Applies to questions of primarily educational value - not only questions that arise from actual homework assignments, but any question where it is preferable to guide the asker to the answer rather than giving it away outright. Please READ THE GUIDANCE IN META before asking homework-like questions.

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0
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0answers
18 views

Solving for the density operator in the quantum Brownian motion master equation

I want to solve for the density operator in the quantum Brownian motion master equation, \begin{align} \begin{aligned} ...
0
votes
2answers
40 views

Orbital Velocity Question

I have a satellite in a stable trajectory around Earth, of a Mass $m$. I know that its velocity is: $$ v = \sqrt{mG/r}\, $$ But now it begins accelerating directly against the gravity vector (i.e. ...
0
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2answers
27 views

Convert cubic feet per sec to feet per sec

Is there a way to convert cubic feet per sec to feet per sec. Or in general volumetric flow to velocity? I want to know the time taken by water to travel from point A to point B. I have the distance ...
3
votes
0answers
31 views

Triangular barrier in infinite potential well

Suppose I am looking to solve the wavefunction for the following 1D potential: $$\psi(x) = \begin{cases}V_0\frac{a-|x|}{a}&\quad\text{for}\quad|x|<a ...
0
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0answers
34 views

How is tunneling probability?

Let's say we have an alpha particle tunneling through a barrier of width 2 fm and height 30 MeV, and the alpha particle has an energy 1 MeV below that of the barrier. The tunneling probability is ...
-3
votes
0answers
13 views

Computational hydraulics [on hold]

Find the emptying time for reservoir that the water level drops from 35m to 25m using following data: - V=15000 h volume-elevation relationship for reservoir - D=2m diameter for outlet works - h' ...
-2
votes
1answer
25 views

Probability with expectational value [on hold]

How can I calculate: $$P(T)=T^2 b^2 |<n| \delta (x) |n>|^2~?$$ Where $P$ is the probability.
0
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2answers
28 views

Show that a function takes the following form using the definition for the function of an operator

If $f(z)$ is a function with a Taylor series expansion $$f(z)=\sum _{ n=0 }^{ \infty }{c_n z^n },$$ then we define $$f(M)=\sum _{ n=0 }^{ \infty }{c_n M^n }.$$ First consider ...
0
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0answers
31 views

Why is entropy in a heat flow process always positive?

Entropy in a quasistatic heat flow process is given by $$\Delta S=C_1\ln(\frac{T_f}{T_{10}})+C_2\ln(\frac{T_f}{T_{20}})$$ Where $C_1, C_2$ are heat capacities at constant volume, $T_{10}<T_{20}$, ...
-2
votes
0answers
15 views

Thermal conductivity of oak wood [on hold]

I have to calculate the thermal conductivity of a natural material (oak wood sample). The heat penetration through a wood sample was evaluated for 4 investigated temperature (55°C, 57.5°C, 60°C and ...
0
votes
1answer
24 views

Equations for a collision between two particles

Say I have two particles on a 2D plane, they have a x and y coordinate, a x and y velocity, a mass, a coefficient of restitution and a coefficient of friction. What formulae would I need to determine ...
-1
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0answers
18 views

Launching a Spring from a meter stick [on hold]

In class we are engaging in 'war games'. We receive a random spring and launch it from a meterstick at a target flat on a nearby table. we will have the variables for: ...
0
votes
1answer
18 views

Influence of a defect on the flow field

I have a long microchannel where flows some water. The reynolds number is much smaller than one. Within the structure of this microchannel there is a big defect. It looks like a bump of size ...
1
vote
0answers
20 views

Decomposition into symmetric and antisymmetric form [on hold]

(a) Given a second-rank tensor Tμν, often viewed as an $N \times N$ matrix (for a space of dimension $N$), show by explicit construction that one can always decompose $T_{\mu\nu}$ into a symmetric ...
0
votes
0answers
33 views

Computing And Finding The Trajectory From The Lagrangian [on hold]

The Euler–Lagrange equations come from extremization of the action. So we expect the "true", dynamical trajectory to minimize (in this case) the value of $S=\int L \,dt$ For free particle motion, the ...
-4
votes
0answers
34 views

Generate the equations of motion for the one-dimensional Lagrangian [on hold]

Generate the equations of motion for the one-dimensional Lagrangian: $$L=\frac {1}{2}m\dot{x}^2-(Ax+B)$$ From the equations of motion (and the implicit definition of the potential), provide a physical ...
-2
votes
1answer
38 views

Perturbation theory of states [on hold]

A particle in 1 dimension with mass $m$ is in potential well with $V = 0$ for $–a/2 < x < a/2$ and infinite potential elsewhere. The particle is initially in the state $n = 10^9 + 1$, with the ...
0
votes
1answer
39 views

Pipe in space outputs gas - what is its angular momentum at $t$ and $t+dt$? [on hold]

Given a pipe in space (neglect gravitational force): The speed of the gas is $v_0$ (in relative to the edge of the pipe) The length of the pipe is $l$ The pipe rests (not moving) at $t=0$ The gas ...
2
votes
1answer
58 views

How to remember the Electromagnetic Spectrum?

