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 questions.

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-2
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1answer
13 views

Amperes Law problems for Homework

Having trouble with homework for an Electromag class. Not sure what the "circulation" is talking about. I know that with the right hand rule, a magnetic field will go in and come back out of loops. ...
0
votes
1answer
17 views

Isobaric expansion work, W = p(V2-V1)

Isobaric process, so $W = p\Delta V$ according to my book. We have 10 kg of saturated steam at $x = 0.9$, P = 200 kPa. We add work at a rate of $700 W$ and we're interested in the time it takes. We ...
0
votes
1answer
38 views

Need help solving a task - laws of conservation (leptons, baryons, charge)

We got a task from our teacher, a multiple choice task; and here it is: Which of the following reactions is correct, considering the conservation laws? Note: We have only learned to use the ...
0
votes
0answers
14 views

Hydrogen Transition Rate from n',l to n',l'

I am trying to solve Problem 9.22 in Griffiths' Intro to Quantum Mechanics (2nd Ed.) that asks to show the spontaneous emission rate for a transition from n,l to n',l' in hydrogen is given by ...
0
votes
0answers
19 views

Quantum physics particle in a box [on hold]

A problem I've been given states: Without calculation, write down and explain the energy expectation value $\langle H\rangle$ and the uncertainty $σ_H$ for the $ψ_n$ energy eigenstate of the ...
0
votes
1answer
68 views

How many years would it take to travel 490 light-years? [on hold]

NASA has discovered so many new planets and galaxies, one may indeed be a habitat in the future for man kind. This new discovery they think by size and location of sun like star, just might have water ...
-1
votes
0answers
13 views

Momentum of a divided sphere [on hold]

I have a problem with a exercise, please, anyone knows where I wrong? I can not find the error in my solution... Basically I consider that the equation of the horizontal momentum is equal the initial ...
-1
votes
1answer
36 views

Two equations for $y$ in projectile motions

I have two equations on my equation sheet: $$y=v_o t \sin \theta - \frac{1}{2} g t^2.$$ And then another: $$y_{\max} = \frac{v_o^2 \sin 2 \theta }{2g}.$$ I understand that the first one is an ...
1
vote
1answer
48 views

Rotation and fictitious forces

A bug eats through an apple and forms a vertical, infinitesimally thin canal parallel to the vertical diameter at a distance $\frac{R}{2}$ from the center. The apple rotates at angular velocity around ...
-1
votes
1answer
25 views

What would be the change in gravitational potential energy? [on hold]

QUESTION ------------ A rope of length L and mass M is connected to a chain of length L and mass 2M and hung vertically from a point. What is the change in potential energy if the system is INVERTED ...
0
votes
1answer
46 views

Transverse part of a current

I am reading Many particle physics (3rd edition by Mahan). On page 51, there is a current of the form $$\vec{j}(\vec{r})=\vec{J}_0e^{i\vec{k}\cdot\vec{r}}$$, the transverse part of it is: ...
2
votes
1answer
79 views

Sum of torque from a sphere

A sphere (grey color) turn in rotation at $\omega$ rd/s. There are 2 walls that prevent sphere to escape. Walls can only turn around center of rotation. The sphere turn only at $\omega$ rd/s too. The ...
2
votes
1answer
20 views

Understanding the displacement of a particle in a wave

I have a question regarding waves and the equation we use to describe their motion. My understanding feels shaky, so i'd like to see if I can get a good explanation/way of thinking about it. The ...
4
votes
0answers
43 views

Show that the Laplace-Runge-Lenz vector is conserved using poisson brackets

(I realise similar Phys.SE questions already exist but there is no answer with the Poisson bracket notation, I'll take this down if someone lets me know I should have commented in the existing ...
0
votes
1answer
41 views

It is a numerical

If the densities of the moon and the earth are related by $\rho_m / \rho_e =3/5$, and if $g_m /g_e = 1/6$, then what is $R_m /R_e$? ($g=$ acceleration due to gravity and $R=$ Radius)
0
votes
1answer
20 views

What is the difference between a linear and non-linear solution in the bending of beams?

I have been working on a simulator for bending of beams and came now to a tricky doubt: What should be the difference between a linear and non linear solution in this case (graphic at bottom)? The ...
0
votes
1answer
17 views

about the gravitation chapter [on hold]

The gravitational field in the region is given by $ g=5\hat{i} + 12\hat{j}$. The change in the gravitational potential energy of a particle of mass $2$ kg when it is taken from the origin to a point ...
1
vote
1answer
58 views

Why don't we need to normalize wavefunction to find probability distribution?

