0
votes
2answers
263 views

Significance of $\pi$ in physics

We all know this magical mathematical constant. My question being , how and why pi just shows up in every other physics derivation or formula or even statistics for that matter . http://en....
1
vote
1answer
67 views

MHD Flow in a channel with and external B field and circuitry determining E. Different circuits and Ohm's law

I was wondering if someone can explain why E is the way it is in cases 2,3,4 in page 9 of these notes ? In case 2 "Short Circuit", do I just have to assume that for a perfect conductor E = 0 for ...
1
vote
2answers
76 views

Which modes are excited when a drum is struck?

I've been searching quite extensively for an answer to this question but I cannot find anything definitive. The most I can see is that "one or several" modes become excited, but this is never ...
3
votes
4answers
3k views

Is an electron/proton gun possible?

In the 1944 SF story “Off the Beam” by George O. Smith, an electron gun is constructed along the length of a spaceship. In order to avoid being constrained by a net charge imbalance, it is built to ...
0
votes
1answer
314 views

Basic Assumption in the Time Dilation Equation Derivation

I am familiar with the way the time dilation equation is derived since I was introduced to it in my sophomore college physics class. It's the standard derivation of a stationary observer watching a ...
-1
votes
3answers
505 views

Can a component of vector be greater than the vector itself?

...we have at our disposal an infinite variety of ways of resolving a given force into components. . . . The fact that any component may happen to be larger than the vector itself doesn't invalidate ...
1
vote
0answers
73 views

Understanding Tensor Product

Consider the operator $$e^{-i \hat{H} t/\hbar} = e^{-i (\hat{P} \otimes \hat{X}) t/\hbar},$$ where $\hat{X}$ and $\hat{P}$ are position and momentum operator of two different systems. We know that the ...
0
votes
1answer
178 views

If kinetic energy is equal to force x distance, does that mean a mass traveling in the vaccum of space keeps getting kenetic energy

This is something I don't understand about it because if the speed remains constant, nothing changes about the objects energy.
1
vote
2answers
723 views

Are sound waves adiabatic or isothermal?

I am doing a presentation on sound waves and I need to know if they are adiabatic or isothermal. I know that they can generate heat, but is the amount of heat created so small that it can still be ...
1
vote
1answer
51 views

Is this cosmological scenario possible?

Is it possible that the universe is infinitely large and contains an infinite amount of mass that is distributed in such a way that gravitational force is never infinite? If so, is it possible that ...
1
vote
2answers
50 views

What dynamical system could this $\dot y = \alpha(y-\lambda), y\geq \lambda$ equation describe?

Just out of curiosity, can anyone identify electrical, mechanical, chemical, etc process that is governed by a differential equation of the form $$\dot y = \alpha(y-\lambda), y\geq \lambda$$ where ...
1
vote
3answers
375 views

Is the work-energy theorem valid for only particles or rigid bodies as well?

Is the work-energy theorem valid for only particles or rigid bodies as well? Most places where I have read this seem to claim the latter. But an example I thought up has been troubling me. Consider ...
2
votes
1answer
75 views

Estimate the maximum distance of water spatter

There is a faucet dripping water from height $H$ onto a smooth, horizontal ceramic surface, and I need to give a upper bound of the maximum distance between the water spatter and the point directly ...
3
votes
3answers
710 views

How much energy is needed to create an electron?

I know how to calculate the electrostatic energy of a sphere (it has a well defined radius). But how can I calculate the electrostatic energy of an electron as it is a point particle? By electrostatic ...
0
votes
1answer
834 views

Emissivity of vacuum for radiation heat

What is the heat radiation emissivity of vacuum? For air as well? What is the difference? I understand that the vacuum has reflectivity of 0, So what is the other two values are in $$ reflectivity + ...
0
votes
2answers
71 views

How do we prove that two conjugate operators $X$ and $Y$ induce $\sigma_x$ and $\sigma_y$ driving terms when restricted to a two level subspace?

Suppose I have a Hamiltonian for a particle moving in a one dimensional potential $$H = H(X,Y) \qquad [X,Y] = i$$ where $X$ is the dimensionless position, $Y$ is the dimensionless momentum, and $\...
6
votes
2answers
423 views

Which collision energy at LHC is better for hunting 125 GeV Higgs, 7 TeV, 8 TeV or 14 TeV?