This may sound off-topic but I am in a severe need of remembering the following shown Electromagnetic Spectrum along with the frequencies and wavelengths. So far I have looked at several mnemonics but ...
-4
votes
0answers
34 views

finding the speed and distance [on hold]

Valerie and Jake are performing the 1-D collisions experiment. They measure the mass of glider A to be $255~\text{g}$ and its length to be $17.3~\text{cm}$. The same properties of glider B are ...
1
vote
1answer
24 views

Variation of infinite grid of ideal one-ohm resistors: what would be the equivalent resistance between 2 points in a 3D lattice?

I'm sure that many here are familiar with this famous problem that popped up on xkcd one day: On this infinite grid of ideal one-ohm resistors, what's the equivalent resistance between the two ...
0
votes
2answers
43 views

Electric field 0 everywhere inside Gaussian surface

Gauss's Law shows that the electric field everywhere inside a spherical shell of uniform charge density is $0$. Suppose we have a surface which divides space into two disjoint regions (an interior and ...
0
votes
1answer
43 views

Lie derivative in this paper [on hold]

In this paper http://arxiv.org/abs/1210.2332 it says in (3.19) p. 8 that $$L_{V}z^A =0$$ but I don't know much about Lie derivatives except what I saw now through wikipedia ...
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0answers
43 views

How to design this material? (in theory) [on hold]

My university task is to design an injectable biomaterial that fills a tissue void and doesn't diffuse away from the site of an injection. I need to describe how I would carry out this design in ...
-4
votes
1answer
39 views

Total Energy Stored in Seven Capacitors [on hold]

The capacitive network shown in the figure is assembled with initially uncharged capacitors. A potential difference, Vab = +100V, is applied across the network. The switch S in the network is kept ...
-1
votes
1answer
26 views

Calculating values related to angular momentum and then their uncertainties [on hold]

Here is the problem: And here is my work (sorry it is handwritten, it would take a while to type this out) My problem is that I'm not sure if it's right or if it makes sense to keep getting 0 ...
2
votes
3answers
128 views

Finding the appropriate coordinate transformation given two metrics

Given the two-dimensional metric $$ds^2=-r^2dt^2+dr^2$$ How can I find a coordinate transformation such that this metric reduces to the two-dimensional Minkowski metric? I know that ...
0
votes
2answers
45 views

Proving the Lorentz invariance of the Lorentz invariant phase space element

I have been looking around for a satisfactory answer to prove that $$\frac{d^3\vec{p}}{2E_{\vec{p}}}$$ where $E_{\vec{p}}=+\sqrt{(|\vec{p}|c)^2+(mc^2)^2}$, is Lorentz invariant. The standard answer ...
-1
votes
1answer
26 views

A 45.0 kg girl is standing on a 163 kg plank? [on hold]

The plank, originally at rest, is free to slide on a frozen lake, which is a flat, frictionless surface. The girl begins to walk along the plank at a constant velocity of 1.47 m/s relative to the ...
1
vote
2answers
145 views

Confused about Gauss's Law for parallel plates

I am having trouble understanding Gauss's Law. Suppose we wish to find the electric field strength between two parallel plates with charge density $\sigma$. I know it should be ...
0
votes
1answer
20 views

Squaring the matrix element and summing over the spins of outgoing particles?

I found the matrix element in a scalar interaction $\nu_ee \rightarrow \nu_ee$ to be $$M=\bar{u}(\nu,q_2)u(e,p_1)\bar{u}(e,p_2)(1-\gamma_5)u(\nu,q_1)$$ But I do not know how to find the $|M|^2$ and ...
2
votes
0answers
28 views

How to solve a difficult equation describing large vacuum fluctuations?

