# All Questions

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### Propagator from Path integral

In class we have proved something like: $$\frac{\partial^2 Z(J,\bar{J})}{\partial J(x) \partial \bar{J}(x')}\frac{1}{Z}|_{J=\bar{J}=0}=\Delta(x-x').$$ That by introducing source terms to path ...
33 views

### Boltzmann brain vs random star formation [on hold]

I can (sort of) understand the Boltzmann brain paradox, but I have a specific question. If the universe does turn into being a thermal equilibrium in a box, wouldn't a scenario consisting of the ...
18 views

### Confusion in understanding Coriolis force example

I am learning Classical Mechanics from a book called Newtonian mechanics by AP. French. The book tries to explain Coriolis force with an experiment as an example which confounds me. The book goes ...
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### Difference in Energy Transfer Between Impacts and Nudges

Say I have a table and I want to produce a vibration in the table. Would I be better off impacting the surface of the table (i.e. smacking the table) or nudging the surface (i.e. leaning on the ...
50 views

### How do we get images of galaxies?

First of all a very basic question upon which my main question is based. "Are the pictures of milkyway galaxy we see on books/mags are real ? ". If they are real then my question is, how we have taken ...
24 views

### Mixed Neumann/Dirichlet BCs in Poisson-Schrodinger self-consistent solver

I'm currently in the process of writing a self-consistent Schrodinger-Poisson solver for a device heterstructure (High Mobility Electron Transistor). I am having a few difficulties however. It seems ...
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### After proving that the photon remains stationary in the fourth dimension, must we conclude that the fourth dimension is moving at c? [on hold]

Firstoff, in his general relativity Einstein showed that dimensions could bend, curve, and move. This is an experimentally proven fact. Dimensions can, and do, move. In an earlier post we ...
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### Moment Of Inertia, Angular Acceleration [on hold]

Consider a pulley and and a string is attached to the pulley with a mass hanging and then dropped. Explain why the acceleration is greater when you attach a string to a larger radius of a cylinder. ...
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I am trying to learn a bit about antenna theory in order to understand how Nikola Tesla may have misunderstood them. (Thank you CuriousOne for your response to my earlier question! It was helpful in ...
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### How do ideal fluids enter narrower pipes?

For an ideal fluid to enter a narrower pipe it would have to compress itself, which can't be true because ideal fluids are incompressible. So how can ideal fluids enter narrower pipes? Below is the ...
240 views

### Wave speed derivation for small amplitudes

The above is a derivation for the wave speed equation in my physics textbook. However, I've read online that this equation is only true for waves with small amplitudes. I do not see where this ...
106 views

### What does a voltmeter actually measure?

For time varying fields (even quasistatic ones) the electric field is given by $${\bf E} = - \nabla \Phi - \frac{\partial {\bf A}}{\partial t}$$ So what does a voltmeter measure? Does it measure a ...
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### Calculating speed in four dimensions

If you are moving at $c$ in 3D space and $c$ in time axis too, What would be your total speed? Edit: Since question has been voted to be closed, I shall make an Edit. In 4D world all objects move ...
174 views

### Why did Albert Einstein receive a nobel prize? [on hold]

We all know that he received the prize due to his explanation of photoelectric effect. But the photoelectric effect, it’s a mere 3-4 page topic and uses a simple equation that typically anyone can ...
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I often read that when you have a deep cut on your arm, you should raise your arm to the point where the cut is above your heart. This should reduce the amount of blood loss because less blood reaches ...
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### How did the Michelson-Morley experiment really confirm the universal speed of light? [on hold]

So we know the earlier physicists largely assumed light moved in the aether as a fixed reference frame, and the Michelson-Morley experiment successfully disproved that. But it appears that the ...
17 views

### L-S Coupling (Atomic Physics) Ground State [on hold]

Can someone please explain to me why in the second configuration the ground state is associated with $L=F=3$ and $J = 7/2$? ** Another question: Should we always take $J$ as even? if yes, why?
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### Proper time of world lines in twin paradox

I was thinking about the twin paradox in Special Relativity and I thought I understood it fine, but when I view the "paradox" in a certain way, I get confused. So we have two twins, John and Jim. Let ...
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### According to Einstein & Brian Greene, does the photon remain stationary in the fourth dimension? [duplicate]

According to Einstein and Brian Greene, does it logically follow that the photon remains stationary in the fourth dimension? In An Elegant Universe, Brian Greene writes: “Einstein found that ...
17 views

### Hankel transformation of Yukawa potential [on hold]

This is a Hankel transformation problem that is used to do the 2-d Fourier transformation of Yukawa potential. We already know that $H_0(\frac{1}{z^2 + r^2}) = K_0(kz)$. Then what should be the right ...
33 views

### Illustrating the definition of Young's modulus from spring factor

The relationship between Young's modulus $E$ and the spring factor $k$ from Hooke's law is $k=\frac{E A}{L_0}$ where $L_0$ is the initial length of the stretched material and $A$ the ...
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### probability of finding the system in the ground state [on hold]

$\renewcommand{\ket}[1]{\left \lvert #1 \right \rangle}$ Assume that a quantum mechanical system is described by two orthonormal states $\ket{+}$ and $\ket{-}$, defined by the property of being ...
57 views

### Height formula of a sphere inside a pipe

I have a pipe with a ball inside it and a blowing air through it. Look this image: The air is blowing in the direction of $F_a$. $F_a$ Is the air force and $P$ is the weight force. What I want ...
31 views

### Does the solar wind correspond to a net electrical current?

The solar wind is a stream of ions emitted from the sun. Are there more positive than negative charges released? That is, is there any imbalance in the charge emitted from the sun in the solar wind?
38 views

