All Questions
235,023
questions
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Why can't we measure different properties of an electron in accordance with its frequency?
sorry if this is a stupid question but its my first one.. Why cant we just observe and electron in accordance to its frequency. Like during one frequency peak we could observe position, next could be ...
- 1,464
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Physics Principle of Balancing Board
Consider one of these
Typically, it is well known that having you legs further apart helps you balance better, the wider the base, the easier it is to balance. I guess a body with a larger base area ...
- 202k
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3
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How do vacuum bubbles "dress" terms in the $S$-matrix numerator?
I am self-studying QFT using the book "A modern introduction to quantum field theory" by Maggiore. On page 124-125 he's doing the calculation in the interaction picture for a process with ...
- 202k
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Relative acceleration (urgent) [closed]
Two car travelling towards each other on a straight road at velocity
10 m/sec and 12 m/sec respectively. When they are 150m apart, both drivers apply their brakes and each car decelerates at 2m/sec2 ...
- 202k
2
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1
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How do the Navier-Stokes equations predict vortex creation in a vorticity-free fluid?
For this question I was looking at the Navier-Stokes equations after you get the curl, this gives you the equation:
$$\frac{\partial \vec{\Omega}}{\partial t}-\nabla \times (\vec{v} \times \vec{\...
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Where does "excess" energy go when a vehicle uses a lower gear to move at a given speed?
So I understand that a lower gear as advantageous when accelerating a vehicle or when driving up hill (e.g because you are having to accelerate against gravity). I know this is because of the ...
1
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3
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Does Bell’s theorem rule out changing hidden variables?
In the EPR experiment, it is said that there are predictions that are made if there are hidden variables determined by something locally in each particle that results in a correlation in entanglement. ...
- 3,386
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Motivation behind reflection positivity
I have taken a look at this physicsSE question, Wikipedia, and this paper by Jaffe which go over reflection positivity. While they all nicely explain the definition behind reflection positivity and ...
- 202k
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How to calculate the force required to create the swivel motion in a caster wheel?
Consider a table/cart with caster wheels mounted on all the 4 legs. The wheels are currently oriented at an angle theta. (Later I'll have to find the same in case of 90°).
The push force will be ...
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2
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52
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What is the difference between temperature and internal energy of a gas?
I came across this doubt multiple times while studying thermodynamics.
Aren't both temperature and internal energy fundamentally due to the kinetic energy of the molecules? An even crazier doubt of ...
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1
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Fermi-Dirac Distribution for Multiple Species
If I have a system containing two types of fermions, what is the probability of a state of energy $E$ being occupied? Is it just the sum of two standard Fermi probabilities for each type of fermion?
- 5,227
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Some concepts do not understand about torque and angular momentum
These are the knowledge points I got from online video.I finally found that there were some things I didn’t understand.
$\sum_n F_iR_i=\tau=I_{axis}a_z$,I learned from the previous video learning that ...
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To find the maximum range for a projectile launched from one end of the inclined plane [closed]
A plane surface is inclined making an angle θ with the horizontal. From the bottom of this inclined plane, a bullet is fired with velocity $v$. Then prove that the maximum possible range of the bullet ...
- 438
2
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2
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313
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Does using thinner glasses have any effect on how the eye look?
My daughter asked me to buy thin glasses for her eyes. So the power of the lenses are the same, but the glasses are thinner because the glasses use plastic with higher refraction index.
Latter I found ...
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0
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Need solution for a rotational motion problem on elevated and depressed track [closed]
[![Question][1]][1]
I am experiencing a problem in this question. Initially I thought the answer is supposedly 5 seconds(After all the calculations). However the answer and solution is entirely ...
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0
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Why are zero-modes preserved on turning on coupling in twisted bilayer graphene model?
In the paper https://arxiv.org/abs/1808.05250 on page 3, they talk about how when the parameter $\alpha$ in the Hamiltonian in Eq. (5) equals $0$, we get zero modes at the Dirac points K and K’. This ...
- 202k
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Dispersion relation with damping force
We consider a linear chain of atom connected by springs with constant $K$. We have the usual elastic force and we add damping force such that the dispersion relation is:
$$
\omega = 2\sqrt \frac K m \...