Suppose that a Quantum System can be described by the wavefunction $\psi(\vec{x},t)$, but due to the occurence of chaotic noise within the Quantum System, only the "filtered" wavefunction ...
0
votes
0answers
34 views

Finding the Expectation Value of basis states [on hold]

I am a Mathematician and I am taking a Quantum Computing class. We have been asked to find the expectation value of $X$ tensor $Z$ and $H$ tensor $H$. X is the not operator and switches the state the ...
0
votes
1answer
42 views

Difference of potential between points [on hold]

I have the following question: What is the potential difference between point A - B? Could somebody please explain me how should I count it? The voltage U=18V. I found out that V1=8V, V2=10V, ...
1
vote
0answers
19 views

Laser travelling through a radially graded index of refraction

A laser beam propagates through a region whose refractive index varies as $\mu=\mu_0(\frac{r}{r_0})$. At a distance of R the beam makes an angle of 30° with the normal. Find the minimum seperation ...
0
votes
1answer
33 views

Galactic Rotation of gas clouds

I need to use the formula for the radial velocities of gas clouds due to Galactic rotation together with the assumption of a flat rotation curve ($V$ independent of $R$) to find the Galacto-centric ...
-2
votes
1answer
37 views

Peskin “An Introduction to quantum field theory”, problem 4.1 [on hold]

I'm trying to solve problem 4.1)c) in Peskin's book "An Introduction to quantum field theory" using this answer sheet. I don't understand why lambda correspond to a line in Feynman diagrams. Any idea? ...
0
votes
2answers
42 views

Calculating the electric field of an infinite flat 2D sheet of charge

I was trying to calculate the electric field of an infinite flat sheet of charge. I considered the sheet to be the plane $z=0$ and the position where the electric field is calculated to be ...
1
vote
3answers
83 views

Can we define the zero potential at an imaginary point?

Consider a force field defined as $$\vec{F}(x) = \left(\frac{A}{x^2}-B\right)\hat{i}\space$$ where $A, B$ are positive constants. We want to get the potential energy function for this field. We can ...
0
votes
0answers
58 views

Quantum mechanical expectation of angular momentum along different axes [on hold]

This is a question from Concepts of Quantum Mechanics by Mathur & Singh, and I don't know where I should start from: Show that, for a state $|j,m \rangle$, corresponding to a definite value of ...
-7
votes
0answers
44 views

vectors, space and coordinate systems, [on hold]

An object moves from the position $r_1 = (1,3,-5)$ to the position $r_2 = (-1,4,8)$ during the time $8~\text{s}$. Find displacement, distance, average velocity, average speed.
0
votes
1answer
61 views

Integral calculation [on hold]

I do some calculation in QED, but I can not calculate such integral $$ I(a_1,a_2,m_1,m_2)=\int\frac{ d^2\mathbf{x} d^2\mathbf{y}}{(1+\mathbf{x}^2)(1+\mathbf{y}^2)((\mathbf{x}+a_1 ...
0
votes
1answer
74 views

How does one normalize this wavefunction? [on hold]

Here is the question: So I could write $ N = \dfrac{1}{{\sqrt{<Ψ|Ψ>}}} $, right? Considering the parentheses in the exponential term, it looks like a good idea to switch to spherical polar ...
1
vote
3answers
51 views

Using a charged capacitor to charge two others [on hold]

Here is the homework question in question: The figure (below) displays a 13.1 V battery and three uncharged capacitors of capacitances C1 = 4.08 μF,C2 = 6.19 μF and C3 = 3.30 μF. The switch is thrown ...
2
votes
1answer
65 views

$SU(3)$ irreducible representations with tensor method

I am dealing with the tensor product representation of $SU(3)$ and I have some problems in understanding some decomposition. 1) Let's find the irreducible representation of $3\otimes\bar{3}$ we have ...
1
vote
2answers
50 views

Would it be possible to make a space elevator only in the atmosphere?

I wonder if with current day technology we could make a "floating platform" which hangs from a satellite by a space elevator. This could allow a "launch process" involving floating a balloon to the ...
1
vote
2answers
74 views

How do you accelerate atoms/particles? [on hold]

Revision of earlier question. So I'm in 11th grade, and I'm writing a theory for my Physics professor and I need help. I need to know how you can make atoms/particles travel faster in the vacuum of ...
0
votes
2answers
62 views

Balancing a pencil

I came across this equation for balancing a pencil while solving some problems: $$ml\ddot { \theta } =mg\theta $$ Where $l=$the length of the pencil, and $\theta$ is the angle it makes with vertical. ...
2
votes
0answers
26 views

Homogeneous turbulence [on hold]

I'm trying to demonstrate the Karman-Howarth expression of isotropic turbulence for the velocity correlation functions: $R_{ij}(r)=A(r)r_ir_j + B(r)\delta_{ij}$ since: \begin{align} A(r) r^2 + ...
0
votes
1answer
26 views

kinematics: parabolic movement [on hold]

How does the formula $ax^2$ is affected by the angle and the power of the launch of, let's say, a cannon ball, (regardless of its mass)? I am trying to create a simulation, and everything I have to ...