Consider an unormalized wavefunction of a rotor at $t = 0$, a combination of $n=0$ and $n=2$ states: $$\psi(\phi) = 3 - 2 \cos (2\phi).$$ Find the probability distribution in angle. The ...
2
votes
1answer
31 views

Dipole matrix elements through parity argument

I am trying to find the following dipole moment matrix element $(|n,\ell,m\rangle)$. $$e\langle1,0,0|\vec r|2,0,0\rangle$$ I believe that I can say this matrix element is zero because of parity. The ...
0
votes
1answer
71 views

Calculating the expectation value of a Hamiltonian

I want to calculate the expectation value of a Hamiltonian. I have a wave function that is $$\psi = \frac{1}{\sqrt{5}}(1\phi_1 + 2\phi_2).$$ I want to know if I set this up properly. The ...
1
vote
1answer
64 views

Question concerning the Feynman Lectures of Physics

I am reading the Feynman lectures and at this point http://www.feynmanlectures.caltech.edu/I_13.html#Ch13-S3 it says as follows: The time derivate of the potential energy is $\begin{equation} ...
0
votes
0answers
20 views

Visualizing Tension in FBD — Rope With Mass

Asked to find the tension of a point that is halfway up a rope (where the rope has mass), given the system: The solution of the problem (FBD included) is as follows. In this diagram (Tm = tension ...
0
votes
1answer
30 views

Rotating frames [on hold]

A bird of mass $m$ is on a merry-go-round of radius $a$ which rotates at constant angular velocity $-\omega_b$ in the $y$ direction. A woman of mass $M$ is on a second merry-go-around of radius $b$ ...
0
votes
0answers
31 views

Stable equilibrium [on hold]

A uniform cylinder of radius $R$ and height $H$ is glued to a uniform hemisphere of a similar radius so that their bases unite. The two bodies have the same uniform mass distribution. The figure is ...
0
votes
1answer
32 views

Isn't $\varepsilon_0\int_{\text{all space}}\vec{E}_1\cdot\vec{E}_2 \,{\rm d}v$ just the potential energy?

I have two metallic spheres each with a charge of $q_1$ and $q_2$ respectively. What is the value of $$\varepsilon_0\int_{\text{all space}} \vec{E}_1\cdot\vec{E}_2 \,{\rm d}v$$ where $\vec{E}_1$ and ...
1
vote
0answers
33 views

How do I get around the fact that boundary conditions don't apply in the equation's region of validity?

A tight string lies along the positive x-axis when unperturbed. Its displacement from the x-axis is denoted by $y(x, t)$. It is attached to a boundary at $x = 0$. The condition at the boundary is ...
0
votes
0answers
39 views

Truss Analysis with multiple moments creates linear combinations

So in my engineering statics course we have been doing a great deal of work with trusses and machines, and a part of the physics behind it has really been confusing me. Just as a note, I also have ...
2
votes
1answer
56 views

Characteristic polynomial of a Matrix

In fact, this problem is more likely to be a math problem. When I read a paper(http://arxiv.org/abs/0707.2875), the author includes the characteristic polynomial for a type of matrix $A_k$ with ...
1
vote
1answer
40 views

Calculate energy from vacuum electromagnetic-field action

I'm reading Rubakov's "Classical Theory of Gauge Fields", and I'm having a little bit of trouble with problem 7, p 15: Using an expression of the type $E = \int d^{3} x \frac{\delta L}{\delta ...
0
votes
0answers
26 views

What is angular speed after 2nd collision? speed of block? time? [on hold]

Question with graph: For a) I got that the mass for rod must equal to block for the block to stop immediately after the collision (perfect velocity transfer) so the initial speed for rod after ...
-4
votes
0answers
29 views

Electromagnetic induction problems [on hold]

1 What will happen to the illumination of the lamp (increase/decrease/remains constant) when decreasing the resistance R. 2 In the given figure a long wire is carrying a current of 5A, what is the ...
-4
votes
0answers
18 views

Electric current problems [on hold]

1 Calculate the equivalent resistance Ans: 8/3 Ω 2 Calculate the equivalent resistance Ans: 5Ω I've posted the answers for these problems to prove that this isn't some sort of homework,all i ...
0
votes
0answers
17 views

Classical Scattering of Slow Neutrons by a Diatomic Molecule [on hold]

Consider an experiment in which slow neutrons of momentum $\hbar k$ are scattered by a diatomic molecule; suppose that the molecule is aligned along the $y$ axis with one atom at $y=-b$ and the ...
0
votes
1answer
24 views

First Order Time Depenent Perturbation theory of particle in magnetic field

So I am dealing with the following hamiltonian, and the following perturbation: $$H=-\mu B_0\sigma_z$$ $$V=\mu B_1(\cos(\omega t)\hat x-\sin(\omega t)\hat y)\cdot{\bf \sigma}$$ I am asked for the ...
1
vote
0answers
37 views

What is the Electric field of an infinite line charge while on the same axis [on hold]

Consider a line of charge (w/ charge per unit length lambda(L)) which extends along the x-axis from $x = -\infty$ to $x = 0$. (a) Find the electric field at any point along the positive x-axis (b) ...
0
votes
3answers
56 views

Why don't the kinematic equations agree with calculating velocity one second at a time?

