# All Questions

647 views

### How does battery cell size affect voltage drop for a fix current load? [closed]

For a fixed current load, will the voltage drop be larger in a small cell or a big cell battery? Why?
6k views

### What justifies dimensional analysis?

Dimensional analysis, and the notion that quantities with different units cannot be equal, is often used to justify very specific arguments, for example, you might use it to argue that a particular ...
75 views

### 1-dimensional Ring geometry - Group of Translations

I considered a Ring-like one dimensional geometry. In this, if we fix an origin (at some point on the circumference), we can think of set of all displacements along the circumference to form a vector ...
576 views

### Show that the circumference of the Bohr orbit of hydrogen is an integral multiple of the wavelength of the electron in it [closed]

We have an electron revolving in a the $n$th Bohr orbit of an atom of hydrogen. The problem is to prove that the circumference of the orbit is an integral multiple of wavelength of this electron ...
179 views

### FQH Edge Theory as decoupled chiral bosons

The action describing the edge theory of the Fractional Quantum Hall effect is given by S = \frac{1}{4\pi} \int \mathrm{d}x \ \mathrm{d}t \left[ K_{IJ} \ \partial_{t}\phi_{RI} ...
290 views

### How far we can see in the thickest possible fog?

There is quite foggy day today here, I cannot see end of the street. This made me think about how much worse can it be. So here is my question: how thick a fog can be? What's the smallest visibility ...
293 views

### Mathematical motivation of OPE?

In Peskin & Schroeder (and also Cheng which I have skimmed through) they motivate the Operator Product Expansion with a lot of words. Is there any way to motivate it mathematically, e.g. Taylor ...
77 views

### Do molecular bounded systems shield or reduce neutron cross-sections?

When talking about neutron cross-sections, literature is usually investigating isolated cases of Neutron + Atom. Here, the abundance of hydrogen is dominating neutron fluxes through material. I ...
1k views

### Heat transfer by conduction is not possible for gases?

Today one of my instructor told me that gases cannot have heat transfer thru conduction because molecules are far apart and so it cannot transfer heat,infact the diffusion process of gases transfers ...
112 views

### Same U(1) charge for the SU(2) doublet

Consider the symmetry $SU_L(2)\otimes U_Y(1)$. The entries of $SU_L(2)$ doublet will have same U(1)-charge. How can this be shown mathematically?
9k views

### Why does the tension on the pulley in an Atwood machine not equal $(m_1 + m_2)g$?

Consider the following simple Atwood machine with an ideal pulley and an ideal string According to my textbook, the tension on the clamp that holds the machine to the wall equals $2T$. I don't ...
473 views

### Synchronizing Pendulums

Assume we have a frictionless pendulum of length $l$ with mass $m$. This pendulum hangs from some weightless contraption, which is itself bolted to a platform. This platform can move horizontally in ...
624 views

### Electrical Potential Energy and Electric Force [closed]

Under certain circumstances, potassium ions $(K+)$ move across the $8.0 nm$ thick cell membrane from the inside to the outside. The potential inside the cell is $−70 mV$ and the potential outside is ...
509 views

### Adjoint of Gamma Matrices - Dirac

I just started to learn how to quantise Dirac field. Meanwhile, as we can write the Dirac equation in terms of gamma matrices : $$(i\hbar\gamma^\mu\partial_\mu - m)\psi = 0$$ where $\gamma_\mu$ ...
126 views

### How to derive energy expressions thinking of it as a conserved quantity only?

By now I understand that "energy is a conserved quantity" and that's all we need to know. Then, the idea of work comes as the change in kinectic energy of a system and we realise that having energy is ...
82 views

### Regarding the centrifugal force in a 3-body system

In our lecture about general mechanics, we have treated the simplified 3-body system consisting of the Sun, the Earth and the Moon. We assumed that the moon does not have any influence on the Sun or ...
222 views

