# All Questions

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### Do lattice gauge theories with discrete gauge groups have sensible continuum limits?

In lattice gauge theories the only gauge invariant observables are constructed from Wilson loops and local field strength observables are reconstructed as zero size limits of Wilson loops. Furthermore ...
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### Examples of measure zero sets in physics

The problem with Riemann integration of functions like ${\chi }_{ℚ}(x)$ (a function which takes the value of 1 if $x\in ℚ$, and 0 otherwise) is usually used to show the need for defining the Lebesgue ...
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### The James Webb Telescope, why are there 5 levels of protecting foil?

From simple curiosity, does anybody know why there are around 5 layers of solar radiation reflecting foil on the James Webb Telescope, rather than one or two? Is it to save weight, or (probably) ...
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### Storing ice for HVAC

Apologies if this question is outside of the scope of the site. Assume my home is a $20 \textrm{ ft} \times 40 \textrm{ ft} \times 10 \textrm{ ft}$ block. Further assume that it has continuous ...
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### Position from a Velocity-Time graph

Find the position at t= 3.0 seconds. The initial position is 2.3 m. I found the average velocity to be 3.33 repeating so I multiplied that by the time (3) to get 10 and then added the initial ...
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### Perturbation theory, eigenvalues and eigenvectors for degenerate case (1st order)

I was trying to understand the perturbation theory, but I was lost in the notation... I have understood that I have to identify the imperturbed kets that are degenerated and find the matrix V, where ...
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### State of a system in Quantum Mechanics and state vectors

I'm taking a course in Quantum Mechanics and there is something I'm not being able to fully understand. On more elementary courses on Quantum Mechanics I've been told that the idea of Quantum ...
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### What is a n-type doped solar cell?

I always thought a solar cell consists of two areas, a n-type and a p-type doped one. So what exactly is meant by n-type doped solar cells? Does it not have a p-type doped area? And how does it work ...
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### Need help on understanding mechanical wave function

My textbook states that, equation 1 : y(x=0,t) = Acos($\omega$t) = Acos(2$\pi$ft), which I understand. However the book goes deeper stating also that, t-$\frac{x}{v}$, and $\frac{v}{x}$-t I am ...
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### Is there a constant decay of particles that maintains the value of the Quantum Vacuum?

By my question I am attempting to understand how the Vacuum of space remains at a constant value, (GR the universe on the large scale matches the “critical” energy density of about 8.5×10-10 J/m3) ...
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### What quantum measurement formalism is easiest to implement physically?

As part of my studies and research, I have learned to work with three different measurement formalism which I define to avoid any ambiguity with the nomenclature: General measurements, which are ...
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### Is the universe a Turing machine?

Reading about Computable numbers I wondered if there is any physical experiment that returns non-computable numbers or if there is any physical theory that needs non-computable numbers. Because if ...
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### $\Delta H = C_p \Delta T$

The definition of enthalpy as a function of heat capacity and temperature change. $\Delta H = C_p \Delta T$. Does it only apply at constant pressure? In my discussions on this board and also with ...
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### Is it possible to have things orbit around us?

Since humans are not large in mass, do we exert enough gravitational force for a small object to orbit around us? (Assuming that the only gravitational force acting on the object is from the human). ...
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### A wave is passing through a potential barrier

Let assume a wave is passing through a potential barrier. Like this: What will be happened to the amplitude or the shape of the wave during the movement through the potential barrier? {assuming ...
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### Mathematics of the Virtual Displacement

So I'm pretty certain this question has been asked to death here, but I still can't find a good explanation of a very particular aspect of the virtual displacements in physics. Background For ...
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My question -which is probably easy to answer for a physicist- stems from trying to understand the repulsive interaction between fermions. For instance the fact that states of multifermion systems are ...
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### Why, in order to obtain distinct interference, is a small distance between the two waves essential?

This is quoted from Concepts of Physics by H.C.Verma: In order to obtain a fairly distinct interference pattern, the path difference between the two waves originating from coherent sources should ...
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### Without friction, what happens in our life?

Generally all are subject to friction. So friction is most a essential part of our life, if friction is not available in our surrounding then what would happen in our life and which technique could we ...
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### why we use Gibb's free energy concept in Ehrenfest's theory of Phase transformation?

Why do we use the concept of Gibbs's free energy in Ehrenfest's theory of phase transformation ? Why don't we use Helmholtz free energy concept?
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### Making a home made projector bur lens an issur

This got booted out if the lifehacker forum so I'll try here I read a nice tutorial here which tells you how to make a homemade projector for two quid. Lovely thinks I so I trotted down to the local ...
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### Electron-positron annihilation

Consider this problem An electron with kinetic energy K=1Mev collides with a positron at rest. In the collision, both particles mutually annihiliate, creating two gamma rays with equal energy, each ...
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### How to start using IDL in 2015?