Increasing collision energy in hadron collders doesn't always improve the abiilty to hunt down the Higgs. I know that if the Higgs mass is just above LEP exclusion, then even 7 TeV is too high to be ...
1
vote
2answers
236 views

Exotic quantum observables

Quantum physics inherited its physical quantities from classical mechanics (energy, momentum, etc). Each quantity is associated with Hermitian operator, but there seems to be a lot more Hermitian ...
3
votes
2answers
184 views

How do you determine the “phase” of a hydrogen eigenfunction?

I've been reading the wikipedia article on the atomic orbitals of hydrogen. They have a nice collection of diagrams, such as this one for n,l,m = 3,1,1 This is apparently showing the wavefunction, ...
0
votes
1answer
100 views

Load resistance adding with minimized power and voltage constraint [closed]

Consider the simple circuit below: Suppose $V$ and $R$ have set voltage and resistance values respectively, and you are to choose the resistance values for $R_1$ and $R_2$. However, you must choose ...
0
votes
0answers
66 views

How would an observer feel the Einstein Thirring Lense Effect?

The Einstein Thirring Lense Effect, also known as Frame Dragging, is what happens when cellestial bodies have rotation. It states that when a body of mass is rotating around an axis it drags space and ...
3
votes
1answer
100 views

Naturalness of tensor fields in general relativity?

In the third chapter of the book The Large Scale Structure of Space-Time, the authors say regarding the matter fields in general relativity: These fields will obey equations which can be expressed ...
1
vote
1answer
80 views

will frictional force work against the vertical direction of velocity of an object?

We assume that an object is placed on a horizontal plane. If this object is accelerated to vertical direction, will frictional force act against the vertical direction of velocity of object? For any ...
1
vote
1answer
200 views

Why can't we run through walls like Harry Potter? [duplicate]

This is a very simple and stupid question but ever since I watched the first Harry Potter and saw Harry and his friends run through a wall at King's Cross Station I have wondered what keeps us from ...
2
votes
1answer
55 views

Does the payload of an Alcubierre drive have to be on board during the preparation phase?

I was thinking about the practical aspect of using an Alcubierre drive, assuming one existed. I'm no expert, but my understanding is that, since the destination has to be in the forward light-cone of ...
0
votes
1answer
72 views

Are end points of an open bosonic strings orthogonal to D-branes?

I'm apologizing for such simple question, I just started to learn string theory. If one end of an open bosonic string is attached to a Dp-brane it means that in directions normal to the brane ...
4
votes
1answer
310 views

How does the notion of topological order relate to the Landau-Ginzburg theory of phase transitions?

As per Landau-Ginzburg (LG) theory, we write down a theory (Hamiltonian) with all possible interactions/operators (in terms of some order parameter) that respects certain symmetries. The ground state (...
0
votes
0answers
60 views

Can someone explain how to do a functional integral through an example?

I want to learn how to do path integrals as used in quantum field theory. I understand the specific type of integral used in qft is called a functional integral. I looked at Wikipedia and checked in ...
1
vote
1answer
415 views

What is the significance of Planck's constant in De Broglie–Bohm theory or Pilot-wave theory?

In standard QM, Planck's constant seems to be a constant that describes the smallest quanta of energy in some way. Does De Broglie–Bohm theory have an alternate interpretation of that constant and ...
3
votes
4answers
409 views

How can we prove that a photon is absorbed only once?

When I first heard about the photons and the double-slit experiment my immediate thought was the following: Alright, energy is not absorbed continuously but in discrete units, photons, but nature ...
9
votes
1answer
201 views

Group notation $\otimes$ and $\oplus$ used for representations of quarks and mesons

I've been trying to figure out this statement from the PDG quark model summary (PDF). Following $\mathrm{SU}(3)$, the nine possible $q\bar{q}′$ combinations containing the light $u$, $d$, and $s$ ...
0
votes
0answers
39 views

Electric dipolar moment of a half wave antenna

We have an antenna ($\lambda = L/2$), of length $L$ with a current density: $$\vec j=I\sin [k(|z|-L/2)]\delta(x)\delta(y)e^{-i\omega t}\hat e_z$$ I want to calculate its electric dipole moment. I ...
3
votes
2answers
2k views

How can I explicit the energy dependence of the Maxwell-Boltzmann distribution?

I'm having a bit of a problem figuring out the energy dependent Maxwell-Boltzmann distribution. According to my book (Ashcroft & Mermin) they write the velocity dependent distribution as: $${{f}...
1
vote
2answers
268 views

Why aren't the energies of two systems in thermal equilibrium fixed?