### A question about canonical momentum and arbitrariness for potential in magnetism

The following question confuses me: There exists magnetic field $B_z =- \beta x$ where $x > 0$, and a particle is incident from origin point $(0,0)$ with pisitive charge $q$, mass $m$, and ...
137 views

### Fine grained vs. coarse grained measurements and MWI

Alright, so it is my understanding that the idea that measuring systems do a coarse grained measurement which give the appearance of decoherence. I understand that the claim implicit there is that ...
505 views

### What would happen if a supermassive blackhole at the center of a galaxy evaporates?

Well the title says it all but I will just explain it in detail a bit. We know that a blackhole eventually has to evaporate due to the hawking radiation. It is also said that the blackholes at the ...
192 views

### Calculate 2D density of states from 3D density of states

I have a 3D DOS which is calculated by this formula: $$D(E) = \sum_n\int\frac{\mathrm d^3k}{(2π)^3}\delta(E−\epsilon_n(k))$$ I do not have any analytical expression for kinetic energy. Is there any ...
141 views

### Does gravity cause spacetime to bend? [closed]

Does gravity cause spacetime to bend even though space has no mass?
159 views

### Massless neutrinos and Chirality

The massless neutrinos can be represented by two component Weyl spinors. Then how does one say that it is an eigenstate of the chirality operator $\gamma^5$, which is a $4\times 4$ matrix and can act ...
174 views

### Admixtures of longitudinal and timelike photons!

In the quantization of electromagnetic field the physical states $|\psi\rangle$ are found to obey the following relation: $[a^{(0)}(k)-a^{(3)}(k)]|\psi\rangle=0$ It is explained as the physical ...
244 views

### What Causes the Noise Floor of Modern CCDs?

CCDs are getting pretty good these days, but all systems are subject to noise. A typical value for a CCD seems to be 20 electrons RMS per pixel. This article from qsimaging.com says CCD Read ...
477 views

### Riemannian manifold: Integration by parts of Lie derivative

How can we apply integration by parts to the Lie derivative? Background: In the Hamiltonian formulation of general relativity, we have the momentum constraint (using abstract index notation, $D_a$ is ...
122 views

### Faraday's law question: wire falling over two rails

I'm having trouble with this problem. The picture shows a wire of mass m and length l which slides on two rails, with certain angle with respect to the horizontal.The whole circuit has a resistance R ...
41 views

### What is $p$ in this candidate solution for the motion of N masses?

In this problem: Consider the system of $N$ masses shown in the figure (see picture 1). a) Using the approximation for small angles, write the equation of transverse movement for the n-th ...
498 views

### Finding Green function using eigenfunction expansion method

Given the Dirichlet boundary condition, I am to show that the functions that satisfy $$(\nabla ^2 + k_{lmn}^2) \psi_{lmn} (x,y,z) = 0$$ are given by \psi_{lmn} = (\frac{\pi}{2x})^{1/2} ...
114 views

### What are the assumptions behind “term symbols”?

In multi-electron atoms, the electronic state of the optically active "subshell" is often expressed in "term symbols" notation. I.e. $^{2S+1}L_J$. This presumes that the system of electrons has ...
332 views

### Is this derivation of Black Hole entropy viable?

This question is motivated by this one. Suppose $l$ is the minimum measurable unit of length. What is entropy of a spinless particle contained in this interval? We know that entropy of a two-level ...
75 views

### Which body position causes the least amount of negative acceleration? [closed]

Let's say you are in the air 500 meters above flat land and you have no parachute. Which pose - forming a sphere or spreading all extremities to make yourself as wide as possible or other - is the ...
118 views

### Uniform Circular Motion: v = v_0 + (a_T)*t as a vector VS as a scalar equation

First, I'll explain my confusion, which may be enough for someone to clarify my misunderstanding. Second, to give context, I will give the homework problem (which I've solved successfully) which ...
290 views

### Matrix representation of a triplet state

The $SU(2)$ triplet state is typically given in the fundamental representation as a column vector, e.g. \vec{\Delta} = \left( \begin{array}{c} \delta^{++} \\ \delta^+ \\ ...
111 views

