1
vote
0answers
36 views

What are the dimensions of the stress energy tensor in relativity? [duplicate]

Can anyone tell me what the dimension of the stress energy tensor is? Also, if it represents energy density, will calling it kinetic or potential energy be appropriate?
2
votes
1answer
126 views

Null geodesic equation

For a null geodesic curve $X^i$, $$0=g_{ij}V^iV^j.$$ When we derive the geodesic equation from E-L equations, will this affine parametrization cause it to blow up? How is it justified to use the ...
-2
votes
0answers
31 views

solving an electromagnetic problem in the frequency domain

I study a course in electromagnetic fields and I don't understand why when I solve a problem in the frequency domain I lose the transient solution. can anyone show me mathematically why does it ...
-2
votes
0answers
38 views

How it is predicted, that a moving charge has to have a magnetic field?

This question appears after this comment: Even electric charges without intrinsic magnetic dipoles moments produce magnetic fields when they move. My respons: All electric charges (electron, ...
0
votes
0answers
38 views

Determining actual dimensions of an object from it's image [on hold]

I have to make an ios app that predicts the actual size (height, waist, chest etc) of a person based on his photo and then suggest clothes of that size. I just want ideas as to how the actual ...
0
votes
1answer
35 views

If a super massive black hole captures a very small black hole, would the smaller black hole be immune to spaghettification?

I think to answer this question we would have to fully understand the nature of black holes. It seems to me that the smaller black hole could have a lower density than the larger one. In this case the ...
3
votes
1answer
184 views

Noether Current when the Lagrangian depends on second derivative of the fields

Let a Lagrangian density for a field theory of $N$ fields $\left\{\phi_i\right\}_{i=1}^N$ be given. Assume that the Lagrangian density depends on the fields, their spacetime derivatives, and their ...
4
votes
1answer
196 views

Bubble in a pipeline

I am just thinking about this phenomenon: We have a horizontal pipeline with a flowing liquid, which contains a small bubble of gas. How do the dimensions of this bubble change when it reaches a ...
1
vote
1answer
35 views

Derive frequency given potential using Newton's laws

A mass with mass $m$ has a potential energy function $U(x)$ and I'm wondering how you would find the frequency of small oscillations about equilibrium points using Newton's laws. I started by finding ...
0
votes
1answer
95 views

Is the fundamental representation of $SU(3)$ irreducible?

I want to check if the fundamental representation of $SU(3)$ is irreducible. The algebra is $$\mathbb{su}(3) = \{ m \in Mat(3,\mathbb{C} )\ |\ m = -m^+,\ Tr[m] = 0 \}$$ and I've found the generators. ...
7
votes
2answers
354 views
+100

Could anyone help explain this current voltage graph for an LED in liquid nitrogen?

I've been doing my coursework investigating LEDs at various temperatures and I've come across an interesting phenomenon which nobody I've asked has been able to explain thoroughly - wheras at room ...
18
votes
5answers
4k views

How to get Planck length

I know that what Planck length equals to. The first question is, how do you get the formula $$\ell_P~=~\sqrt\frac{\hbar G}{c^3}$$ that describes the Planck length? The second question is, will any ...
-3
votes
0answers
22 views

relations opf space-time to gravitation as an evolutionary process [on hold]

just thoughts - 1 -gravity - could it be that it is not a fundamental force after all - could it be that it evolved after the big bang? space time was evolving and things [lets call them string but ...
1
vote
1answer
148 views

A few questions on wave packets and uncertainty relations

According to Cohen-Tannoudji the wave-function for a one-dimensional free particle can be written as $$ \psi (x,0)=\frac{1}{\sqrt{2 \pi}} \int g(k) e^{ikx} dk.$$ While $g(k)$ is not specified, there ...
1
vote
3answers
215 views

Dielectric constant of water

I need the dielectric constant of water from $10^{-2}$ Hz to $10^4$ Hz. As stupid as it may seem, I cannot find much info. I've googled for days. All I can find is close to GHz. And the only info ...
3
votes
1answer
313 views

Why do we ignore rotational energy in monatomic gases? [duplicate]

I understand that the average energy of each degree of freedom in a thermodynamic system is $\frac12kT$. And so, for an ideal monatomic gas, there are three degrees of freedom associated with the ...
1
vote
4answers
125 views

Why is $F=-\nabla V$?

