All Questions

0
votes
1answer
8 views

Confusion about average KE in free electron model via Density of States (DOS)

Assume a low temperature regime in which levels up to the Fermi Level, $E_F$, are populated. I have evaluated the density of states in energy space as $$D(E)=\frac{L^3}{\pi^2\hbar^3}(2m_e^2E)^{1/2},$...
0
votes
0answers
15 views

Group and phase velocity for a superposition of 2 monochromatic waves

A wave packet in one dimension with a dispersion relation $\omega$ is a function of the form: $$f(x,t):=\sum_{i=1}^n\lambda_i\exp[i(k_i x+\omega[k_i] t)]\hspace{1cm}\forall i\leq n\colon\lambda_i,k_i\...
3
votes
1answer
48 views

Explain why the magnets move towards eachother in this setup

I have two bar magnets (actually stacks of small magnets, but I believe they should behave the same) attached to a stack of (presumably conductive) coins, and not diametrically opposite. When I ...
0
votes
0answers
43 views

Light path in a Schwarzschild spacetime

This might be a silly question for the physicist as I'm not one but I've have watched this YouTube video which shows the steps needed to simulate light path in a Schwarzschild spacetime, what I failed ...
0
votes
0answers
30 views

A (probably silly) doubt regarding motion of bar magnet

So my instructor was teaching electrodynamics the other day, and he analysed the electric and magnetic field of the bar magnet moving with velocity v in x direction, first from the ground frame and ...
0
votes
1answer
37 views

Materials with good thermal insulating properties

I'm doing a research for a university project. In particular I'm looking for a "commercial" material (so a material that is available on the market or can be home made) that has good thermal ...
0
votes
0answers
31 views

What is an axial and polar vector?

Can someone please explain this type of vector to me, I can not understand it. Axial vectors have an inner orientation, i.e. the direction of the vector indicates the positive orientation. For ...
1
vote
1answer
52 views

Uniqueness Theorem and the 1D Infinite Square Well

Consider the 1D infinite square well problem: $$\frac{d^2\psi (x)}{dx^2} = -k^2\psi (x)\tag{1}$$ along with the boundary conditions $\psi (0) = \psi (L) = 0$. This seems to be a well posed problem ...
-1
votes
0answers
31 views

Difference between DC and AC liquid resistivity measurement

If I was to try and measure the resistivity of an insulating liquid (>10^14 ohm.cm), which type of voltage should i apply, AC or DC, and why?
0
votes
0answers
54 views

Static electric field which admits no potential

Conservative condition for (static) electric field $\mathcal{E}$ is usually defined as $\mathcal{E}$ being closed (curl-free). Now this clearly holds when for the given manifold $X$ we have $H_\text{...
1
vote
0answers
33 views

Symmetry transformations that are self-inverse and global symmetries of the Hamiltonian

I have the simplified Ising model. The Hamiltonian is given by $$ \mathcal{H} = -\mathrm{J}\sum_{<ij,i' j'>} \sigma_{ij} \sigma_{i'j'}. $$ Where the sum over $<ij,i'j'>$ means just the ...
2
votes
1answer
73 views

Is Lorentz and Galilean invariance mutually exclusive?

I know that the classical mechanics stays valid under Galilean transformation. The same argument applies to relativistic equations and Lorentz transformation. My question is, can a set of equations ...
-1
votes
0answers
26 views

Question on Statics [on hold]

TO WHAT LENGTH SHOULD A STEEL TIE ROD of given length and diameter BE BORED CENTRALLY SO THAT IT'S EXTENSION UNDER a given tensile load will INCREASE BY, 15% as compared to extension of normal rod(...
0
votes
1answer
28 views

Properties of Expectation Values Under Variable Substitution

I am working on a homework problem from Griffiths QM (Problem 2.11, 3rd Edition). Specifically, I'm working on finding $\left<x^2\right>$ for the ground state and the first excited state of the ...
1
vote
0answers
36 views

How is weight dispersed among multiple points?

