# All Questions

2answers
1k views

### Moment of inertia of a coin

I have a a coin infinitely thin, rotating along the diameter. How to derive the formula for it's moment of inertia passing through the diameter. I was suggested to use the surface density and ...
2answers
3k views

### Why is the Higgs boson spin 0?

Why is the Higgs boson spin 0? Detailed equation-form answers would be great, but if possible, some explanation of the original logic behind this feature of the Higgs mechanism (e.g., "to provide ...
2answers
1k views

### What is the physical or mathematical meaning of the Gibbs-Duhem equation?

The Gibbs-Duhem equation states $$0~=~SdT-VdP+\sum(N_i d\mu_i),$$ where $\mu$ is the chemical potential. Does it have any mathematical (about intensive parameters) or physical meaning?
2answers
173 views

### Does locality emerge from (classical) Lagrangian mechanics?

Consider a (classical) system of several interacting particles. Can it be shown that, if the Lagrangian of such a system is Lorenz invariant, there cannot be any space-like influences between the ...
0answers
25 views

### Does it make sense to speak of amplitudes of finite closed boundaries in QFT?

A example of amplitude in Relativistic Quantum Mechanics or specifically in QFT is the amplitude of a field configuration on a space-like hyper-surface of space-time to "lead" to another field ...
1answer
119 views

### Describing the surface of a sphere in Huygens' Principle

$U(P) = \frac {iU(r_0)}{\lambda} \int_{S} \frac {e^{iks}}{s} K(\chi)\,dS$ Where $S$ describes the surface of the sphere, how is the surface of the sphere described? Sorry if you feel this is quite a ...
3answers
5k views

### “Find the net force the southern hemisphere of a uniformly charged sphere exerts on the northern hemisphere”

This is Griffiths, Introduction to Electrodynamics, 2.43, if you have the book. The problem states Find the net force that the southern hemisphere of a uniformly charged sphere exerts on the ...
1answer
139 views

### Experiment to find structure of water

Who first determined the structure of water (two hydrogen atoms stuck to an oxygen atom at approx 105 degrees), and, more importantly, how was this done?
2answers
738 views

### Microwave oven + water: dielectric heating or ion drag?

When you place a water or food in a microwave oven, it heats. Which process commits more energy to that: dielectric heating, or ion drag i.e. resistive heating? AFAIK, in distilled water (which is a ...
3answers
5k views

### How to calculate the evaporative cooling rate needed to protect a house from forest fire

Recently in our area there has been a large forest fire and I've been looking into home defense from such things. I am not a physicist - but can do some basic math. I was wondering how I could ...
1answer
2k views

### Why is light described by a null geodesic?

I'm trying to wrap my head around how geodesics describe trajectories at the moment. I get that for events to be causally connected, they must be connected by a timelike curve, so free objects must ...
1answer
123 views

### States diagonal in the tensor product of Bell states.

Bell-diagonal states are 2-qubit states that are diagonal in the Bell basis. Since those states lie in $\mathbb{C}^{2} \otimes \mathbb{C}^{2}$, the Peres-Horodecki criterion is a sufficient condition ...
1answer
372 views

2answers
350 views

### Question with Einstein notation

Let’s consider this equation for a scalar quantity $f$ as a function of a 3D vector $a$ as: $$f(\vec a) = S_{ijkk} a_i a_j$$ where $S$ is a tensor of rank 4. Now, I’m not sure what to make of the ...
1answer
393 views

### Are two polarization states of light coherent?

Let's consider a situation: we have distant point source of unpolarized light in certain non-zero range of wavelengths (it's polychromatic). Let's divide this light into 2 beams depending on ...
2answers
468 views

### When “unphysical” solutions are not actually unphysical

When solving problems in physics, one often finds, and ignores, "unphysical" solutions. For example, when solving for the velocity and time taken to fall a distance h (from rest) under earth gravity: ...
2answers
2k views

4answers
2k views

### why ammonia is not a competitive energy carrier?

reading this article about Germany planning to store renewable energy as hydrogen gas, i'm left wondering why they deem so great this plan, compared with other alternatives like ammonia as a energy ...
2answers
118 views

### Heuristics for definitions of open and closed quantum dynamics

I've been reading some of the literature on "open quantum systems" and it looks like the following physical interpretations are made: Reversible dynamics of a closed quantum system are represented ...
2answers
373 views

### Computing the path of photons near a black hole

For a simulation, I want to compute the path that light follows near a black hole. Non-relativistically, a massive point particle in a central newtonian gravitational field follows either an ellipse, ...
2answers
4k views

### Flow rate of a syringe

Suppose a syringe (placed horizontally) contains a liquid with the density of water, composed of a barrel and a needle component. The barrel of the syringe has a cross-sectional area of $\alpha~m^2$, ...
1answer
1k views

### How can a Higgs decay to heavier products than its mass?

How is it possible that a higgs at ~125 GeV can decay into 2 W bosons @ ~ 80 GeV a piece (for example)? Shouldn't a particle only be allowed to decay to lighter particles + energy? Diagram copied ...
2answers
333 views

### What is the interaction with Higgs field(s) that give the quarks so much different masses?

The masses of quarks are: mu 2∼3 MeV md 4∼6 MeV mc 1.3 GeV ms 80∼130 MeV mt 173 GeV mb 4∼5 GeV
0answers
231 views

### Modeling the trajectory of a particle in an Electric field [closed]

[This problem has been resolved, sorry for posting in the wrong forum!] (I was trying to model the trajectory of a particle in an electric field.)
4answers
1k views

### can one introduce magnetic monopoles without Dirac strings?

To introduce magnetic monopoles in Maxwell equations, Dirac uses special strings, that are singularities in space, allowing potentials to be gauge potentials. A consequence of this is the quantization ...
1answer
572 views

### Energy band diagram of a system of Silicon Quantum dots

Suppose that we have a system of Silicon nanoparticles embedded in ZnO dielectric matrix. i'm thinking about how to construct the energy band structure of this system , suppose that we already have ...
3answers
2k views

### Flying a toy helicopter inside an accelerating train [duplicate]

Scenario: You ride in a train, you have this helicopter toy. The train is not yet running when you flew your helicopter on a constant altitude (say 1 meter above the train's floor). Question: What ...
1answer
425 views

### Do spacelike junctions in the Thin-Shell Formalism imply energy nonconservation and counterintuitive wormholes?

The Thin Shell Formalism (MTW 1973 p.551ff) is used to properly paste together different vacuum solutions to the Einstein equations. At the junction of the two solutions is a hypersurface of matter – ...
1answer
46 views

### Time Evolution of a Manifold Embedding

Given a smooth manifold $\mathcal{M}$ with a simplicial complex embedding $\mathsf{S}$, what specific tools or methods can be used to give an analysis of the time evolution of the manifold given some ...
2answers
115 views

### The relativistic mechanics of a battery that is being charged and accelerated at the same time

This might be an interesting question: Let's attach a battery into one end of an electric cable. Then we rotate the battery around, with accelerating speed, using 100 Watts of power, while the ...
2answers
6k views

### What are all the equations we use to calculate how bounces work?

I mean, what is the object's final displacement, or the function that describes the object's height over time (see [1]) of an object thrown by a height $h$ with a speed of $\vec{v_0}$, a mass of $m$, ...

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