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### Classical toy models of particles with intrinsic spin

Related to my question here (spacetime torsion, the spin tensor, and intrinsic spin in einstein cartan theory), I'd like to be able to put test particles on a manifold with non-zero torsion and see ...
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### What will happen if I throw chocolate figure on hot red fan?

What will happen if i throw chocolate figure on hot red fan. Fan, of course, is sucking in my figure, but will it evaporate, or it will be thrown away in liquid form? Lets take fan with steel blades, ...
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### Energy in a string [on hold]

A nylon string of length L = 50 cm and under a tension of T = 70 N is plucked with an amplitude of 5 mm. What is the energy in the string? Please give me the relationship between energy of the ...
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### Why is cooling much harder than heating?

I'm trying to invent a distillation apparatus that runs solely on electricity. Suddenly, I realized that cooling things is really hard, while heating them up is so easy. Actually, it seems that there ...
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### What happens to the physics when you scale-up a water and air bubble in zero-gravity to gigantic size? [duplicate]

How to the effects of the various forces in equilibrium vary as you scale up a bubble of air and water in zero gravity from 10cm diameter to planet size? Note that there is only the gravity of the air ...
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### Why do bubble wands have ridges?

The bubble wands supplied with children's bubble mixtures seem to fairly universally have a set of ridges around the hole used to create bubbles - for example, see the circle in the upper right of ...
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### surface tension versus gravity in a zero-gravity environment, size limit of water and air bubble [on hold]

surface tension versus gravity in a zero-gravity environment, size limit of water and air bubble. When do the forces balance?
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### Why does my wooden door disperse light into a rainbow color spectrum?

On a clear morning, light comes through this window (viewed from the inside) It then hits the door on the opposite side (so viewing from the outside the door straight on the other side) There is ...
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### What is the difference between $N=(2,2)$ with $N=(2,2)^*$ in 2d?

What is the difference between $N=(2,2)$ with $N=(2,2)^*$ in 2 dimensional theory? In some sense, i heard, they are totally different theory. I heard from breaking of $N=(4,4)$ supersymmetry it ...
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### Is there, and if there is, what is the name of the concept that matter is made out of void/space?

I can't recall where I heard about this, but it was rather intriguing. I recall the notion that matter is compressed void/space, where I remember an example of a bedsheet representing space, and the ...
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### Thermodynamic equilibrium- mixing of gases

What is the final temperature and pressure after removing diaphragm of a cylinder which divides the cylinder into two equal components each of volume $0.1\,m^3$ and each component contains air at ...
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### Why do we use less fuel by driving at constant speed?

It is common practice to drive at constant speed as much as possible to reduce fuel consumption, or at least to avoid "pumping" on the throttle (at frequencies high enough that the lowpass filter does ...
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### Unmixing of gases: What is the relevant temperature for my Entropy?

This answer to a question about unmixing gases states: However, let us now assume we have to hand a source of mechanical work, and a large heat reservoir at temperature $T$. I'll assume that ...
163 views

### Joules to do something

If my very limited understanding is correct, then, not accounting for gravity: 1 Newton can move 1 kilogram 1m But, can 2 Newtons move 1 kilogram 2 meters? Is this because 1 Newton = acceleration ...
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### Why does a bubble take a spherical shape?

I suspect this has something to do with thermodynamics and the isoperimetric inequality and I'm interested in a mathematical derivation of this result.
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### How to calculate heat generated from cosmic dust in space? [duplicate]

Say I have a spaceship, and I'm flying it through space at a constant high velocity V. The spaceship is pretty large (big surface area). In space however, there is cosmic dust scattered throughout ...
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### How to visualise the direction of torque?

Torque is analog of force in rotational motion and it has one of two directions I.e clockwise and counterclockwise . How I can visualise these directions . For example r and F are on same plane(x and ...
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### why do many appliances not run on A.C? [on hold]

I know the basic difference between D.C and A.C but why do many appliances run on D.C and not on A.C? How can I visualize A.C? Does a fan run on A.C because it has a capacitor which does not allow D.C ...
347 views

### Why some thunders sound deeper while others sound snappier?

Why does some thunder sound deeper and lower in pitch while others sound snappier and higher in pitch? I don't think there should be any difference between the lightnings and so the thunders sound be ...
146 views

### Particle Outside the Box

What prohibits, mathematically, that a particle cannot be found outside the box ? Here, I am referring to particle in a box problem (infinite potential on both ends & zero potential along the ...
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### Radiative mass generation in QED?

