# All Questions

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### Kinetic energy change in a non-uniform magnetic field?

I have a square loop (sides L) with a current $i$ that produces a non uniform magnetic field which I'll call $\mathbf B$. In the central axis of that loop, I have placed, at a distance $d$ a postive ...
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### When can the work performed by subsystems not equal the total work by the system?

In his $Thermodynamics$, Fermi mentions the following: “The entropy of a system composed of several parts is very often equal to the sum of the entropies of all the parts. This is true if the ...
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### How do I obtain the equations for the trajectory of a particle acted by the forces in a vector field?

Suppose we have a vector field, let's say $F(x,y)=(-x,-y)$ and I release a massive particle in the coordinate $(1,1)$. This particle is going to be acted by a force in the direction $(-1,-1)$ and will ...
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### What are some theories where time emerges from a graph of events?

I'm not really interested in reality, but more so, I'm interested in novel theories or systems or constructions which are inspired by reality, but may or may not accurately reflect reality. I am ...
64 views

### How does Fermi jump to this conclusion in Clausius inequality?

In his $Thermodynamics$, Fermi proves beautifully the following (rephrased): For a system undergoing a cyclic process, $$\oint {\delta Q\over T}\leq 0,$$ and for a reversible cyclic process, it is ...
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### Why does a layer-like structure occurs in a cold plasma? See pictures

I've been experimenting with cold plasma created by a high voltage source at a frequency of about 23kHz. I observed that sometimes the plasma forms layers within the bulk plasma. Following picture ...
57 views

### What will be the effect on the water level? [duplicate]

I came upon this question recently while studying about buoyancy. It goes like this: If an iron block is covered with ice and floats in a tub of water, what will be the change to the water level ...
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### How do we explain the Idea that pressure increases with height in a liquid, With the molecular level (microscopic) reasoning? [duplicate]

We know that in uniform gravitational field, in a liquid, the pressure increases with height. How do we explain this phenomenon with the reasoning at the molecular level? What difference is coming ...
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### Partial Derivative of the Energy-Momentum Tensor [closed]

How do I calculate the partial derivative of the energy momentum tensor $∂_𝜇 𝑇_{𝜇𝜈}=0$? and in what conditions its zero? here are the Lagrangian density and $𝑇_{𝜇𝜈}$ formulas, respectively.
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### Is it possible to determine the motion of a vehicle moving at a constant speed of $0.8c$?

Suppose you are in a rocket with no windows, traveling in deep space far from other objects. Without looking outside the rocket or making any contact with the outside world, explain how you could ...
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### Isobaric Process

I was studying the thermodynamic processes, particularly the isobaric process where the pressure remains constant. While solving a particular question (which I couldn't) I saw that for a gas enclosed ...
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### Is relativistic statistical mechanics “foundationally” relativistic?

Does relativistic statistical mechanics average over particles that exceed the speed of light? This article seems to imply as such: It is interesting to note that the classical and quantum ...
31 views

### SXS gravitational wave data question

I need to do some analysis for a project with the SXS gravitational wave data: https://data.black-holes.org/waveforms/catalog.html but I am a bit confused about the initial conditions of their ...
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### Definition of metric tensor and line elements

I know that the metric tensor $g_{ij}$ is defined as: $$g_{ij} = X_i \cdot X_j$$ where $x_i$ and $x_j$ are the covariant basis vectors ($X_i = \frac{\partial X}{\partial X^i}$) (the definition is ...
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### Relativity Of Simultaneity Confusion

I have currently been introduced to relativity of simultaneity with the following thought experiment: Consider a train in which there are two presidents, one sitting at the front and the other at the ...
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### Noether's current to invariant transformation (scalar theory) [closed]

I need to find the Noether's current in this case: Thank you very much in advance!
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### Is a brachistochrone a straight line in curved space?

Please bear with me, and don't get upset if i have lack in knowledge about spacetime. Brachistochrone: Given two points A and B in a vertical plane, what is the curve traced out by a point acted ...
1k views

### Why isn't current carried through a vacuum?

The empty vacuum of space shouldn't offer any resistance to a travelling electron, and so a large collection of electrons should similarly travel through a vacuum without resistance. As a result, the ...
665 views

### In 4 spatial dimensions, would motion under a central force law be confined to a plane?

In dimension $3$, we have the angular momentum $\omega = q \times v$, and since $$\frac{d}{dt} \omega = v \times v + q \times (F/m)$$ from the fact the force is central (so $F$ is parallel to $q$) we ...
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### What does an external observer see if you shine a laser pointer while crossing an event horizon?

If I understand correctly, the event horizon of a black hole is the boundary beyond which we cannot perceive or describe any events, and all objects appear to freeze when falling in until their light ...
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### Observed speed of a receding light source

Let’s say there’s a planet 4 light years away from Earth and we send a rocket ship towards that planet at 99.9% light speed. We stay behind on Earth and watch the rocket ship travel towards the other ...
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### VMI image analysis

I used VMI for the photodissociation of O2 and I wonder how I can correlate the radius with the actual velocity of the fragments. Thanks a lot!
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### Noise generated by ball impacting a rigid obstacle

How can I estimate the noise of an elastic non-deformable ball that hits an elastic plate that is simply supported?
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### Where can i practice my analytical mechanics?

Does anyone know a good source ? Personally looking for Q & A but cant seem to find
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### Thermal expansion of a rectangular metal shape

This question is inspired by this interesting answer which left me somewhat uncomfortable for the following reasons: Suppose we have the following notched metal shape: Following the linked answer, ...
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### Projection of speed of particles moving close to speed of light [closed]

Let's describe the velocity of the massless particle in two-dimensional plane with an absolute value $v$ and direction $\phi$. How fast should travel the particle in direction $\phi+\Delta\phi$ so ...
11 views

### What form(s) of radiation are least attenuated by the atmosphere?

Given the composition of Earth's atmosphere, what is the most efficient way of wirelessly beaming energy from the stratosphere down to the surface?
16 views

### MOSFET Mobility

The surface mobility is lower than the bulk mobility because of surface roughness scattering What is surface roughness scattering? How does it affect the mobility?
25 views