# All Questions

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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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12 views

### 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 ...
0answers
5 views

### Repulsive potential for free fermions

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 ...
0answers
7 views

### 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 ...
1answer
16 views

### 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 ...
1answer
5 views

### 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?
1answer
8 views

### 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 ...
0answers
20 views

### 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 ...
0answers
13 views

### 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 ...
0answers
23 views

### 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 ...
0answers
16 views

### 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.
0answers
5 views

### 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 ...
0answers
7 views

### 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?
2answers
202 views

### 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 ...
0answers
18 views

### 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 ...
0answers
14 views

### 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
0answers
6 views

### 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 ...
0answers
15 views

### 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). ...
0answers
7 views

### 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?
1answer
16 views

### 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 ...
1answer
25 views

### 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: ...
0answers
9 views

### 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 ...
0answers
15 views

### 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. ...
1answer
18 views

### 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, ...
0answers
22 views

### 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 ...
0answers
21 views

### 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 ...
0answers
21 views

### 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 ...
3answers
43 views

### Can't we consider that EmDrive is Pushing against Space itself?

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 ...
1answer
19 views

### 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 ...
1answer
115 views

### Would the hot windows break? [on hold]

I have a question that I was thinking about for a long time. I live in Kuwait and in the summer season the weather reach 50°C which is extremely hot specially inside a car. What if I was able to ...
0answers
11 views

### chemical thermodynamic [on hold]

Calculate U and H for each of the following changes in state of 2.50 mol of a perfect monatomic gas with CV,m 1.5R for all temperatures: (a) (1.50 atm, 400 K) → (3.00 atm, 600 K); (b) (2.50 atm, 20.0 ...
1answer
14 views

### Expression for spontaneous emission in solids

I have yet to find a reliable source for the expression of the rate of spontaneous emission in a solid. Can anyone confirm if the following is correct? The basic ingredients of the calculation are ...
1answer
41 views

### Do wormholes violate causality [duplicate]

Does opening and closign a wormhole to send a message within seconds across the galaxy violate causality? Wouldn't different observers disagree about the order of events and what effected what? Is ...
1answer
47 views

### Different electrons, why aren't they all the same?

Why do we say that there are different kinds of electrons when discussing different situations in physics? For instance the Weyl electron, Dirac electron etc. From my exceedingly basic knowledge isn't ...
1answer
65 views

### M-theory and Mathematics

If the framework of M-theory is the right path for a unification of gravity with quantum mechanics, is it obligatory to believe that, at a certain level, reality is not just easily describable by ...
0answers
39 views

### Hydrogen is the ONLY Element?

Let's say you have Hydrogen and it's isotopes, Can you fuse them together to create all the 118 elements, by fusion or something else, Can anyone give me any research paper or anything which gives ...
1answer
46 views

### Completeness relations of eigenstates in the Heisenberg picture

I've been reading Srednicki's introduction to path integrals and I'm slightly unsure of the notation that he uses for the completeness relation of position eigenstates in the Heisenberg picture. In ...
0answers
5 views

### Magnetic field lines for perpendicular current carrying wires [on hold]

Given two straight current carrying conductors, carrying the same current I kept perpendicular to each other. What would the magnetic field lines look like?
0answers
22 views

### if you feel that a wire is warm,what will you check or do? [on hold]

pls,answer ASAP homework to be passed the other day
0answers
12 views

### Is Earth's magnetic field moving? [on hold]

Is the Earth's magnetic field moving? Why is the Earth's magnetic field said to be moving?
1answer
17 views

### Maximum horizontal speed and vertical speed interdependently [on hold]

I need to limit hspeed and vspeed taking direction in count, I know how to calculate the maximum speed as a whole, but not limiting that speed by setting the hspeed and the vspeed
1answer
23 views

### Do you know any graphing software for graphing equipotential lines?

well, do you know any software for graphing equipotential lines? I only know how to graph straight lines :( Thanks for any information.
0answers
16 views

### Concerning topology of BPS states of the the M5-brane

My question is about the M5-brane in M-theory. I would like to know whether the BPS states of the M5-brane worldvolume theory (especially the 1/2 BPS and 1/4 BPS ones) are independent of the topology ...

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