### could bacteria perform nuclear fusion of hydrogen atoms [on hold]

Consider a bacteria that produces a protein that reacts in such a way as to force two hydrogen atoms to fuse. Is such a protein possible? Could the energy produced be harnessed. Is this sort of thing ...
39 views

### Why does an oscillating electric field couple more strongly to electrons than to ions?

As stated in Principles of Plasma Discharges and Materials Processing, by M. A. Lieberman and A. J. Lichtenberg, about capacitively coupled plasmas: (...) the light and heavy charged particles in ...
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### X-ray Diffraction: Difference between characteristic radiation peaks and diffraction peaks (hkl)

I'm having hard time understanding the charasteristic X-Pay peaks. I have XRD data which I plotted in the figure below. I'm supposed to find the X-Ray tube material by finding the wavelengths of ...
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### Relationship between angular magnification and how much bigger the image appears

How is the angular magnification of a magnifying glass related with how much bigger the image appears to the user? Is it simply the same? If yes, why? (Let's assume that the object is placed in the ...
42 views

### Why do you feel “hot” at a certain temperature? [on hold]

I think it's safe to say that in general 90$^\circ$F or even 85$^\circ$F is pretty hot. Your body ends up having to sweat or use some mechanism to release heat. I looked up the surface temperature of ...
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### In an infinite universe, would there be an infinite amount of matter as well?

Or would you eventually reach a point past which there is no matter? How would an infinite amount of matter work? Would it sort of "loop back" to the beginning, or would it be entirely unique the ...
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### Four particles arranged in a square and rotated on a turntable [on hold]

A question reads. Four particles, each of mass 3kg, are connected by light, inextensible strings, each 0.08m long, so that they form a square with the particles at the corners and the strings ...
31 views

### Pauli Spin Matrices [on hold]

i) Determine the effect of the pauli spin matrices $\sigma_x$ and $\sigma_y$ on the eigenstates of $\sigma_z$. Which physical process is conveyed on the eigenstates of $\sigma_z$ by applying ...
25 views

### Pressure Drop in Water Fountains

In first grade, my class made a list of things that our school could improve on. One of the things on the list was to increase the water pressure in the drinking fountains because it was very poor and ...
25 views

### Calculating mechanical motion/velocity of a pendulum/swing? [on hold]

I am a bit new to this so sorry if I mess anything up. Just needed some help with this question. A mom puts her 20kg son into a swing seat and pulls him back so he is 3.00m vertically to the lowest ...
3k views

### If we feel it's hotter when humidity increases, then why do we feel it's colder when inside water?

When the humidity in the air is high, we sweat more and feel it's hotter than when the humidity is lower. So why don't we feel it's hotter when we go inside water, where the water content is much ...
84 views

### Period of a pendulum [on hold]

In the book 'Calculus the Early Transcendetals' at page 776 (7th edition) they give that the period of a pendulum with length $\text{L}$ that makes a maximum angle $\theta_0$ with the vertical is: ...
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### As planets orbit around the Sun, does the force of gravity due to the Sun remain constant?

I know that the force of gravity is: $$F = GMm/R^2$$ Where $G$ is gravitational constant, $M$ is mass of sun, $m$ is mass of the orbiting planet and $R$ is the distance between the center of their ...
25 views

### Calculate the total impedance of this circuit [on hold]

I'm not very familiar with impedance calculations using complex numbers and I would like to ask for a verification for my reasoning in the the following problem. The problem is to calculate the total ...
134 views

### Is time an observable in Relativistic Quantum Mechanics?

Relativistic Quantum Mechanic is based, as far as I know, in the Dirac Equation. Now, the Schrödinger equation, in the abstract state space takes the form: i\hbar ...
21 views

### Different width of spectral lines for different groups of stars

As seen in an HR-diagram, a certain stellar classification can correspond to more than one group/sequence of stars (G5 could for instance be either a giant, main sequence star or a white dwarf and so ...
24 views

### Electric field inside an insulator's cavity [on hold]

I'm trying to find the electric field inside insulating sphere's cavity, and I don't know how. However, if there was an conductor I'd know that $E=0$ from Gauss's Law.
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### The sun glows bright white, but what color of light does it reflect?

Basically if you take a gray iron bar, and heat it red hot, it would still reflect a dullish white light, even though all you could see is the red light it emits as radiation. In fact if you watch ...
34 views

### If two cars have the same acceleration at time $t$, are the velocities of the cars the same at time $t$?

So when I look at the question logically, I reason that if I had two cars: Range Rover and BMW, at time t, I could pump the gas and make them accelerate at the same rate at 0.5 m/s^2 but I could be ...
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### How exactly does osmosis or any other related phenomena work?

How do the molecules of one type 'know' there are less number of the same type on the other side of a semipermeable membrane? Does it have to do with the overall energy state of the system?
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### Are all dualities in physics either S or T dualities?

S Duality and T Duality are usually associated with string theory. However, by definition they seem quite general. Does this mean that ALL other dualities, such as the Kramers Wannier duality are one ...
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### What does it mean that the laws of physics are time reversible?

The Universe, as far as we can tell, only operates according to laws of physics. And just about all of the laws of physics that we know are completely time-reversible, meaning that the things ...
39 views

### Refractive index of red and blue light

if $\frac{\sin(I)}{n}=\sin(r)$ where $n$ is the refractive index, $I$ is angle of incidence and $r$ is angle of refraction (these are relative to the normal). How come blue light refracts at a ...
82 views

### Gravitational wave and string theory

I'm new to physics and have been reading about fundamental and textbook physics text, which is the Young & Freedman University Physics (good book). I'm little skeptical towards string theory as ...