I was recently viewing remembrance photos from the 1995 bombing of the Alfred P. Murrah Federal Building on MSN news where slide nine shows a photo of firemen teaming up to carry a $40'$ steel bar ...
2
votes
0answers
29 views

Entropy and temperature of small systems

I've been struggling for a while now in understanding the concept of entropy as a function of the internal energy. Textbooks typically call $g(E)$ the number of microstates compatible with an energy $...
0
votes
2answers
36 views

Angular collision equations help

The situation is a particle of mass m striking a rod of the same mass and length $L$ at an angle with an initial velocity $v$ elastically. Now to describe this situation I wrote the following ...
0
votes
0answers
30 views

Current in the Photoelectric Experiment

A photocell is connected in series with an ammeter such that the collector is connected to the negative terminal of the battery (emf > 0). Therefore there exists a stopping voltage between the anode ...
0
votes
1answer
21 views

MOSFET Body terminal

What is the point of the body terminal in a MOSFET. If we assume Vsoure=0, Vdrain>0 then we will have a linearly changing voltage from drain to source. Then if Vgate>0 , depending on the difference ...
0
votes
0answers
26 views

Einstein's thought experiment about a train

Observers: - person 1 on the train - person 2 on the ground person 1 sees the two lights instantly person 2 sees person 1 at 2 different instances based on their position person 1 would see person 2 ...
0
votes
1answer
26 views

Rotation matrix product and angular velocity

I'm solving the following problem from the book "Analytical Mechanics for Relativity and Quantum Mechanics" by Oliver Davis Johns: Let $R(t)=R_a(t)R_b(t)$ be a rotation matrix. Show that the ...
-1
votes
0answers
20 views

Bernoulli question from a layman

So my understanding is that pressure and velocity are inversely proportional. If I where to go from an 8" pipe to a 4" pipe the velocity would increase thus a decrease in pressure. Pretty straight ...
0
votes
1answer
42 views

How can sunlight cause skin cancer? [on hold]

If the atmosphere absorbs the UV wavelengths, then what’s left is visible, IR and maybe a really small amount of UV that transmits it’s way to earth. IR and visible are non ionising, so is it the case ...
1
vote
2answers
29 views

Speed of a video and Doppler effect

While watching videos at 1.5x or 2x the original speed I often notice a change in the speaker's voice. I am not sure if that is a change in pitch or something else, and was thinking how it could ...
-1
votes
0answers
22 views

Explain why the angle of incline of the ramp affects friction between the surface of the ramp and the object [on hold]

Experiment: the object is placed on the ramp and the angle between the ramp and the floor is changed.Explain why the angle of incline of the ramp affects friction between the surface of the ramp and ...
0
votes
0answers
25 views

Stability of a 10km planetoid with a micro-black hole in its centre

Let's assume there is a tiny, 10km-radius planetoid with a micro-black hole in its centre of about 10^19kg, so the surface of the planetoid is at 1g. How it appeared (or was constructed) would be the ...
-1
votes
0answers
17 views

MOSFET operation

I have 2 questions about the the MOSFET operation. When we apply voltage at the gate is the depletion region between the drain and the p type semiconductor destroyed? If we put a resistor to the ...
2
votes
0answers
23 views

Normal force at junction [on hold]

Question is to compare normal forces at A,B and C. I found out theta subtended by arc AB to be $30°$. Problem is at point B. As for a point just before B, normal force is $mg\cos30° + mv^2/100$. But ...
0
votes
2answers
29 views

What is Work Done against the Atmosphere

{Assume it's a Spherical Balloon} The answer on the book states (7pV), but I'm definitely missing something. I've looked up online on what 'Workdone against atmosphere' means but there is little to ...
1
vote
0answers
10 views

What is self narrowing atomic beam pantography?

Can anyone please explain what is this technique. This is a topic in second unit of GTU syllabus for Nanotechnology. I googled a lot but I don't understand what is that.
0
votes
0answers
31 views

Gravity and propulsion [on hold]

If a body (m) weighs 200 lbs (90.7185 kg) at sea level and gravity diminishes as the distance between it and the earth (M) increases, what would the body's relative weight be at an altitude of 128,000 ...
1
vote
0answers
16 views

Radius of curvature of a spherical mirror using only magnification?

Given that an object of heigh 1cm is placed in front of a spherical mirror and produces an upright image of height 0.2cm, find the radius of curvature of the mirror. I know that $ M = \frac{h_i}{h_o}...
0
votes
1answer
62 views

Integration by parts, Weinberg Cosmology p.526

How do I perform this integration by parts done explicitly? $$0 = \delta I_m = \int d^4 \sqrt{-g} T^{\mu \nu} \left[- \frac{\partial \epsilon^\rho}{\partial x^\mu} g_{\nu \rho} - \frac{\partial \...
0
votes
0answers
31 views

Solving the problem using Many Body Perturbation Theory

I am trying to solve the following Hamiltonian using Many body perturbation Theory. $$H=\sum_{i=1}^{N}\Bigg[\frac{P_{i}^{2}}{2m} -\sum_{i,j}\frac{1}{|\vec{r}_{i}-\vec{R}_{j}|}\Bigg]$$. I split this ...
0
votes
0answers
21 views

