5
votes
2answers
421 views

Why does this equation refer to magnetic tension?

The force density in a plasma, according to MHD, is $$ \mathbf{F} = -\nabla P -\nabla(\frac{B^2}{2\mu_0}) + \frac{1}{\mu_0}(\mathbf{B}\cdot\nabla)\mathbf{B} $$ where first term is the thermal ...
0
votes
1answer
46 views

Change in volume of air held at the top of a tube with upper end closed and holding some liquid [closed]

Consider a tube both end open.dip one end in a container with water..close the top with your thumb..(make sure some air remains between your thumb and the liquid in the tube)..now take the tube out of ...
11
votes
2answers
542 views

The difference between The Dilaton and The Radion?

I have read this question on the Dilaton, but I am a little confused with the distinction between the Dilaton and the Radion. I definitely have the feeling that these two scalar fields are different ...
0
votes
4answers
1k views

Gravity force between two objects with different mass

We know thanks to Newton that: $$F=G\frac{m_1\cdot m_2}{r^2}$$ Where $G$ is the Gravitational Constant that is about $6.673\cdot10^{-11}$ $m_1$ and $m_2$ are the masses of two different objects ...
1
vote
2answers
2k views

What is the effect of surface tension on an air bubble inside water?

When an air bubble is formed inside water, and while it's still inside, does a force due to surface tension act on it? Why and where does this force act? I read somewhere that a bubble would detach ...
2
votes
2answers
426 views

What is wrong with considering the Atwood machine as a system?

I am confused about a method used in the following problem. There is an arrangement as shown below. The surface is smooth, and the pulleys are light. We have to find the acceleration $a_0$ of $m_1$. ...
1
vote
3answers
341 views

Good math books for physicists [duplicate]

In his first lesson (transcripted in "Tips on Physics"), Feynman talks about math for physicists in a very cool and practical way. And at the end of the section he talks something like "so the first ...
0
votes
1answer
1k views

How is this the graph of induced emf against angle for a rotating coil in a magnetic field?

So a coil is rotating in a magnetic field, and at $\theta=0$ the coil is perpendicular to the field lines. At $\theta=90$ the coil is parallel to the field lines. The the angle, theta, is the angle ...
15
votes
3answers
2k views

Wouldn't angular momentum of a binary star system decrease?

Consider a binary star system, as these stars go around one another they would emit gravitational waves. Since, the graviton is a spin 2 particle. Wouldn't the angular momentum of the stars decrease? ...
2
votes
1answer
127 views

Why can't we detect redshift of Galaxies/Clusters moving parallel to us?

If a moving Galaxy/Cluster doesn't have radially outward motion component from us, we are unable to detect its redshift. The answer may look obvious: There's no wavelength squeezing/expanding ju-ju. ...
3
votes
3answers
1k views

Calculating the mass of a wave of water

I am assuming it is possible to calculate the mass of water if the volume is known. My assumption is that doing this for stagnant water, in a container perhaps, is easy. However I want calculate, ...
6
votes
1answer
502 views

How to sum over final, and average over initial color states?

Consider the $s$-channel mediated top quark production process $$ d + \overline d \rightarrow t + \overline t$$ Using the Feynman rules for QCD, the amplitude contains a color factor $$[c^\dagger ...
0
votes
1answer
205 views

Acceleration ratio

Can anyone please explain why: $$a_1=a_2\tan\alpha $$ We have 2 same cubes placed on a platform, between them there is a wedge which is pushing the cubes in opposite directions. The hard part of the ...
5
votes
2answers
448 views

Why isn't there a potential difference across a disconnected diode?

I know this question sounds silly, as if there was a potential difference a current would be created when the terminals are connected together and this would mean energy has come from somewhere. The ...
2
votes
1answer
154 views

What kind of problem in quantum mechanics can have an algebraic method of solution?

For example, a harmonic oscillator can have an algebraic solution, and hydrogen potential can also have an algebraic solution. Here the algebraic method of solution means that we can use the similar ...
0
votes
1answer
201 views

Coordinate System vs. Angular Properties vs. Centroid

Please help me check my understanding related to the rotational motion of a 3D rigid body after reading some Physics textbooks and googling for some more materials (e.g., Wikipedia's Torque, ...
3
votes
3answers
441 views

What is a particle?

I posted this elsewhere also and just found this place so copied it down but yeah. I've always wondered this cause I like wondering bout things but I wanna know and it's simple so I should. I got an ...
1
vote
2answers
15k views

Loop-the-loop question [closed]

Problem: The marble rolls down the track and around a loop-the-loop of radius R. The marble has mass $m$ and radius $r$. What minimum height $h$ must the track have for the marble to make it around ...
7
votes
1answer
229 views

Why does Einstein say contradictions arise from treating the EM field as lines of force?