In the derivation of the Boltzmann distribution they consider a system $A$, enclosed by a diathermal wall in a heat reservoir $R$. Then they calculate the probability that the system $A$ is in an ...
6
votes
3answers
1k views

Why do we still not have an exact definition for a kilogram?

I read that there is an effort to define a kilogram in terms that can exactly be reproduced in a lab. Why has it taken so long to get this done? It seems this would be fairly important. Edit Today I ...
0
votes
1answer
146 views

Are the value of velocity and height constant for geostationary satellite?

We know that generally for geostationary station T=24 hours, h=3.6x10⁴km, R=6400 km, and velocity is 3.1 km/s. I think this values are stationary for geostationary satellite. Let we want to place a ...
2
votes
1answer
58 views

When I'm in a warm room how much of the warmth I feel is due to radiation from surfaces?

Say I'm in a room heated to 23C, once the temperature is in balance all the objects will also be 23C and radiating heat towards me. How much of the warmth I feel is due to this radiation, and how much ...
5
votes
4answers
5k views

Why does grinding steel create sparks but aluminium doesn't?

Grinding steel produces sparks. Grinding aluminium does not. Why? I found this article about why grinding steel produces sparks but would like to confirm if it is true. I would also like to know why ...
0
votes
1answer
41 views

isotropy of 3-space and spacetime metric

The most general spacetime metric is given by $$ds^2=g_{\mu\nu}dx^\mu dx^\nu=c^2dt^2+g_{0i}dtdx^i-g_{ij}dx^i dx^j$$ Why does the second term said to violate isotropy of 3-space? It is true that, ...
4
votes
4answers
1k views

Accuracy of the Boltzmann equation

I have had this question for some time now. Hopefully someone can answer it. I know that the Boltzmann equation is widely regarded as a cornerstone of statistical mechanics and many applications have ...
1
vote
2answers
53 views

Particle motion characteristic

I'm making a particle motion raffling normal numbers. The normal random numbers raffled are the angles of the directions that the particle is going. The particle speed is constant. Look how this is ...
6
votes
0answers
619 views

Entanglement entropy and area law

I am currently reading a review "Area law for the entanglement entropy" by Eisert, Cramer and Plenio (2010). From what I understand: In one dimension, for local gapped models, we have an area law ...
3
votes
2answers
343 views

Feynman's $i \epsilon$ prescription in loop expansion

I have some questions about the $i\epsilon$ factor in Feynman diagrams. First, what is the physical meaning of $i\epsilon$ in loop amplitudes. Second, how does it ensures unitarity? And third, Dyson ...
4
votes
2answers
79 views

How do you measure neutron energy?

I think you can detect a neutron event with a scintillator (neutron to photon), photomultiplier tube (photon to electric signal) system, but how would a fusion neutron's energy (2.45 MeV) be typically ...
-4
votes
3answers
115 views

Newtons third law and unbreakability [closed]

We all know Newton's third law. But then why this law doesn't work when we place a 1000KG heavy container on a small weak table. The table will break. Pressure is exerted on table by container, and ...
4
votes
3answers
337 views

How to block neutrons

What is a good way to block neutrons and what is the mechanism that allows this? It's my understanding that polyethylene is somewhat effective. Why?
2
votes
1answer
5k views

Is force the derivative of energy?

In my lecture today my professor briefly mentioned that force is the derivative of energy but I did not really get what he meant by that. I tried to express it mathematically: $$\frac{d}{dt}K_E=\frac{...
0
votes
2answers
571 views

How does a magnetic field “push” charges?

I was under the assumption that a magnetic field acts similar to that of the normal force in mechanics; both affect the path of the object, but do no work. So now suppose that we have a rectangular ...
2
votes
1answer
705 views

At what rate does light 'bend' around the surface of the earth?

Since the g force of earth is 9.8 m/s*2 does that mean light 'drops' at that rate as it travels past earth? Or is general relativity a lot more complex than that?
1
vote
0answers
151 views

How do I prove the equivalence of chirality and helicity operators acting on a massless Dirac spinor?

I have massless Dirac equation and chirality and helicity operators which are given as $$ \hat {P}_{ch}\Psi = \gamma_{5}\Psi, \quad \hat {P}_{h}\Psi = \frac{(\hat {\mathbf S} \cdot \mathbf p)}{|\...

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