-1
votes
1answer
34 views

What does it mean by “infinities” when dealing with quantum mechanics?

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 ...
3
votes
2answers
56 views

Two inertial frames (different angles)

The motion of the earth relative to a star changes the angle at which the star is perceived. Consider to inertial frames of reference I and I'. I' moves with velocity w relative to I along the ...
0
votes
0answers
8 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 ...
0
votes
2answers
125 views

A plane electromagnetic wave - phase change - amplitude

A plane electromagnetic wave has the shape: $\vec{E}(\vec{r},t)=E_0\cdot cos(\vec{k}\vec{r}-\omega t)\cdot \vec{e}_y$ $\vec{B}(\vec{r},t)=(B_1\cdot cos(\vec{k}\vec{r}-\omega t)+B_2\cdot ...
0
votes
0answers
11 views

Two points of view on the Lorentz transformations

Currently as I know there are two possible ways to define the Lorentz transformations. One way is the original approach by Einstein, based on the two postulates. In that case, the postulates implies ...
2
votes
1answer
63 views

How to write the Clebsch-Gordan decomposition in tensor notation

Let be $G$ a Lie Group and $\textbf{N}$ its complex representation. It is known that any state $|\ ab\ \rangle\in \textbf{N}\otimes\textbf{N} = \oplus_I\textbf{r}_I$ may be decomposed through the ...
-4
votes
0answers
11 views

Conservative Energy & Ramp

Hello there I had this following question: For the initial velocity in the picture I wanted to use sin(37) * 10 m/s to get the vertical component and then plug it in to find the height for part A. ...
-2
votes
0answers
14 views

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

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!
0
votes
1answer
143 views

Nuclear to Electricity Energy Conversion

Currently nuclear power generates heat, which heats water into steam that turns conventional turbines. The energy conversion is as follows: photonic->heat->kinetic->electric This would result in low ...
0
votes
1answer
18 views

Questions about null geodesic

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 ...
43
votes
4answers
22k views

Why glass is transparent?

Once I asked this question from my teacher and he replied "because it passes light", "and why it passes light" I asked and he said "because it is transparent". Same question again, Why glass is ...
0
votes
0answers
15 views

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

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 ...
67
votes
10answers
5k views

Are units of angle really dimensionless?

I know mathematically the answer to this question is yes, and it's very obvious to see that the dimensions of a ratio cancel out, leaving behind a mathematically dimensionless quantity. However, I've ...
142
votes
9answers
36k views

Don't heavier objects actually fall faster because they exert their own gravity?

The common understanding is that, setting air resistance aside, all objects dropped to Earth fall at the same rate. This is often demonstrated through the thought experiment of cutting a large object ...
0
votes
2answers
45 views

Why an impact exerts so much force?

If an object of velocity $v$ and mass $m$ moves towards a resting object of mass $M$, then if the object which is hit might break. Why? What is the reason that a collision has more power than a ...
0
votes
0answers
19 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 ...
0
votes
2answers
167 views

Coriolis force in free fall: Directions?

Inspired by this Phys.SE question, Coriolis force in free fall, I have to ask: Why does the Coriolis force act in the east-direction? I would say, if I jump from a high distance and look at the ...
0
votes
2answers
34 views

Use Runge Kutta method to solve schrodinger equation

The schrodinger equation in spherical coordinates after seperation of variables as a solution of hydrogen atom is given by $$ \frac{-\hbar^{2}}{2 \ m} \left[ \frac{1}{r^{2}} \frac{d}{dr} \left(r^{2} ...
2
votes
2answers
130 views

Can you use infra-red goggles (or similar principle) to see through mist and fog?

As per title really... fog is obviously quite opaque to visible light yet transparent to radio waves. What is the range of frequencies at which fog is opaque, and is either end of this range at a ...
1
vote
0answers
27 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 wavenumbers (cm^-1). In order to prove this conversion, one must ...
0
votes
1answer
14 views

Show the Berry phase is invariant under U(1) unitary transform

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 ...
6
votes
3answers
46 views

Degrees of freedom and temperature

I quote the following lines directly from the Wikipedia page titled "Heat capacity": "...rotational kinetic energy of gas molecules stores heat energy in a way that increases heat capacity, since ...
0
votes
2answers
14 views

Discrepancy between the rate of heat flow

I'm in a introductory college physics class, and my professor recently gave us this formula for the rate of heat flow: $\frac{dQ}{dt} = \frac{A \Delta T}{R_{factor}}$ where $R_{factor}$ is ...
0
votes
0answers
9 views

Production of entangled photons by spontaneous down conversion

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 ...
0
votes
0answers
9 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 ...
0
votes
2answers
33 views

Circular motion and inclined friction

I'm given the following picture: A car with the mass m passes the top of a hill with the height H. The coefficient of friction is $µ$. The speed of the car is $v_0$ in the moment of braking. ...
1
vote
2answers
155 views

Does charge of a metal charged by induction determine by which ends of the metal is grounded to?

Does charge of a metal charged by induction determine by which part of the metal is ground to? I draw a diagram to make it simple to understand: Right diagram: When ground is touched with the ...
0
votes
1answer
18 views

seeking an intuitive way to apply/understand Laplace's law in surface tension

[question edited] The question I encountered is as follows- Two bubbles/balloons of different sizes but with the same surface tension are filled with air and connected by a valve. If the valve gets ...
-4
votes
1answer
115 views

If systems minimize their potential energy, why do people climb mountains?

