All Questions

25
votes
2answers
2k views

Why didn't Lorentz conclude that no object can go faster than light?

Based on Lorentz factor $\gamma = \frac{1}{\sqrt {1-\frac{v^2}{c^2}}}$ it is easy to see $v < c$ since otherwise $\gamma$ would be either undefined or a complex number, which is non-physical. Also, ...
0
votes
2answers
40 views

Why will a guitar string “ring” when I resonate my vocal chords at the same pitch as the string?

From a resting state, a guitar string will begin to ring as I resonate my vocal chords. If I make a fairly loud, and high-frequency "oo" sound at 440hz, then the A string on my guitar begins to ...
-1
votes
1answer
17 views

Direct current electric circuit

Why when I reverse one of the three batteries in a torch light, the bulb does not light up? The three batteries (one of them is reversed) and the bulb suppose to form a closed loop. According to ...
5
votes
1answer
37 views

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 ...
0
votes
1answer
12 views

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 ...
0
votes
1answer
36 views

$W$, $Z$ 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 ...
1
vote
1answer
20 views

Why use a cube for deriving kinetic theory of gases

Is it because it simplifies it or its more symmetry Aren't things more easy with spheres?
1
vote
1answer
18 views

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 ...
0
votes
0answers
21 views

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 ...
0
votes
0answers
11 views

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 ...
0
votes
1answer
25 views

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?
-3
votes
0answers
15 views

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 ...
2
votes
1answer
29 views

How to obtain the quantization of a simple pendulum using Bohr-Wilson-Sommerfeld rule?

How to obtain the quantization of a simple pendulum using Bohr-Wilson-Sommerfeld rule?
-2
votes
1answer
29 views

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 ...
1
vote
0answers
19 views

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?
0
votes
0answers
8 views

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 ...
0
votes
1answer
37 views

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 ...
0
votes
1answer
33 views

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 ...
0
votes
0answers
14 views

Global $U(1)$ Symmetry in GSW Model

Consider the theory of a single generation of $e, \nu_L$ matter content. The initial lagrangian is $$ \mathcal{L} = i\bar{\ell}\not \partial\ell + i\bar{e}_R\not \partial e_R \tag{1} $$ where $$ ...
0
votes
1answer
32 views

Does time dialation cancellation occur between two observers moving in opposite directions

This is the question that has been on my mind lately: If an object A is moving away from an object B which is presumed stationary by A, then the time observed from the perspective of object B will ...
-4
votes
0answers
32 views

How to convert $\ kWh/m^2 $ to $\ Joule/kg$. Comparing sun radiation over a location with the radiation produced by a CT scan

How to convert $$\ kWh/m^2$$ to $$\ Joule/kg$$ I want to compare the radiation produced by the sun over a location (where I live in, measured in $\ kWh/m^2$), to the radiation a person gets from a ...
1
vote
0answers
13 views

What are inelastic elastic-bands?

In the third episode of Julius Sumner Miller's physics series he sets up an experiment with two masses connected by an array of elastic-bands. He comments that the elastic-bands are inelastic. ...
2
votes
4answers
75 views

How to interpret this construction of the states in QFT?

Non-Relativistic Quantum Mechanics To make this question clear it might be useful to contrast with non-relativistic quantum mechanics. In any quantum theory, the states of a system are unit rays in ...
0
votes
3answers
42 views

harmonic oscillator position expectation value

I'm trying to get the expected value as a function of time for the position, of a harmonic oscillator hamiltonian and a state vector $|\psi\rangle=a|0\rangle+b|2\rangle$. I have $$|\psi(t)\rangle=...
2
votes
0answers
15 views

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 ...
1
vote
1answer
44 views

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 ...
-1
votes
0answers
32 views

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 ...
1
vote
0answers
31 views

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. ...
-1
votes
0answers
32 views

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.
1
vote
2answers
24 views

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 ...
5
votes
1answer
59 views

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). ...
4
votes
0answers
42 views

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 $\left|\psi\right>$ ...
-1
votes
0answers
47 views

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 ...
2
votes
0answers
71 views

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 ...
0
votes
0answers
20 views

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 ...
-3
votes
0answers
11 views

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 ...
1
vote
1answer
52 views

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 \begin{equation}P_k=\frac{\partial U}{\partial X_k}.\end{equation} For entropic ...
0
votes
0answers
24 views

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 ...
-3
votes
1answer
86 views

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 ...
-2
votes
1answer
39 views

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 ...
-1
votes
1answer
22 views

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 ...
3
votes
1answer
70 views

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 ...
2
votes
0answers
51 views

Need help with Gauss Bonnet gravity

I am trying to find the equations of motion for a coupling Gauss-Bonnet gravity. The action is : \begin{equation} S=\int{d^4x \sqrt{-g}\left[\frac{R}{2k}+f(\phi)(\alpha R^2+\beta R_{\mu\nu} R^{\...
0
votes
0answers
38 views

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 ...
2
votes
0answers
23 views

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 ...
-1
votes
2answers
46 views

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 ...
1
vote
2answers
66 views

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 ...
1
vote
0answers
35 views

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? ...
7
votes
2answers
95 views

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 ...
0
votes
1answer
46 views

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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