I came across this equation $F=-\nabla V$ where $V$ is gravitational potential. I do understand that $F(r)=-\frac{dV}{dr}$. Hence does this mean the nabla operator in this case means derivative? ...
3
votes
2answers
134 views

Is the Legendre transformation a unique choice in analytical mechanics?

Consider a Lagrangian $L(q_i, \dot{q_i}, t) = T - V$, for kinetic energy $T$ and generalized potential $V$, on a set of $n$ independent generalized coordinates $\{q_i\}$. Assuming the system is ...
-5
votes
3answers
2k views

If there is no gravity, does that mean there's no mass as well?

I have demonstrated that weight only is measured based on the gravitational pull of where you live. For example, the gravity on the surface of Mars is three times smaller than the gravity on the ...
-3
votes
0answers
48 views

How can I understand Anderson's work “More Is Different”? [on hold]

Recently, I am reading P. W. Anderson's famous paper "More is different". I know it is a masterpiece, and many people have their thinking on it. Is there anyone can tell me something about it? ...
2
votes
1answer
27 views

Water well fluid dynamics

In my home village everyone has their own water well at the foot of the mountain (limestone) . The wells are over 100 feet deep but they are not cemented or have the area between the hole and the ...
3
votes
1answer
122 views

Is it possible to eliminate Van der Waals interactions?

I came to know that the friction force actually depends on the surface contact area due to weak interactions (adhesion due to Van der Waals forces) between the atoms of both materials increasing in ...
1
vote
1answer
111 views

reactions in molecular dynamics simulations

EDIT: Apparently this is called reactive molecular dynamics. It seems that the ReaxFF potential function is used for some reactions like this. I am interested in adding support for reactions to a ...
1
vote
1answer
27 views

How do I calculate the change in water temperature as a function of depth?

Let's say I have ground water at a depth of $3000m$ at $50^{o}C$. If I now pump this water up to the surface, the water pressure decreases from the $\pm 300 atm$ at that depth to $1 atm$ at the ...
0
votes
0answers
22 views

Symmetry considerations in Plane Poiseuille Flow

I'm taking a first course on fluid dynamics, and I have this (sort of) conceptual question that's been nagging me for a moment now. I can completely follow the mathematics behind the derivation of the ...
36
votes
2answers
3k views

Wind generators - why so few blades?

Why commercial wind generators usually have just 2-3 blades? Having more blades would allow to increase power OR decrease diameter. Decreased diameter would also reduce stress due to different wind ...
2
votes
3answers
176 views

Does energy conservation not hold in fission and fusion processes?

I have read that during fission and fusion processes, there is some kind of equilibrium between the single nucleus and the disintegration products, so they are constantly being converted into each ...
-8
votes
2answers
1k views

Is it a problem with radiometric dating that carbon 14 is found in materials dated to millions of years old?

The preferred method of dating dinosaur fossils is with the radiometric dating method. And the result of this accepted method dates dinosaur fossils to around 68 million years old. However: Consider ...
4
votes
1answer
137 views

What kind of free energy do we use for a superconductor in a magnetic field?

My reasoning is as follows (using Gaussian units): Start from the second law: $$dU=TdS+dW,$$ where $dW$ is the work done by the magnetic field. To derive $dW$, we consider a solenoid with current ...
0
votes
4answers
50 views

Why, when and where is Gauss's law applicable?

Why is it said that Gauss's Law is mainly applicable for symmetric surfaces/bodies? Why not for asymmetric surfaces? I want a logical explanation! BTW my teacher said that Gauss's law is ...
2
votes
2answers
399 views

Why does a wind turbine have only three blades? [duplicate]

Why not four or five or even more? Intuitively, the more leaves the more power. So, what is the reason?
6
votes
2answers
2k views

Why is pressure in a liquid the same in all directions?

