# All Questions

134,310 questions
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### Why does the classical continuous partition function blow up as $T \to 0$?

At $T = 0$, we'd expect Entropy to be zero because there's only one microstate and the $\log(1) = 0$. However, when I take the limit as $T \to 0$ in the classical canonical ensemble, it goes to ...
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### How or where I could experience sonoluminescence in daily life?

Sonoluminescence is a known phenomenon, even if a conclusive explanation for its origin is not given. I wonder if I could ever hear and see sonoluminescence somehow or somewhere in daily life, or if ...
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### $W$, $Z$ bozons and fermions gained their masses WHEN?

I have read these questions: How does the Higgs mechanism work? How does the Higgs boson give mass to other elementary particles like electrons? What is the difference between the Higgs Boson ...
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### Why use a cube as a reference volume in the kinetic theory of gases?

In basic kinetic theory we often use cubes. For example, the derivation of the ideal gas law is done with a cubical volume. Is a cube necessary for it to work? If not, wouldn't a sphere be more ...
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### Why do we subtract the voltage of the resistor from the emf voltage to find the terminal voltage?

I attached an image of a question from homework. I calculated the current in step 1) and then I calculated the voltage of the resistor. But why do I need to subtract that voltage from the emf voltage ...
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### The rise of BCI

If EEG and MEG are measuring different aspects of neural oscillations, why can't we measure the combined signals as a whole, when we are doing the same for higher frequencies? Why did the us navy ...
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### Has there been any R&D(let alone progress) in the development of gamma voltaic cells?

I ask this because it is my understanding that gamma rays form ion tracks and oxygen radicals when gamma emitting material is immersed in an aqueous solution. Cesium-137 is a very strong gamma ray ...
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### How is aerogel made? [on hold]

Aerogel is a promising material capable of a high thermal insulation. What is the production process of aerogel, is it a cheap technology?
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### Physics Elecromagnetic Induction [on hold]

A series combination of L and R is connected to a battery of emf E and negligible internal resistance; then the final value of current depends on : Options: 1. L and R 2. E and L 3. E and R 4. L, R ...
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### How to obtain the quantization of a simple pendulum using Bohr-Wilson-Sommerfeld rule? [on hold]

How to obtain the quantization of a simple pendulum using Bohr-Wilson-Sommerfeld rule?
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### How to find the efficiency of a pulley system [on hold]

Find the efficiency of a pulley system which has a mechanical advantage of 2.5 and where the lond lifts by 2.5 meters on pulling the rope by 10 meters. I have never done any problems related to the ...
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### How can you block ELF in the range of 2hz - 20hz?

If our brains are emmiting em waves(https://engineering.mit.edu/engage/ask-an-engineer/can-brain-waves-interfere-with-radio-waves/), how can we effectively shield our brains from emitting such waves?
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### Why is the orbital part of the ground state ket of two spin 1/2 particles a direct product?

In Shankar QM on page 406-407 he says that both electrons are in the lowest orbital state $|n=1, l=0,m=0>$ and have opposite spins so the orbital part of the ground-state ket is just the direct ...
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### Two types of electric field

My textbook mentions that electric field produced by electric charge and that produced by changing magnetic field are different in nature. After searching from various sources I found that electric ...
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### Downward force applied on a wedge

If a vertical (downward) force is applied on a wedge (on the sloping surface) then what would be its component in horizontal direction? It should be zero as the angle between them is 90 degrees. But ...
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### What should be the shape of an axisymmetric solar reflector that provides a uniform flux on the surface of a circular cylinder placed on its axis?

I was wondering about creating a solar concentrator with an antisymmetric concave shape that is able to focus the collected light onto a circular cylinder positioned on the concentrator axis. The ...
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### How can the exponential generator apply to all Lie groups (not just rotation)?

How can it be shown that any element of a Lie group can be represented as $A=e^{ig_A V^A}$? I think this results from the exponential map. In the case of $SO(3)$ it can be shown through the Taylor ...
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### A material point of mass m slides on a curve [on hold]

Can someone help me solve this question, please? In the Oxy plane, where the axis Oy is vertical and oriented upwards, a material point of mass m slides on the curve xy = 1, 1/2 ≤ x ≤ 2, under the ...
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### Pressure spike due to the water hammer

I'm trying to calculate pressure spike when valve is open in the depicted system. We have two volumes separated by a valve. We have pressure on one side of the valve (P1). Other side is at 0 pressure. ...
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### Resources for learning general physics? [duplicate]

The title is the entirety of my question. I could easily go on coursera or something along those lines and look up, but I want to see what people who actually know physics think is best.
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### Clarification on the Lid-Driven Cavity Problem in CFD

I need some clarifications on The Lid-Driven Cavity Problem. What does it actually mean? I know cavities are bubbles created when a fluid moves through liquid in low pressure zones, but what does the ...
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### Where does the sound we hear from short circuits come from?

Title is self explanatory. Whenever I see or watch a short circuit happening, there is always a humming/buzzing sound going on (e.g., the sounds we hear from this short circuit compilation video). ...
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### Could a non-unitary time evolution violate the no-cloning theorem?