A car is moving at a velocity of $10 \, \text{m}/\text{s}$. After point $A$ no acceleration is provided. By simple measurement, the acceleration is found to be $-1 \, \text{m}/\text{s}^2$. Using ...
2
votes
1answer
97 views

Sum of forces with liquid in rotation

It's not homework (I'm teacher). I would like to compute sum of forces on this study : The shape is symmetrical like that I'm sure the center of gravity is in the center of the shape. I compute ...
0
votes
1answer
36 views

Distance travelled given velocity at various moments of time [on hold]

I am trying to solve a problem where I need to find the distance travelled at the end of the nth second and my input data contains only velocities at different time instances. Say for instance I ...
0
votes
1answer
16 views

Capacitance of a parallel plate capacitor with joint plates [on hold]

What is the capacitance of a parallel plate capacitor with plates joint with a thin metal strip diagonally like this |\| I am confused. Is it infinte or 0.
3
votes
1answer
63 views

Feynman Green Function in D=2 for D'Alembertian

I'm trying to obtain the Feynman Green Function (i.e. I'm using the Feynman Causal prescription to compute the green function) for the D'Alembertian in D=2, I'm finding $$G^{(2)}_F (t; \vec x) = ...
-3
votes
1answer
17 views

Work done from force-extension graph [on hold]

The answer is C, but I got B... F at 3cm = 10, F at 6cm = 20 $W=F\cdot s$ $(20-10)\times 3=30$ I must have made a mistake or stupid assumption then, how am I meant to get 45 N cm?
-2
votes
0answers
13 views

electric fields - point charges [on hold]

A +15μC point charge Q1 is at a distance of 20mm from a +10μC charge Q2. Explain why there is a point along the line between the two charges at which the electric field strength is zero. After ...
0
votes
0answers
20 views

Begginer Physics - Interpreting displacement-time graphs [on hold]

d Hello, I am given the question to find the displacement when t = 9.0 s from the above graph The answer is 2.5m, but I cannot see how to calculate it on this graph? I've completely forgot!
0
votes
1answer
15 views

The force of a weight tied in a vertical circle striking an object at its lowest point when dropped from a 90˚ Angle [on hold]

So there is a fixed point, and a string is tied to it. On the other end of the string is a weight. I've set up the apparatus so that at the lowest point of the vertical circle made by spinning the ...
0
votes
0answers
15 views

Carrier charge density [on hold]

A conductor has increasing cross sectional area. Will the carrier charge density change? And will the drift velocity change according to change in magnitude of carrier charge density or will it ...
1
vote
0answers
38 views

Degeneracy, spherical harmonics

In a 3D oscillator, the energy levels are known to be $(n_x + n_y + n_z + \frac{3}{2})\hbar \omega = (n + \frac{3}{2})\hbar \omega$. Say for $n = 1$, any of the $n$'s can be $1$ and the rest are $0$. ...
0
votes
0answers
27 views

2D Harmonic Oscillator - Degeneracy, Symmetry, Perturbation [on hold]

Part (a): Derive Ehrenfest's Theorem. What is a good quantum number? Part (b): Write down the energy eigenvalues and sketch energy diagram showing first 6 levels. Part (c): What's the symmetry of ...
0
votes
0answers
24 views

Transition between two states probability - Perturbation Theory [on hold]

Part (a): Show probability to transit from state i to j is given by: Part (b)i: Use answer in part (a) to find probability Part (b)ii: Use time evolution to find probability Attempt at question: ...
2
votes
1answer
58 views

How do I simulate this simple quantum circuit in MATLAB

I want to simulate a circuit similar to the one below in MATLAB. If you have a state matrix describing the state of 3 qubits, I understand that you could apply a CNOT matrix tensored with and identity ...
0
votes
0answers
20 views

Breakdown voltage

Capacitor (Geiger counter) is made out of wire with radius $R_1 = 5 mm$ and coaxial tube with radius $R_2 = 5 cm$. What is the maximum voltage on the capacitor, given the breakdown voltage of air ...