EDIT - I have included the context of the quote I am interested in, as people seem to be as baffled by Einstein's quote as I am: In a 1920 address Einstein says this: Think of waves on the ...
1
vote
1answer
456 views

First Order Correction to wave function in ground state

I am looking at a spin 1/2 particle in a magnetic field. This has Hamiltonian $$H=-\mu s\cdot B_0$$ For simplicity, assume $B_0=B_0\hat z$ so $H=-\mu B_0$. I then apply a perturbative magnetic field ...
3
votes
0answers
88 views

String Vertex Operators in Light Cone Gauge

I have a basic confusion about how to compute string scattering amplitudes in light cone gauge, using the operator formalism reviewed in GSW. I am familiar with the covariant gauge formalism, where I ...
0
votes
1answer
125 views

Perturbation of coupled spin

I am given a system with Hamiltonian (all 1/2 spins) $$H_0=\alpha(S_1\cdot S_2)$$ I broke it down and found that there were four eigenstates: $|1,[0,\pm1]\rangle$ and $|0,0\rangle$. Each has an ...
1
vote
0answers
132 views

Is it possible to define a notion of temperature in a microcanonical ensemble?

I am thinking of a mircrocanonical ensemble as a finite system for which the number of particles, volume and the total energy is fixed. Is there a more refined view of this? Can I think of ...
6
votes
2answers
582 views

How come a whistling kettle starts whistling only when water boils, and not long before - due to hot air escaping under pressure?

A whistling kettle will start to whistle when the water boils and turns into a jet of steam which then exits the small aperture in the spout. But why doesn't this happen much earlier - when the air ...
4
votes
0answers
153 views

What real experimental systems are well-described by Glauber-Ising spins?

I'm hoping for references to actual physical systems in which all or at least most of the following can be simultaneously characterized: the spin flip rate, the temperature, and a relaxation or ...
3
votes
2answers
1k views

RMS Free Path vs Mean Free Path

I am trying to determine the mathematical difference between mean free path and root-mean-square free path. For an ideal gas, the relaxation time is $$\tau=\frac{1}{\sqrt2 \pi nd^2 \bar v}$$ and the ...
5
votes
3answers
111 views

How to measure the effects of the rotation around the Sun?

It is well know that we can measure the spinning of the Earth with a Foucault pendulum. But, is there a similar experiment for the rotation of the Earth around the Sun? I would like to know if we ...
2
votes
1answer
147 views

Out-of-Plane Phonons

I am trying to derive the out-of-plane phonon dispersion relation for a membrane. As far as I can tell, one of the simplest ways to do so is with a Lagrangian of the form: ...
0
votes
1answer
91 views

Confusion about units of angular momentum

According to multiple sources the SI units for angular momentum are kg * m$^2$ / sec I am confused about the derivation for this. Here is what I have done: $$L = I \cdot \omega \\ = m \cdot r^2 ...
1
vote
1answer
80 views

$\mathrm{d} \Omega_{CM}$ for a $1\rightarrow 2$ particle decay?

The differential solid angle is described in e.g. Srednicki's QFT text but only for the case of scattering. Because in the case of scattering it's defined with respect to the incoming three-momentum ...
3
votes
1answer
186 views

Is obtaining the coordinate representation of momentum operator from commutator more fundamental than generator of translation

Related post: What is the most general expression for the coordinate representation of momentum operator? There are two methods of obtaining the coordinate representation of momentum in quantum ...
3
votes
2answers
2k views

How does throttling effect the upstream flow?

Let's assume we have a simple duct with a fan (compressor) situated somewhere along it. The fan runs at the same speed always. At the end of the duct, downstream of the fan, there is a throttle valve. ...
1
vote
2answers
1k views

Why does the induced EMF oppose the change in magnetic flux? Lenz's Law question

Can anyone explain to me why the induced magnetic field will oppose the change in magnetic flux? Is it an energy thing? I know that the induced emf is $$ emf= - \frac{d\phi}{dt} $$ but my book ...
5
votes
3answers
399 views

Newton's principle of determinacy

I am a mathematician. I have a (somewhat long term) goal of understanding some of the physical insights that have influenced my area of research. To this end I read Arnold's Mathematical methods in ...
1
vote
0answers
101 views

Proving two forms of atom-field interaction perturbation Hamiltonian are equivalent

In the presence of an electromagnetic field in the dipole-approximation (${\boldsymbol A} = {\boldsymbol A}(0,t)$) we have the two forms $$H_{{\boldsymbol d}\cdot {\boldsymbol E}} = - q {\boldsymbol ...
1
vote
2answers
3k views

Temperature in an isobaric process

I have a certain conceptual issue - I'm solving heat engine problems and I found something difficult to understand. Let's take an isobaric part of a cycle of an engine, let's say that the ideal gas is ...
2
votes
2answers
117 views

How to get conserved currents of a theory which are not Noether currents?

