All Questions

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Velocity of (un)rolling toilet paper

Consider a roll of toilet paper. It consists of an inner cardboard core of radius $r_1 = 2cm$ wrapped in 5m of paper so the external radius is $r_2 = 3cm$. The paper is unrolled by holding on to the ...
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How much part of an iceberg sinks in water?

How much part of iceberg of water sinks in water. Well I know the answer is 92%, I want anyone to prove it for my homework. (water is not salty and is at 4 degree celsius)
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How do you calculate the change in location of the focal point by rotating the lense?

Image you have a lens with a certain focal length. Light will be focused into the focal plane on the Optical Axis if the light is a collimated beam which travels parallel to the Optical Axis. But I ...
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Difference between PIN photodiode and silicon surface barrier in beta spectography

In a physics undergraduate lab we performed an experiment in spectography of beta decay. We used two detectors to measure the energy spectrum of Bi207: PIN photodiode Silicon surface barrier ...
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Is drift velocity related to diameter of conductor?

Please help me out of the confusion regarding relation of drift velocity of electrons with dimensions of conductor .
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Water Moving Through Coffee Grinds

Like many of my peers, I rely on coffee as if it were prescribed to me. It might even be classified as an addiction if it weren't for the cultural norms surrounding a cup of coffee or tea in the ...
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Deriving Rabi oscillations using the Heisenberg picture of QM

The semiclassical treatment of an simple two level atom in a resonant electromagnetic field is usually done in the Schrodinger/Interaction picture of QM, by assuming that the wavefunction of the atom ...
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If Black holes are maximal entropy how can they evaporate?

According to Hawking/Bekenstein a black hole represents the highest amount of entropy for a given volume, (actually the entropy is related to the surface area of the black hole but the fact that they ...
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Electromagnetic waves and photons?

Electromagnetic waves are photons or photons cause electromagnetic waves ? Its said that when charges are accelerated we get electric and magnetic field that carries energy but then they say that this ...
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Photon absorption in a medium - basic principle?

I am trying to understand the basic physics of photon absorption in a medium. Take the example of oxygenated hemoglobin (red curve). Clearly at different frequencies/energies relatively more or less ...
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Calculate the speed of Halley's comet at his perihelion

Knowing that Halley's comet orbita the sun every 75,3 years, that the semi-major axes is 17.83 AU, the semi-major axes is 4.53 AU and at his perihelion he is 0.586 AU away from the sun. With these ...
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Why are black holes black? [duplicate]

If heard two explanations for this. One explanation is that the gravity is so strong that space is being stretched inward faster than the speed of light and thus no photons could possibly escape. The ...
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Calculate Temperature change in a solid due to inductance heating

Trying to calculate temperature increase in a solid due to a current flow, is this along the right lines? First I have calculated resistance due to resistivity, then calculated power from the given ...
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Inductance heating, Temp Change

I am trying to find the temperature increase of a solid(copper) in a gas (air) due to an electric current passing through the solid (inductance). I need to encompass any temperature losses from the ...
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Why the motion of rotating magnet doesn't stop if we use a superconducting wire?

I would like to know why magnets rotated,wouldn't stop their motion if we place a superconducting wire as said by William Beaty at MadSci Network.
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Where is the mistake in the following rationament

Well... kind of hard to translate in English so bare with me :). Let's consider a wheel that spins in the void. Each point of the wheel has the speed $v = ω r$. That means that for any $ω$, there is ...
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Guide for a program that calculate operator equations

Is there any program that calculate operators equations just with having operator commutator relations? For example if $A$ and $B$ are two operators and $$[A,B] = m$$ that $m$ is a number this ...
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Why does holding an object cost energy while no work is being done?

I was reading the discussion here: Why does holding something up cost energy while no work is being done? I feel as though the question is being avoided. Suppose instead of holding an object by hand ...
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How electromagnetic waves carry energy?

Its said that electromagnetic waves carry energy. Is this because these waves are made up of electric and magnetic field which can cause changes to the stuff that falls with in their range ? That is ...
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Entropy always increases in a closed system - what if the universe is open?

An interesting question I was asked: Entropy always increases in a closed system - what if the universe is open? Does that mean that entropy can decrease in such a system? Of course, I think there is ...
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Relation between interlayer tunneling amplitude and external potential in Bilayer graphene

What is the relation between interlayer tunneling amplitude ($\gamma_1$) and external potential applied ($U_{ext}$)? Is there any mathematical relation?
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Heisenberg's uncertainty principle derivation in a ring

The standard derivation But now suppose the space is a ring of length $L$, it seems the derivation could work out exactly the same and we get $$\Delta p \Delta x \geq \hbar/2.$$ But since $\Delta x$ ...
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What causes the screening length of metals to be so small?

