All Questions

9 views

problem in making hologram (3 sided) [on hold]

why the image become illusion in transparent glass ? (in making hologram).
37 views

Pauli matrices and $SU(4)$ [on hold]

together with the 2-dim identity matrix, the pauli matrices build a basis of the vector space of the 2x2 hermitian matrices. The set of i*pauli matrices build a basis for the SU(2). Assuming, we ...
37 views

Can a theoretical object exist as such that its mass overcomes gravitational collapse

A daydream the other day led me to the question if "a universe filled with water (or any other substance for that matter), instead of being mostly void" could exist within the rules of our current ...
70 views

Derivation of Euler-Lagrange equation from principle of least action

When deriving the Euler-Lagrange equation for a field $\phi$ the term $$\int\textrm{d}x^{\mu}~\partial_{\mu}\left( \dfrac{\partial \mathcal{L} }{\partial(\partial_{\mu}\phi)}\right)\delta\phi$$ is ...
12 views

Tidal heating of body

I am trying to make a plausible, though not necessarily accurate model of tidal heating of a body moving close by a gravitational attractor, not necessarily in orbit and not considering the spin of ...
9 views

Infrared Camera spectral filtering with an uncooled camera? [on hold]

I have been doing experiments with thermal cameras and stumbled upon CO2 detection. Having ordered a filter I was full of joy and immediately tried it out - nothing happened. So I talked to the ...
34 views

Thermodynamics : Heat transfer : Conduction or convection?

I have an hot object, that is place in a room of 20°C. My goal is to be able to estimate how warm the object will be in T seconds. To be able to do that, I must first understand what is really ...
39 views

Simple harmonic motion angular frequency [on hold]

I'm struggling with this physics question. "A simple harmonic wave of wavelength 16 cm and amplitude 2.5 cm is propagating along a string in the negative x-direction at 35 cm/s." a) Find its angular ...
25 views

What kind of other light sources can disturb the detection of LASER

My project objective is to detect different lasers , for example Laser Range Finder: pulse width 10ns and repetition rate is ...
100 views

Torsion-free, symmetric connection and non-coordinate basis

The torsion tensor is defined as (Hawking p.34) \mathbf{T}(\mathbf{X},\mathbf{Y}) = \nabla_{\mathbf{X}}\mathbf{Y} - \nabla_{\mathbf{Y}}\mathbf{X} - [\mathbf{X},\mathbf{Y}]. ...
22 views

Under what conditions does a beam splitter entangle two input photons?

There is a dispute on PhysicsForums related to what are the conditions necessary for two photons to be entangled by a beam splitter. Lots of references given by the forum users but they never arrive ...
12 views

How to Calculate Engine Torque from Drive Torque @ The Wheels for All wheel drive and Front wheel drive? [duplicate]

Would it be possible for someone to explain how engine torque is calculated from drive torque at the wheels and how this differs between AWD and FWD. Thanks.
34 views

How would one attach black hole to starship in the Black hole Starship model? [on hold]

A black hole starship is a theoretical idea for enabling interstellar travel by propelling a starship by creating an artificial black hole and using a parabolic reflector to reflect its Hawking ...
34 views

Majorara zero mode in Ising chain, not exactly zero subtlety

We know the transverse field Ising model with N sites(open boundary), can be mapped into N free fermions(there are 2N modes if including the negative energy counterparts) With property: ...
30 views

Intuitive understanding of vector spaces [on hold]

I want to understand the intuitive meaning of vector spaces (Hilbert, Banach, metric, normed). I have read a lot of texts and understand the mathematical formalism given but how does it correspond to ...
69 views

How do I find the relation between the accelerations of the ring and the disc (see image)? [on hold]

What is the relation between the accelerations of the ring and the disc (see image)? Both the ring and the disc have mass $M$. The ring has a radius $R$ and disc has radius $2R$. They are connected ...
24 views

Scale invariance and stress energy tensor

I have seen in a paper [1] that in a quantum field theory scale invariance takes place provided the stress energy tensor is traceless. How this is true? References: "INFINITE CONFORMAL SYMMETRY IN ...
18 views

On a AC to DC adapter there are +12v wire with 24A and -12v have 0.5A. How many amp this adapter will provide in total? [on hold]

I have a PSU of a pc which is 500w. I want to charge a 12v 100ah battery which need 20 amp of current. Is this psu can provide current that I have wanted? Main question is On a AC to DC adapter ...
896 views

Question about the apparent loophole in principle of least action

In Lagrangian formalism, given two points $(x_1,t_1)$ and $(x_2,t_2)$, we ask the question which paths $x(t)$ make the action $S=\displaystyle \int_{t_1}^{t_2}L\ \mathrm dt$ stationary and satisfy the ...
11 views

When light reflects from a medium of lower index of reflection to a medium of higher index of refraction, why does the light undergo a phase shift? [duplicate]

I learned in my physics class that there is a phase shift when light reflects off a low $n$ from a higher $n$, but never got the explanation.
34 views

Dipole moment of a single point charge

Kindly refer to the Multipole Expansion section (chapter 3) of David Griffith's Introduction to Electrodynamics. Let us first discuss about why multipole expansion is needed. As far as I understand, ...
29 views

Room with open door and heater : use inside boundary conditions or heat source for PDE?

I want to model the stationary temperature in a room (=square) that has one open door (=side) is otherwise perfectly insulated is heated through a (circular) heater that is placed somewhere in the ...
4k views

How is 6W equal to 40W in LED bulbs adverts?

Every advert I come across for LED bulbs advertise them as the equivalent of a higher W regular bulbs. This makes no sense to me, if the room requires 40W to lighten it up then it'll always require ...
36 views

About periodicity of coordinates given a metric

If I am given a metric how do I decide which coordinate is periodic? Eg. can I look at metric in plane polar coordinates and tell that θ direction is periodic. Also How do I calculate the period of ...
41 views

Difference between stiffness and damping?