In the first SuSy lecture last week following theory of two real scalar fields has been considered as a first example: $$\mathcal{L}=(\partial_\mu \phi_1)^2/2+(\partial_\mu ...
0
votes
2answers
91 views

Find the tangential acceleration

A racing car driver drives his car on a flat circular track of radius $\frac{25}3 m$ and a coefficient of friction $0.5$. Find the magnitude of its tangential acceleration (in $ms^{-2}$) at an ...
0
votes
1answer
62 views

Electromagnetism and the principle of relativity

I'm reading the book "Fundamental Physics 2: Electromagnetism" by Alonso and Finn. I understand everything up to the point where everything is "unified". The following example is given in the book: ...
4
votes
1answer
220 views

Time-Energy Uncertainty Principle and Operators

In most of examples, I notice that uncertainty principle for time & energy is given between mass & lifetime. The UP for time and energy is $$ \Delta t\,\Delta E\geq\frac h{4π} $$ where $$Δt ...
2
votes
3answers
804 views

Charge distribution on plates

$A,B,C$ are $3$ identical metallic plates. Initially, charges $Q$, $4Q$ and $2Q$ were given to $A$,$B$ and $C$ respectively. Find final charge distributions when $B$ was earthed and $A$ and $C$ were ...
17
votes
2answers
2k views

How can I read off the fact that gravity is associated with spin-2 particles from the Einstein-Hilbert action?

I have often heard that the gravitational field has spin $2$. How can I read the spin of the field from the Einstein-Hilbert action $$S=\int \! \mathrm{d}^4x \,\sqrt{|g|} \, \mathcal{R} \, \, \, ?$$
6
votes
2answers
7k views

Why is the divergence of a magnetic field equal to zero?

We know due to Maxwell's equations that: $$\vec{\nabla} \cdot \vec{B}=0$$ But if we get far from the magnetic field, shouldn't it be weaker? Shouldn't the divergence of the field be positive? If ...
4
votes
1answer
165 views

How use the Higgs branching ratio plot to extract information about the Higgs mass compared to experiment?

What does the plot of higgs branching ratio (see figure below) say about the higgs mass anyway? How can one use it as a guide to find the higgs mass experimentally? If we e.g. go to $M_H=126$ GeV ...
2
votes
2answers
576 views

From Quantum Mechanics to Classical Mechanics [duplicate]

Is it possible, and has it been attempted, to use quantum mechanics to deduce Newtonian, macroscopic level mechanics laws as was the case of statistical mechanics deriving thermodynamic relations?
2
votes
4answers
697 views

Lowering and Raising Kronecker Delta

When an index of the Kronecker-delta tensor $\delta_a^b$ is lowered or raised with the metric tensor $g_{ab}$, i.e. $g_{ab}\delta^b_c$ or $g^{ab}\delta_b^c$, is the result another Kronecker-delta ...
2
votes
0answers
247 views

Collision of Discs and Snooker Kicks

I woke up this morning thinking about spinning discs. Could someone verify whether my reasoning below is correct? Problem 1 Suppose have two identical uniform discs constrained to move in a plane. ...
1
vote
0answers
33 views

Applications of low-dimensional topology to physics [duplicate]

As a mathematics graduate student whose research area lies in low-dimensional topology (more precisely, invariants of 3-dimensional topological manifolds), I heard that there exist multiple ...
13
votes
2answers
1k views

What are all the lines on a double circuit tower?

I understand what three-phase power is. But when I look at some pictures of a double-circuit-three-phase-power-line I see two or three lines close together? What is the purpose of these lines close ...
15
votes
2answers
1k views

What's the relationship between $SL(2,\mathbb{C})$, $SU(2)\times SU(2)$ and $SO(1,3)$?

I'm a beginner of QFT. Ref. 1 states that [...] The Lorentz group $SO(1,3)$ is then essentially $SU(2)\times SU(2)$. But how is it possible, because $SU(2)\times SU(2)$ is a compact Lie group ...

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