It is written in some places that the unitarity of time evolution is what prevents quantum cloning. However, consider the typical definition of a cloning operator $A$. For all $\left|\psi\right>$ ...
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### I found a contradiction in the formula for thermal expansion [on hold]

So I was just toying around with thermal volume expansion and found something strange, not sure weather this can be called a contradiction. What assumption of mine was wrong? It turns out that Vi = V1 ...
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### Why did this bottle crumple?

These two empty 2-liter soda bottles were left forgotten in the back of a closet for three years. When they went in, they were both in good shape. They were both resting on top of something, with ...
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### Finding the Buoyancy force acting on the piece of aluminum

I have problem finding the Buoyancy force acting on the piece of aluminum. Problem to solve A 0.48 Kg piece made with aluminum is immersed in water. Find the Buoyancy force acting on the piece of ...
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### relation between distance and data rate [on hold]

if i have a good LOS for distance up to 100 km, working frequency 5500 MHZ , amplifier 30db, ant gain 34 db TX , ant gain 34 db RX , - what is the relation between data rate and distance ( if the data ...
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### Question on Partial Differentiation in Thermodynamics

For energetic fundamental relation $U=U(S,X_1,\ldots)$ where $X_k$ represent extensive parameters $V$ or $N_j$, let $$P_k=\frac{\partial U}{\partial X_k}.$$ For entropic ...
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### Sending elliptically polarised beams into a reversed beam splitter

If I send two elliptically polarised beams into the outputs of a (say 50:50) beam splitter which is at some angle $\theta$ to the $x$-axis, what sort of beam emerges? As far as I'm aware they do not ...
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### How is it possible to see a full moon at midday? [on hold]

I’m currently doing an observational study of the moon that’s been a passion of mine for the last few years. I’ve found I can see the moon all the way to almost full at about midday). Added I can see ...
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### quantum eraser question

I have a question concerning the quantum eraser: Let's assume you have two envelopes with one having the printed detector data and the other one having the printed measurement screen data, for a ...
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### Comprehensive Nature of Light: Transverse vs Longitudinal Waves and Ray Optics

----------I understand that we consider light as a transverse wave in the sense that light is composed of E&M fields oscillating perpendicular to each other and the direction of propagation. And I ...
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### How do ion thrusters create a force against the spacecraft?

I recently saw an old thread, How do reaction engines create a force against the rocket?, get bumped up, and it asks a good question: in a chemical rocket, the fact that the rocket exhaust gets ...
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### Need help with Gauss Bonnet gravity

I am trying to find the equations of motion for a coupling Gauss-Bonnet gravity. The action is : S=\int{d^4x \sqrt{-g}\left[\frac{R}{2k}+f(\phi)(\alpha R^2+\beta R_{\mu\nu} R^{\...
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### Interaction picture: why the Hamiltonian describing the dynamic doesn't change with the same law as other observables?

First: what happens in a general change of picture? If I have the following equation: $$A | x \rangle = | y \rangle .$$ To do a change of picture is to apply a unitary $U$ on all vectors of the ...
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### Kinetic term for a Majorana fermion in curved Weyl geometry

I am trying to write the action for a Majorana fermion on a curved Weyl-gravity background. Since I am considering a fermion in curved space, the tetrad formalism is appropriate and the kinetic term ...
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### What is the direction of the electricity flow in a DC circuit?

I know that in AC, the direction of the flow of electrons is constantly changing, but this question is for a DC circuit like an LED with a battery. Does current in such a circuit flow from the - side ...
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### How can be $\frac{1}{4\pi R^2}\int_{S}V_{ext}(R)da= V_{ext}(0)$ physically explained?

I was working out problem 3.1 (4th edition) of Introduction to Electrodynamics by Griffiths, which asks you for the average potential over a spherical surface due to a charge located inside the sphere ...
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### Distance of rainbow from observer

I have been reading about rainbows and I know that there is a different rainbow for each observer. Also the rainbow is centred on a line joining the head of a person to his shadow. What does it mean? ...
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### Why are symmetries in phase space generated by functions that leave the Hamiltonian invariant?

Hamilton's equation reads $$\frac{d}{dt} F = \{ F,H\} \, .$$ In words this means that $H$ acts on $T$ via the natural phase space product (the Poisson bracket) and the result is the correct time ...
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### Energy formula for finite potential well [on hold]

The energy formula for infinite potential well is $$E=\frac{n^2h^2}{8ma^2},$$where $m$ is the mass of the particle, $a$ is the width of the well but in the case of finite potential well, I actually ...
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### Infrared pass material thermal camera

I have a situation in which I need to use a thermal camera in a rain enviroment, the camera is mounted on a gymbal and can rotate 360 degrees, so I tought it may be an option to build a dome in order ...
According to everywhere work done on system of a gas is equal to $p\,\mathrm dV$ but here $p$ is just the external pressure. Shouldn't internal forces be considered while calculating work done thus ...