Fourier transformation of a discrete function; conclusion of having a finite sum

For a function $g$ defined at discrete points $x_n = n a$ with $n \in \{0, \ldots, N\}$ with periodicity $g(x_0) = g(x_N)$ the discrete Fourier transformation reads $g(x_n) = \frac{1}{Na} \sum_{q \in ...
0
votes
0answers
16 views

The equality of fermi energy and chemical potential at high temperatures

Through the duration of my equilibrium statistical mechanics course, I came across a point in ideal Fermi systems where the Fermi energy was approximated as chemical potential when we derive the ...
0
votes
4answers
55 views

Deriving the formula of potential difference duo to a solid conducting sphere

The doctor in university was deriving a formula and I can't understand how it works A sphere with charge Q The Sphere's radius is R, and we are trying to derive a formula for potential difference at ...
-1
votes
0answers
27 views

Women who are science communicators for quantum physics? [on hold]

I've been asked about books or other sorts of materials (video, etc.) on quantum physics produced by women. If you can recommend any, thanks in advance!
0
votes
0answers
21 views

Intensity, power and efficiency question help

The solar Intensity in the Netherlands is $1.0e3\ W/m^2$ at a maximum sunshine and a perpendicular incidence. On average, about $10\%$ of this peak power can be utilized. The efficiency of the ...
0
votes
1answer
30 views

Normalising squeezed position eigenket?

I want to find the effect of squeezing operator $S(r) = \exp \big[r(\hat{a}^2 - \hat{a}^{{\dagger}^2})\big]$ on $|q\rangle$ i.e. $S(r)|q\rangle$. I proceed as follows: $$S(r)\hat{q}|q\rangle = S(r)...
1
vote
1answer
39 views

Is the Electric and Magnetic field in an EM wave interacting in the same way static E and B fields?

Sorry by the title, I don't know how to ask this: What happens to a charge when some EM wave passes across it? If we have an EM wave with really low frequency, is this basically the same as having a ...
1
vote
1answer
30 views

Sum over real photon polarizations. The minus sign

Ok for real photons there is the formula when summing over the polarizations: $$ \sum_{\lambda=\pm}\epsilon^{*\mu}_\lambda\epsilon^\nu_\lambda = -\eta^{\mu\nu}$$ But if I have a matrix element of ...
0
votes
0answers
8 views

About electrode self-energy and the relation between transmission functions and Green’s functions

I am reading Electronic Transport in Mesoscopic Systems by Supriyo Datta. I got stucked when deriving some formulas. On page 147, the book says "see exercise E.3.3" when it gives the formula (3.5.18), ...
0
votes
1answer
39 views

Calculate the ratio of heat losses from two different bodies at different temperatures

In the above question, I used Newton law of cooling, and got answer 3:2, but this option is not given in the above question. What is the correct method to solve this question?
0
votes
0answers
29 views

Convective derivative vs total derivative

I was wondering what is the difference between the convective/material derivative and the total derivative. We were introduced to the notion of material derivative $$ \frac{D\vec{u}}{Dt}=\frac{\...
3
votes
3answers
173 views

Why is there Net Work Done on a Pressure/Volume Cycle?

The area enclosed by the rectangle is said to be the Net Work Done. My question is why would there be any net workdone if it returns to its initial position (W). Shouldn't it act like a force-...
1
vote
0answers
28 views

Electron-positron annihilation into proton-antiproton

How can I obtain the differential cross-section of the process $e^{+}e^{-}\rightarrow \bar{p}p$ from the cross section of $ep$-scattering? I have calculated the amplitude of $ep$-scattering and see $...
0
votes
0answers
25 views

Rayleigh-Schrödinger series and pertubation theory

If we look at the time-independent perturbation theory and the non degenerate case where $H=H_0+\lambda H_1$ and $H_0$ has eigenvalues $E_i^{(0)}$ and normalized eigenfunctions $|\Psi_i^{(0)}>$. ...
0
votes
0answers
9 views

How do Oort's constants relate to shear and vorticity?

I am told to show that Oort's constants represent shear and vorticity in the velocity field of an orbiting star. I know that Oort's constants are defined as: $A = \frac{1}{2}(\frac{v_{\bot,0}}{R_{0}}...
1
vote
0answers
70 views

Earth's surface area

Here we are trying to calculate the earth's surface area via geodetic coordinates: \begin{align} x&=(R\,p(\lambda)+h)\sin\lambda\,\cos\phi\\ y&=(R\,p(\lambda)+h)\sin\lambda\,\sin\phi\\ z&...

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