I understand stiffness as the extent to which an object (e.g. a mass spring) resists deformation from an applied force, or the rigidity of an object. And I understand damping as the energy ...
21 views

What are the calculations specifically needed for High Altitude Ballooning? [on hold]

I want to know the lifting gas needed to launch the vehicle including the positive lift I want to predict the possible retrieval site I want to know the burst altitude AND MAYBE ALL WITHOUT ...
28 views

First Law of Thermodynamics and Signs

I have a question in my text book about the first Law of Thermodynamics. It gives 3 situations with the systems for each in brackets: 1) Rapidly pumping up a bicycle tyre (air in the pump) 2) ...
38 views

A question about the Fermi acceleration

In high school physics, we know that a charged particle cannot gain energy from a magnetic field. But, it seems that in the so-called Fermi acceleration, the particle is accelerated by the magnetic ...
18 views

Ratio of the change of internal energy under the specified condition given below [on hold]

What will be the ratio of the change of internal energy of a diatomic gas U1 & U2 for a unit rise of temperature in constant pressure and volume respectively.
20 views

What is an overview of secondary school astronomy textbooks for students who have taken some calculus?

I teach mathematics at the secondary level. This year my students have founded an astronomy club. Most have finished two semesters of "college-level" calculus and are familiar with the derivative, ...
13 views

Velocity matrix and non-local pseudo potentials

It is known that velocity of bloch wave functions are related to band energy derivatives: $$v(k)=\frac{1}{\hbar}\frac{\partial \epsilon}{\partial k}$$ However, in the following paper, it is given ...
35 views

Does the energy inside a capacitor has a variable dependency of the capacity?

While studying energy inside capacitors, I came up with something: If we can calculate the energy inside it as: $$U=\frac{1}{2}QV=\frac{1}{2}CV^2=\frac{1}{2}\frac{Q^2}{C}$$ Then, why in some cases ...
20 views

How does cavity resonance produce EM waves?

My understanding is it acts like a capacitor and inductor in a loop. The capacitor releases stored energy which is absorbed by the inductor through a magnetic field which then returns it to the ...
37 views

While deriving Coulomb's Law from Gauss' Law, the power of 'r' comes out to be exactly 2 while experiment shows that it differs about 10^(-16). Why?

While deriving Coulomb's Law from Gauss' Law, the power of 'r' comes out to be exactly 2 while experiment shows that it differs about 10^(-16). Why is it so? I'm referring to the book 'Physics by ...
19 views

The existence Water nanoparticles [on hold]

Has anyone investigated the properties of water nanoparticles? If yes what are those? If not, why not?
40 views

Free fall in non-uniform field

Imagine I'm a space-diver, with mass $m_1$, 500 miles above the Earth's surface at $x_i$. I want to calculate my position, velocity, and acceleration as a function of time, accounting for the Earth's ...
52 views

Questions about null geodesic [on hold]

Show for the null geodesic in 3D flat spacetime using polar coordinates so the line element is $ds^2=-dt^2+dr^2+r^2d\phi^2$. Do light rays move on straight lines? My question is that I only learned ...
85 views

Why is $np$ always equal to $n_i^2$?

For you guys who studied semiconductor physics must be familiar with the equation: $$np=n_i^2$$ I can understand why this is true for the intrinsic case (the broken bonds would always provide ...
84 views

Is meters per second equivalent to seconds per meter?

I know this question is probably ridiculous, but bear with me for a moment. This thought emerged while I was converting between nm and wave numbers ($\rm cm^{-1}$). In order to prove this conversion, ...
91 views

What does it mean by “infinities” when dealing with QFT? [on hold]

I found this PDF online here while browsing Nobel Prize winner contributions, which explains a bit about renormalization (a concept for which Kenneth G. Wilson won the Nobel). However I was somewhat ...
51 views

Force on a charged particle in a field of an EM wave

For example if there is a charge q with mass m travelling with a velocity v "in the field" of a plane electromagnetic wave that is travelling in the z direction (in free space). My question is what ...
35 views

Show the Berry phase is invariant under $U(1)$ unitary transform [on hold]

Recall that $$\gamma_n = \oint A_n(R) \cdot dR = \oint \langle\psi_n(R)|i\nabla_R|\psi_n(R) \rangle \cdot dR.$$ Under the $U(1)$ transform, $$\psi_n \to \psi'_n \equiv e^{i\xi_n(R)}\psi_n,$$ where ...
20 views

Immersing a copper sphere into a conducting liquid that is itself in a conducting bath

Question A small copper sphere of radius a is half immersed at the surface of a poorly conducting liquid of resistivity $\rho$. The liquid is contained in a spherical copper bath of radius b, where b ...
23 views

Production of entangled photons by spontaneous down conversion [on hold]

I have found that the conditional uncertainty product of position and momentum of any one photon produced in the spontaneous parametric down conversion process using a gaussian pump beam decreases ...
27 views

Newton's 2nd law for rotation (accelerated rolling and inertial frame of reference) [duplicate]

I need help in understanding why, in accelerated rolling, the center of mass must be at the origin of an inertial frame of reference in order for the second law to be applicable. Thanks!
56 views

66 views

Are there any equations in physics that will always equal 1?

I am sorry to ask such a layman question but I am really interested if there are any equations in physics that will always equal one or that will always equal a number very close to one. If there are ...
In my programming project I calculate the minimal energy of an atom with 2 electrons in the $L=0, S=0$ state, using a Hylleraas wave function. The values I find for $Z=2$ (He) and $Z=1$ (H$^-$) are ...