- 202k
18
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8
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Do we understand chemistry from particle physics? [closed]
My chemistry knowledge is of a high-school level. In high-school, the properties of atoms were mostly presented as empirical phenomena. We learned some physical principles such as the idea that ...
- 10.4k
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Lens in tilted image formation [closed]
Consider a lens and on this lens rays are incident parallel to the principal axis now if the lens is slightly tilted by an angle theta then by how much does the focus shift?
- 12.1k
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Why there is steady DC current in a metal under static $E$ field?
One explanation for a steady current formation under $E$ field is due to a shift in Fermi-sphere: in equilibrium, there are same number of carriers of velocity $v$ and $-v$, and therefore the net ...
- 202k
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Fictitious forces in generalised coordinates (e.g. polar coordinates) with relation to tangent bases
I came across the claim on Wikipedia here and here that there are two separate definitions or uses of the term "fictitious force", in particular the centrifugal force - one related to ...
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1
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Why the wavefunction of a set of non-interacting particles is a product of the individual wavefunctions of each particle in the set?
In Quantum mechanics, before we get to Slater determinants and all that, it is said that many-particle wavefunction for the case of non-interacting particles is just a product of all the particles' ...
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What is the observable for the optical field?
Typically, observables in quantum mechanics are associated with Hermitian operators. However, Glauber argues in 1963 ([1]) that the electric field operator $\hat{\mathbf{E}}(x,t)$ is not the relevant ...
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Can a single lens have a fixed focal point that does not shift in the focal plane when the angle of incidence changes?
Can a single lens produce a fixed image that does not shift in the focal plane when the angle of an incident light beam changes?
For normal simple lens, the point that an incoming parallel beam of ...
- 3,386
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1
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Thought experiment: EM fields and coils
Hello Physics Exchange,
I have been thinking a lot lately about EM fields. I thought about something the other day and wanted to check with the experts if my intuition is correct or not. I know this ...
- 27.6k
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In Maxwell’s Fluid Analogy, is the Magnetic Field the “Wake”?
Does Maxwell's analogy of incompressible fluids extend so far that the magnetic part of a wave created by a moving electric charge is like the "wake" of a moving body through fluids, only ...
- 2,306
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3
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380
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Physics of devil's barrel
The physics concepts here are inertial and non-inertial frames of reference. Also rotational equilibrium.
On the picture, you can see a person performing the stunt in so called "devil's barrel&...
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Physical Interpretation of BEC Formation
I have recently worked through a theoretical argument for why Bose-Einstein Condensation occurs. If we are comparing an ensemble of distinguishable versus indistinguishable particles, we can compute ...
- 1,223
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0
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13
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Relation between velocity distribution function and microstate numbers in statistical physics
I know Boltzmann's $H$ is proportional to the entropy ($S$) with the sign difference. And, I know about the entropy $S$ is proportional to the logarithm of inverse probability of microstates appearing ...
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2
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75
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Can you reason the equivalence principle from SR?
So for the past few days I've been reading on "why is special relativity incompatible with gravity?".
Most answers are something along the lines of "special relativity doesn't have ...
- 3,386
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3
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173
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Equations of motion for a force in special relativity seem to require a differential equation — in this context, what does Kinetic Energy *mean*?
I was faced with a situation where I suddenly realized that although Kinetic Energy in Special Relativity is defined as
$KE=\gamma m_0 c^2 - m_0 c^2$
The work energy theorem says
$\Delta KE +\Delta ...
2
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1
answer
39
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Problem from Electromagnetic Induction of IE IRODOV
According to Faraday's law, a change in the flux associated with loop generates an EMF in the loop and when we are bringing the loop into the region containing field obviously the flux associated with ...
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Why is entropy a state function in Joule expansion? [duplicate]
Consider Joule expansion, where a $1$ mole gas initially with parameters $P_0, V_0, T_0$ expands freely to a volume $2V_0$. We say that $\delta S = R\ln 2$ by claiming $S$ is a state function and thus ...
- 202k
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1
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How would the field in between two semi-infinite placed next to each other look like?
Suppose two semi-infinite solenoids placed next to each other are separated by a small gap as shown below
If the solenoids are identical to each other, What would you expect the field in between the ...