I'd like answers both in the more intuitive side an on the more precise side. Thinking of water as "cubes" of water, for example, would allow pressure in the z axis to be independent of the y or x ...
0
votes
0answers
19 views

Find an expression for S(T,x) from tension and specific heat

I'm working on a problem from a Statistical Mechanics lecture series online, and on the homework, I hit a bump in this problem. Here is the problem set, and I'm asking about #1.c. Short version, we ...
0
votes
1answer
101 views

Viscosity calculation of a rarefied gas

I am studying the rotational-translational relaxation of a diatomic gas (like oxygen) using a GPU in order to accelerate the calculations; during the calculations I get the translational temperature, ...
10
votes
4answers
8k views

How can time be relative?

I don't understand how time can be relative to different observers, and I think my confusion is around how I understand what time is. I have always been told (and thought) that time is basically a ...
3
votes
2answers
139 views

Green's function for the Klein-Gordon equation diverging?

I'm trying to work out the propagator for the free scalar field theory (i.e., the Green's function for the Klein-Gordon equation). On pages 23 and 24 of Zee's Quantum Field Theory in a Nutshell (you ...
1
vote
1answer
197 views

Calculating phase diagrams (Calphad)

I am calculating the phase diagrams for Li-Mg binary alloy with reference to the following text: http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=5600701 In the equation used in the above ...
1
vote
4answers
108 views

Do moving charges produce magnetic fields?

As I understand it, a moving charge produces a magnetic field. Now it may be that I have have a misunderstanding here and if so I need to have the proper understanding of this. So if I do have a ...
0
votes
0answers
38 views

Constructing an equation of motion [on hold]

I have the following problem. Two particles initially located at $(0,0)$ and $(-1,0)$ start moving with constant speed ($v_b$ and $v_c$, respectively) towards $(0,1)$ such that at any time $t$ the ...
0
votes
0answers
24 views

Why we see a distant object more closer from us with a telescope?

The side magnification in the image obtained by a telescope its always less than 1, then, Why we see a distant object more closer from us?
2
votes
2answers
190 views

How to use the first law of thermodynamics for simple mechanical systems?

I'm confused about what exactly is $Q$ and $U$ and their signs. Consider a block initially having some kinetic energy which we stop and we want to find by how much amount its temperature increases. ...
4
votes
1answer
83 views

White Noise Generator Through Low Pass Filter - Autocorrelation of normalized data dropping below 0 (showing negative correlation)

I am doing the following experiment: http://physlab.lums.edu.pk/images/a/ab/Correlation.pdf It is basically a white noise generator running through a low pass filter. Sample voltages are taken and ...
3
votes
1answer
65 views

Redefinitions of Lagrangians using EOM

I am trying to understand an statement of this paper. In section 2 this Lagrangian is introduced ...
1
vote
1answer
38 views

What causes continuous errors in a qubit?

I read that due to decoherence a qubit in a superpositon gets destroyed or put into one definite eigenstate. This kind of error seems to occur due to interactions with other stuff like the environment ...
0
votes
2answers
163 views

Compute distance between planes in a crystal

I want to compute the distance between two (111) planes in a cubic crystalline structure, in order to do some computations involving Bragg reflection. I have a sketch of which the (111) planes are, ...
1
vote
1answer
24 views

Light / TEM wave in a hollow waveguide

I'm cramming for my EMF exam, and a question came up: Light in free space is an example for a TEM wave, isn't it? (Maybe thats already my mistake) And we learned that in a hollow waveguide only TE ...
-3
votes
1answer
64 views

What is time and time dilation? [on hold]

What is time? If it is nothing and just our understanding of the change then why does time dilation occur? If time is relative I think that time dilation shouldn't be possible. I just can't ...
1
vote
1answer
136 views

Ampère's law and superconductors

I have been reading about superconductors and emerge me a inquietude about the explanation of existence of currents inside the superconductor while their magnetic field (inside too) is zero. Based in ...
2
votes
4answers
592 views

Confused about Newton's 3rd law

I am confused about Newton's 3rd Law. If a person jumps off the ground a force is applied both to the person and to the ground. However, as $F=ma$ acceleration experienced by the Earth is much less ...
3
votes
5answers
337 views

Does coordinate time have physical meaning?

I have always been a little confused by the meaning of the "$t$" which appears in spacetime intervals or metrics in general relativity. I concluded that $t$ was just a mathematical thing which allow ...

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