Unfortunately it looks like I have to run a bit of old astronomy software on IDL. I keep running into statements that "I need to buy a license" in order to run IDL. Is this true? How can I ...
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### Is this solvable? Time-dependent perturbation theory

The question is A hydrogen atom is placed in a time-dependent homogeneous electric field given by $$\varepsilon(t) = \varepsilon_0(t^2 + \tau^2)^{-1}$$ where $\varepsilon_0$ and ...
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### How does space expand near the speed of light?

As you travel near the speed of light relative to the Earth, does the rate of space expansion change in some way? If so please specify in which directions it does so and by what amount.
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### Can I use an IV curve alone to differentiate between metallic, semiconducting and insulating materials?

Assuming the size of the bandgap is expressed as resistance in the IV curve, could I tell between metals, semiconductors an insulators by only doing an IV sweep? For example very high slopes (large ...
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### What's the difference between semiconductor and insulator (besides band gap)?

The typical classification of electronic materials is metal-semiconductor-insulator. Is there any actual difference between a semiconductor and an insulator, besides the size of the bandgap?
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### Why can we see through rain?

I am gazing through my office window into a heavy rain. I am thinking that raindrops are like small lenses that bend the light. Thus I am surprised, that I can clearly see other buildings through the ...
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### Imaginary time formalism

Consider a function of energy-scale $\tilde{F}(k)$ and its fourier transform in time as $F(t)$ (as energy-scale is the fourier-conjugate of time in Minkowski space). Then we have ...
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### Magnetic field due to closed donut

How to find magnetic field due to a torroid? Thanx in advance. Im studying for jee and our prof tried to teach it but i didnt understand.help please
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### components of vector regarding the direction of acceleration

The steps of how to calculate th direction of acceleration Example Calculate the direction of the acceleration when acceleration has an eastward component of 2.5m/s squared and nortward component of ...
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### Unable to solve cable theory equation correctly

First time posting, let me know if I'm in the wrong board. I tried this over on the math board and was redirected here. In my spare time I enjoy reading outside of my field (software development). ...
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### Potential of a point charge in dielectric

My configuration is a point charge embedded in a finite dielectric geometry. I have to calculate the potential of the point charge only within the dielectric device. How should I do this?
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### Equipartition Theory at room temperature.

I read a statement about participation theory. The equipartition theorem is generally valid only at high temper- ature, so that the thermal energy is larger than the energy gap between ...
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### How can I prove using data provided that resistance is inversely proportional to cross sectional area?

I know that resistance is inversely proportional to the cross sectional area, but I need to explain why with the following data: ...
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### If the surface tension is reduced to half(say by using a surfactant), what would be the effect on the size of an air bubble in the liquid

If I reduce the surface tension to half of the original value, what would be the effect on the size of an air bubble in the fluid? What would be the effect on buoyancy and drag forces? Also, if I ...
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### Why does the weaker force deform the solid body? Elasticity problem

It's about a problem in which two forces are pushing a cylincer in different directions. Ïnside the cylinder some reactions forces appear.Here's the representation of a mass dm since it's very small. ...
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### Density and Wave Speed

For a string, the formula for wave speed is $v=\sqrt{\frac{T}{\mu}}$, where $\mu=\frac{m}{L}$. The greater the linear density, the more massive the string is per unit length, the more inertia it has, ...
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### Directional derivative of the Ricci scalar in the direction of a Killing vector field

Do I need the Levi-Civita connection for the directional derivative in the direction of a Killing vector field of there curvature scalar to be 0? EDIT: Let $K$ be a Killing vector field and $R$ the ...
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### What is the time dilation effect in the presence of two equal and opposite gravity sources?

If one were between two equal and opposite gravity sources, would the time dilation effect cancel out, since the acceleration is cancelled out?
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### What does $(I + iδH^t)(I-iδH)$ equal to? [on hold]

$I$ is equal to 1, $δ$ is a small change. According to 43:04-43:10 of this video http://theoreticalminimum.com/courses/quantum-mechanics/2012/winter/lecture-4 \begin{align} \ i{ δ }(H^t-H)=0 ...
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### Proof of transformation between contravariant and covariant components of vector [on hold]

What is the proof of the relation ${g^{ir}g_{rj}=\delta^i_{j}}$ where ${g}$ is the metric tensor and ${\delta}$ is the Kronecker delta. EDIT:The operations is known as equivalent to raising and ...
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### Can't we consider that EmDrive is Pushing against Space itself? [on hold]

When something move through space it pushes against something else in space with equal force. However, EmDrive warps space around. So can't we say it's pushing against space itself, and thus momentum ...
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### Find the separation distance for a line of oil being squashed between two flat plates

I was wondering if someone could give me some help on how to start this problem, I'm really struggling to get my head around it. A long line of oil is being squashed between two flat plates of length ...

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