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Correlation functions zero for repeated creation or annihilation operators
In the derivation of the damped harmonic oscillator, at some point, see for instance here one has to compute certain correlation functions for the bath of harmonic oscillators in thermal equilibrium. ...
- 20k
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2
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Converting laser irradiance into volume energy density
If I want to convert a laser's irradiance of $10^{20} \,\text{W}/\text{cm}^2$ into a volume energy density, do I just divide it by the speed of light $c$?
- 33.7k
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Coils in parallel
Lets say we have 2 coils in parallel, hooked up to a voltmeter. A permanent magnet is then pushed in and pulled out of the first coil, in such a way that there is still flux through the second coil, ...
CommunityBot
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Net flux through insulating cylinder
In a non-uniform electric field, say with field lines curving away from each other along x, will the net flux on a cylinder be negative or zero?
CommunityBot
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Do non-quantized values of the Zak phase mean anything?
We know that in 1D models, the Wannier centers ($W_c$) are related to the Zak phase as $$W_c = Zak \cdot \frac{a}{2\pi}$$ for a unit cell of length $a$.
My question is related to special inversion ...
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2
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46
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Why is a bigger camera sensor better at low light than a smaller one?
People say a bigger camera sensor is better for light performance as the photo sensors are bigger and hence let in more light. But couldn't you just let the same light in and focus it down onto the ...
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2
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Given gravitational lensing, why would black holes visually appear black?
This image from Wikipedia, also referenced in this question, is an artist's vision of how a black hole should look like:
However, given that a black hole is a gravitational lens strong enough to ...
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Is a fermionic boson possible?
We know that bosons need an overall symmetric wavefunction. So is it possible for a boson to have an anti-symmetric spatial wavefunction and an anti-symmetric spin wavefunction? Such that upon ...
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557
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Converting $\vec{E} = - \nabla \phi$ into spherical coordinates
Let's say I have an electric field $\vec{E} = (0, 0, E_z)$, where $E_z$ in constant. Then the electric potential is $\phi = - \vec{E} \cdot \vec{r}$, where $\vec{r} = (x, y, z)$.
Calculating $\vec{E} =...
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Help in demonstrating formula for internal pressure in a curved pipe [closed]
I know this question may seem very odd for most of you. Also I hope it doesn't fall under the banned "Do my homework" type. I'm not looking for an answer I just want advice. Also sorry for ...
- 8,921
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3
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129
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Basic doubt in quantum mechanics
Do entities like electrons, which are considered point particles in Classical Mechanics, actually have a definite position at a particular time (irrespective of it can be measured or not)?
- 20k
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345
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How do "air nozzles" in airplanes concentrate air flow so well?
I'm working on a project that requires concentrating air flow into a beam such that it can travel longer distances through an unconstrained medium (i.e. open space, not a hose).
Something that came ...
CommunityBot
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2
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Is $T^{\mu\nu}=-\frac{2}{\sqrt{-g}}\frac{\delta S_m}{\delta g_{\mu\nu}}$ a true tensor or a density?
The energy-momentum tensor is defined by
$$T^{\mu\nu}=-\frac{2}{\sqrt{-g}}\frac{\delta S_m}{\delta g_{\mu\nu}}$$
where $S_m$ is the matter action
$$S_m =\int d^4x\sqrt{-g}\mathcal{L}_m$$
and $\mathcal{...
- 202k
3
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4
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684
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In this conservation of momentum problem, where is the sign error coming from?
Say I have a particle travelling in the $x$-direction and breaking into two pieces:
Conservation of momentum in the $x$-direction obviously gives:
$$mv = m_1 v_1 \cos(30) + m_2 v_2 \cos(60).\tag{1}$$
...
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1
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How this equation arises in free body diagram of rolling motion? [duplicate]
Concerning rolling motion for a sphere (without slipping)
I have read somewhere that here solving using free body diagram we can write$$F-f_r=ma_{com} \space\space\space\space(1)$$ where $a_{com}$...
- 202k
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1
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Derivation of transformation law for the Hilbert Stress-energy tensor [duplicate]
The Hilbert stress-energy tensor is defined as
$$T_{\mu\nu}=-2 \frac{1}{\sqrt{g}}\frac{\delta S_M}{\delta g^{\mu\nu}}.$$
Given the name one expects that it transform as a tensor, but how to prove this ...
- 202k