A person climbs a mountain. They convert the stored chemical energy of food in their body and back pack into heat energy on the mountain, and gravitational potential energy as they move towards the ...
-1
votes
2answers
54 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 ...
2
votes
0answers
17 views

Physical meaning of the chiral condensate in QCD

Considering the QCD Lagrangian in the chiral limit, where all the quarks masses are set to zero. Then the Lagrangian has the following chiral symmetry: $$ SU(L)_{V} \times SU(L)_{A} \times U(1)_{V} ...
4
votes
3answers
149 views

Velocity in a turning reference frame

I often see the relation that $\vec v=\vec v_0+ \vec \omega \times \vec r$ in a turning reference frame, but where does it actually come from and how do I arrive at the acceleration being $$\vec ...
2
votes
3answers
130 views

Why engines don't melt?

The temperatures of internal combustion engines are well above the melting points of the component materials. For example, temperatures can reach 2200° C whereas the melting point of aluminium is ...
0
votes
4answers
68 views

Action is equal to reaction. Then how is a horse able to pull a cart? [duplicate]

Please answer the question in detail. I have tried other websites for answers but was not able to understand properly.
2
votes
1answer
46 views

Confusion over plant's “magnetic field” being used to reduce wastage of pesticide

As far as I am aware, a plant does not have a magnetic field of any significant magnitude. What I wonder is, is there any electromagnetic related basis for the crop spraying technique outlined in ...
1
vote
0answers
18 views

reconstruct the wave-function from one body reduced density matrix?

Given a many body wave-function for a Fermion system, we can calculate the one-body reduced density matrix straightforwardly. Now suppose we know the one-body reduced density matrix, is there a way to ...
1
vote
0answers
8 views

Lower limit for atomic number in case of imaginary ion with Z < 1

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 ...
0
votes
0answers
4 views

What determines the sign of phase dislocations?

I am studying the nonlinear Schrodinger equation $$A_t+iA_{xx}+i|A|^2A=0$$ for $A=ae^{i\theta}$ a complex valued function, with $a,\theta$ real. I am trying to figure out what sets the signs of the ...
0
votes
2answers
17 views

Diminishing solar temperature and its effects on earth

This is a hypothetical question; considering both the earth and the sun as black bodies. If the temperature of the sun decreased N times, what would be the effect on the radiation intensity received ...
24
votes
3answers
3k views

How can an electron fake a jet?

This is a question for experimentalists. I have seen in several ATLAS papers (see for example chapter 4 in arXiv:1602.09058, 6th paragraph), that after objects have been correctly identified, any jet ...
12
votes
3answers
5k views

Why do we use Hermitian operators in QM?

Position, momentum, energy and other observables yield real-valued measurements. The Hilbert-space formalism accounts for this physical fact by associating observables with Hermitian ('self-adjoint') ...
0
votes
2answers
177 views

Find the force needed to accelerate body to a certain velocity for a certain time with respect to drag force

So, the problem is straightworward when we suggest that air resistance force is constant: $$ \vec F = \frac {\vec V_1 - \vec V_0} {t} m / b $$ $$ \vec V_0, \vec V_1 - \text {initial and final ...
0
votes
2answers
37 views

Calculating the surface integral over a charged box

In a question that I'm trying to solve, we're told that there is a box with dimensions 20cm long, 4cm high, and 3cm deep. we're given that the electric field on every surface of the box is pointing ...
1
vote
0answers
51 views

What is really sought when we purpose Einstein's postulates in Special Relativity?

Special Relativity can be motivated by looking at Maxwell's Electrodynamics and noticing that there is some kind of inconsistency between it and Newtonian Mechanics. Indeed, as Einstein pointed out on ...
3
votes
1answer
31 views

What happens to the electric field at the boundary of a dielectric?

The question in the title is a broader one, but for now, I want to confine myself to the following problem. I have the task to find the potential difference between points A and B shown in the ...
12
votes
2answers
851 views

How precise can current technologies measure the mass of an object?

Masses of various objects are listed on this wikipedia page: Orders of magnitude (mass). For example, mass of an HIV-1 virus is on the order of 1 femtogram. Are these data actually measured (which I ...
0
votes
0answers
12 views

Should CSCO only be consider for hermitian operators?

$$\hat A=\left( \begin{array}{ccc} 1 & 0 \\ 8 & 1 \end{array} \right)$$ is not hermitian, but it only has one eigenvector. It seems to satisify the condition of CSCO, however. Namely, ...
2
votes
1answer
21 views

Energy conservation around a black hole

In the Schwarzschild black hole, the Killing vector "time translation" $k^a$, so that the following quantity is conserved along a geodesic: $$E = -g_{ab}k^au^b = (1 - ...
1
vote
1answer
24 views

100 lbs of drag? Really? Chain running physics

Recently came across this video of an endurance technique involving the strapping of a 100lb chain to one's back and running along the grass dragging it behind. What is the true force experienced by ...
1
vote
0answers
8 views

Kitaev chaing, time reversel symmetry, particle hole symmetry

I was wondering if the Kitaev chain has time reversal symmetry. I think it probably doesn't because by staking Kitaev chains it is possible to create a so called Chern insulator with propagating ...

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