### (Euclideanized) QFT on $S^d$ vs $S^{d-1}\times S^1$

Broadly I would like to understand what is the difference in the physical interpretation of a (Euclideanized) QFT which is on space-time $S^d$ and which is on a space-time $S^{d-1}\times S^1$. In ...
223 views

### Learning about super-symmetric quantum mechanics and integrable systems

I'm an undergraduate interested in theory. I recently asked one of my professors, a physicist in the particle theory group at my school, if he'd be willing to take on an undergraduate for a senior ...
583 views

### Which is smallest distance measurement which makes sense talking about [duplicate]

This might seem a silly question, but here it goes: Of all possible values for a measure of distance, which is the smallest that makes sense talking about? I mean, I could talk about $10^{-100000}$ ...
308 views

### Can a spinor be defined as any quantity which transforms linearly under Lorentz transformations?

Recently I’ve come across a few papers from China (e.g. Xiang-Yao Wu et al., arXiv:1212.4028v1 14 Dec 2012) that make the following statement: ...any quantity which transforms linearly under ...
266 views

### Wormhole traversability in general relativity

It is well known that stable, traversable wormholes require exotic matter that violate the dominant energy conditions. For the purpose of the question, let's assume enough exotic matter can be ...
294 views

### Calculate integral of motion condition with Poisson brackets

Problem statement: The Hamiltonian of a system is given by the formula: \begin{equation*} H = \frac{p_r^2}{2m} + \frac{p_\theta^2}{2mr^2} + V(r,\theta). \end{equation*} Under what condition is ...
452 views

### Must every isometry have an associated Killing vector?

I understand that the flows of Killing vector fields are isometries, and that one-parameter groups of isometries have an associated Killing vector which generates them, but are your Killing vectors ...
105 views

### Force needed to change momentum, from fixed position

I have a situation where I want to change the velocity of a mass, by applying a force from a fixed position. For example in the diagram below, the mass starts with the initial velocity in the upper ...
48 views

### What are the constituents of communicated time differences between moving objects?

In "The Elegant Universe," two individuals, George and Gracie, are noted to be moving relative to one another in space and wearing clocks. They sync their clocks upon passing each other. Each observes ...
355 views

### Finding the ground state of the toric code Hamiltonian

How do I write by proof, the ground state of the toric code (by Kitaev) Hamiltonian $H=-\sum_{v}A(v)-\sum_{p}B(p)$ where $A(v)=\sigma_{v,1}^{x}\sigma_{v,2}^{x}\sigma_{v,3}^{x}\sigma_{v,4}^{x}$ and ...
2k views

### Why does a faraday cage protect you from high currents?

In an electrostatic case it is clear that that in a space enclosed with a conductor (without charge in it) the electric field is zero. This is often demonstrated in physics shows like on the ...
221 views

### Can you read and write a quantum state to a particle without collapsing its probability field?

I know the main problem with a quantum computer is that you can't read and write data from it until after it has run though all of its calculations. But would reading and writing quantum data to ...
309 views

### Does decreasing the length of the line between a harness and zip wire increase the velocity at which you travel?

My friend is an instructor at an outdoor adventure centre which has lots of zip wires. The instructor's harness has a shorter line that attaches to the zip wire than the normal harness does and she ...
912 views

### If electrons behave as standing waves when they are bound to an atom then how do they carry charge?

Today in my physics lesson we learnt that the best way of describing the behaviour of an electron that is bound to an atom is to treat it as a standing wave. I understand that this is the ...
270 views

### Do black holes exert an infinite amount of energy at the event horizon?

An interesting thought crossed my mind when reading about Hawkings' apparent horizon theory. If we assume that event horizons do actually exist, it would seem that black holes violate basic laws of ...
Why is the ground state assignment $0^{+}$ whenever the number of neutrons and protons are both even?