Does self-energy correction leading to a modification in the mass of the electron can be called radiative mass generation? In Zee model of radiative neutrino mass generation, the helicity of the ...
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### On solving a two lens system

A common method to solve for the image formed due to a two lens system consisting of two thin-lenses separated by a distance is: Locate the (intermediate) image formed by the first lens, ignoring ...
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### Intensity of Brightest Fringe of a multi-slit Diffraction

Let's say we have a multi-slit experiment with N slits.Since the central fringe has the most intensity, is there an equation to calculate the intensity of central fringe in terms of the intensity of ...
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### Classical Hamiltonian involving product of factors whose quantum analogues don't commute

Dirac remarked in his quantum mechanics book: One can usually assume that the Hamiltonian is the same function of the canonical coordinates and momenta in the quantum theory as in the ...
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### Ideal gas law in open volumes

I often find in text books that the ideal gas law given by $p\cdot V=m\cdot R\cdot T$ is applicable in a closed system, still, some use it for open systems (like freezing the system at a given ...
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### What will be the time taken by a stone to reach the ground if it is thrown with velocity $v$? [on hold]

I was studying from my reference book and I got this question. I tried solving it by finding the total distance n average velocity but it didn't work please help me out.
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### Why can a ball tossed up object free fall upwards?

I've found many websites that say that a ball tossed upward would be free falling upwards, but none of them say why. If the definition of free fall is an object that is falling while only under the ...
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### Can time be interacted with? [duplicate]

Astronauts come back to Earth younger than they would have been had they stayed on Earth for that same period of time. They are traveling so fast relative to the Earth that time slows down for them. ...
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### What happens when a photon reaches the “edge” of the universe? [duplicate]

As I understand it, there are several models for what the universe could look like from the “outside”. I don’t know if any of these are outdated, so if they are please tell me if they are. Expanding ...
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### FBD of a spring damper system

Does it matter which direction the spring and damper forces point? In one of the examples in my professor's lecture, he stated that if we define y to be positive when pointing up (see image below), ...
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### Hawking Radiation: Couldn't virtual particles make black holes more massive? [duplicate]

From what I understand about Hawking radiation, a virtual particle comes into existence very near the event horizon of a black hole. This causes one part of the particle to fall into the black hole, ...
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### Could an object higher than the stationary orbit be returned to the outgoing point on this orbit?

In the question Centrifuge speed of an object higher than a stationary orbit, I asked: Does an object higher than the stationary orbit move slower or faster around the world than the top of a tower ...
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### Identitity operator-state mapping in CFT

In a general CFT, how can the identity operator be mapped into a state? Can you give examples to demonstrate the systematic way to do the mapping? Take 2D CFT (in String Theory) as examples. For ...
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### Finding equation for acceleration using mass1, mass2, gravity, theta, and Mu [on hold]

So, the only question I could not solve in this chapter for AP Physics, and I really want to understand it. We have a situation like this: There is a box sitting on an incline. We are trying to ...
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### Will an un-scattered photon go to the edge of the universe? [duplicate]

Will an unhindered (un-scattered) photon go to the edge of the universe?
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### Can beamer presentation can be used at APS meetings? [on hold]

I am asking this question because I couldn't find the answer in the APS site. Can we use a presentation created with latex beamer at the APS (American Physical Society) meeting?
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### Joules To Move A Kilogram In Space [on hold]

If roughly ten joules lifts one kg on earth one meter, how many in space? Meter / joule/kilogram Kilogram/meter/joule
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### Milky Way Density Profile

Does anyone know where I can find either a printed version or, better, a downloadable dump of data giving me the density profile of the Milky Way? I'm looking to put together a quick-and-dirty ...
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### Correlation functions scale as powers of external momenta

In the 1st paragraph of section 12.2 on page 406 of Peskin and Schroeder, it says "the correlation functions turn out to scale as powers of their external momenta, with power laws that do not appear ...
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### Fresnel lens magnification, convex divergence

Is it possible to use a series of lenses to divert some of the energy output of a large Fresnel lens to two distinct focal points?
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### A few questions about photons

I have a few questions about photons. I know the emitting and absorbing of photons is related to quantum leaps. Does a quantum leap cause emitting/absorbing, or is it the other way around? Do ...
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### New coordinates in terms of old from Jacobian— Software? [duplicate]

Fellow geometry enthusiasts: I am studying a particular geometry and I would like to change from one set of coordinates to a new set that makes the metric unimodular (i.e. it's determinant is one). ...
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### Sturm Liouville Problems [migrated]

I'm currently having a class at university that discusses Sturm Liouville Problems. We have to solve a few problems one of which I can't seem to solve. We use Advanced Engineering Mathematics by Erwin ...
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### Does magnitude of a charge influence magnitude of force that individual charge exerts on another charge [on hold]

two point charges, q1 and q2, are placed 0.3m apart on the x-axis, as shown in the figure above. Charge q1 has a value of -3 nano Coulomb and q2 has a value of +4.8 x10^-8 C. The net electric field ...
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### What happens to the gas in the container in this picture when the container is tilted

So the test question asked what would happen to the gas if this container were tilted on its side. The piston shown here is 10 kg and moves frictionlessly. I said that the pressure inside the ...
### Ising model on lattices with (vertical side length) $\neq$ (horizontal side length)
Consider the Ising model with nearest neighbours interactions on a rectangular lattice $L\times M$. If $L=M$ (2-dimensional square lattice), it is known (e.g. by Peierls argument or Onsager explicit ...