I am currently studying semiconductors and metal junctions. Most of the journals I have come across mention that the screening length of metals is small. As a result, the bands of the metal do not ...
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In an RLC series circuit let applied EMF be given $V=V_0\sin\omega t$, $$Z=Z_C+Z_R+Z_L=R+i\left(\frac{1}{\omega C}-\omega L\right)$$ $$|Z|=\sqrt{R^2+\left(\frac{1}{\omega C}-\omega L\right)^2}$$ Then ...
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Standard-model flavor symmetry

If we consider the chiral Lagrangian after the spontaneous symmetry breaking, we have got fermion masses and Yukawa couplings to the physical Higgs boson. So it follows global symmetries in flavor ...
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Is the momentum of a particle both uncertain and, independently, also random?

Is momentum of a particle "random" because it is uncertain, or is it uncertain in addition to being random? Is the uncertainty principle and quantum randomness different names the same physical ...
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Can bending spacetime produce electromagnetic fields?

Let's say that we can somehow bent spacetime, can this produce electromagnetic fields? or EMF can only be emitted from charged particles of matter?
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Eigenvalues, Eigenvectors [on hold]

what is the physical meaning of eigenvalues and eigenvectors? Is there any relation between them and the energy states if there is any reference please provide it
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Conflicting answers gained from different equations

First time posting I hope this is the place. if someone could help me sort out formatting for the equations that would be nice "As a result of being hit from behind by a toy truck, a 50g toy car ...
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Does curved spacetime arise from inhomogeneity of gravitational field?

In general relativity textbooks such as Sean Carroll's Spacetime and Geometry, there is often a line of reasoning that goes like this: Strong equivalence principle states that free falling frames ...
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Find the acceleration of the bead [on hold]

Two identical, uniform large rings, each of mass $\text{m}$ are connected through a bead of same mass, which can move freely. When bead is released, it starts sliding down. The large rings roll ...
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Are symmetries of a degenerate ground-state manifold always broken?

If a Hamiltonian has a global symmetry and a degenerate ground state, then in the thermodynamic limit, the ground states $| \psi \rangle$ that are eigenstates of the symmetry operator typically become ...
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Pressure required for gas to flow through an orifice

Imagine I have an upside-down cone, which is filled with a gas - let's say it is air. At the bottom of this cone is an extremely small hole (a few microns in diameter). How do I go about calculating ...
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Compton Effect Demonstration using MATLAB

I have recently been taught about Compton Effect and I learnt about the Intensity variation with the wavelength . Now I also know a little bit about MATLAB so I was wondering if I could create plot ...
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Antimatter and Annihilation [on hold]

I have two proposition, both pertaining to the physics behind antimatter's annihilation with matter. My first proposition is simpler: Given an atom of Helium (Two Protons, Two Electrons), and an atom ...
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Gravitational collapse (conservation of energy)

During gravitational collapse, gravitational potential energy of the gas is converted to its internal kinetic energy so the internal energy of the clous of gas is said to be increased But isn't ...
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Evaluating a function with divergent probability outcomes

I apologize for any vagueness here but i am trying to learn a general solution for a problem I am working on for fun. Getting the final answer is therefore not my goal but getting unstuck is. Alsi of ...
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How would I find the velocity of an object after it doesn't fully enter the gravitational pull of a planet and is kind sling shot out of its gravity?

Obviously that question doesn't have a lot of jargon having to do with math and physics. I am not a physicist, I'm not a mathematician, I'm trying to make a game and simulate some physics found in ...
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translational invariance property of the Feynman propagator

I saw a translational invariance property for the propagator from a lecture note, which is: $$G(q_3,T_2+T_1;q_1,0)=\int{d{q_2} {G(q_3,T_2;q_2,T_1)G(q_2,T_1;q_0,0)}}$$ it says a particle propagate ...
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Vibrating string - wave speed and tension relation

I was studying vibrating strings and in my teacher's notes I found that, generically, if I change the tension on the string by $\Delta T$ then, the speed percentage change can be written as: ...
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Generalized Eigenvalue Problem from linear stability analysis

I am trying to solve a generalized eigenvalue problem raised by linear stability analysis $$AV=\lambda BV.$$ $A$ and $B$ are non-symmetric complex valued matrices. The set of equations I am trying to ...
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Non-linear friction tsunami modelling

Can you proof the following equation  \frac{\partial u}{\partial t} + u \frac{\partial u}{\partial x} + g \frac{\partial \eta}{\partial x} = - \frac{k}{2 h \left( x \right)} u \left| u \right|\\ ...
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Why the incessant confusion between diffraction and interference?

Feynman said, "No one has ever been able to define the difference between interference and diffraction satisfactorily. It is just a question of usage, and there is no specific, important physical ...
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Protecting astronauts from G's when taking off/landing

When landing from orbit or launching from the ground to orbit (with chemical rockets or other means of fast acceleration), could one place the astronauts in a centrifuge and spin it to protect them ...
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Refraction of light rays

My textbook says that the greater the refractive index of a medium, the lower the speed of a light ray passing through it. Seawater has a slightly larger refractive index than fresh water. Light ray ...
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Is the distortion of spacetime caused by frame dragging continuously changing, or fixed?

Consider a very heavy star and let´s take a look at the distorted spacetime accompanying the star. Does the spacetime gets increasingly wind up, or is it stationary? If it´s not stationary, then we ...
After taking the second derivatives of both $u(r)$ and $v(r)$ I'm stuck.