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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21 views

Calculation of Water Temperature

In a close looped process the soft water (approx. $200\frac{m^3}{h}$ at $14$ barg) gets heated gradually from the ambient temperature to $\Delta$T of 20°C in $13-14$ minutes, i.e. if amb. temp is 45°C ...
0
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0answers
20 views

Motion equations [on hold]

i'd like to develop motion equations for the following using newton's laws, and i can't seem to identify all the forces involved, especially when it comes to the ball. The ball is rolling freely ...
0
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0answers
35 views

Why toroid has no magnetic pole?

Our teacher taught us about Gauss law of magnetism, $\phi_B=\oint_s{\small{}}\vec {B} .\vec {ds}=0$(being valid because magnetic field line form closed loop, right?) which denies the existence of ...
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1answer
38 views

Where does this commutator relation come from?

What is the origin of this relation: $$ [H,a_n^\dagger] = \epsilon_n a_n^\dagger $$ for Hamiltonian $H$, creation operator $ a_n^\dagger $, and eigenvalue $ \epsilon_n $. The square brackets denote ...
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3answers
38 views

General question about the potential barrier problem: Why does $\exp( kx)$ diverge when $x>0$ in the case when $E < V(x)$?

For the two images below, the first potential barrier has particles approaching it where $E > V_o$ & the second has a particle that has $E < V_o$, where $E$ is the energy of the particles ...
1
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1answer
30 views

Wigner's Theorem and discrete Symmetries

According to my skript: A pure state is a ray: $\quad$ $\{λψ\}$, where $ψ ∈ \mathcal H$, $||ψ|| =1$ fixed and $λ ∈ \mathbb C$, $|λ| = 1$. Pure states are uniquely given by 1-dimensional orthogonal ...
0
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0answers
10 views

Transistor circuit question [migrated]

I'm trying to solve the transistor question. But , I'm stuck on the I2. Is I2 equal I0 ? The ...
0
votes
3answers
68 views

Which formula is correct for the velocity of a mathematical pendulum?

Suppose I have a mathematical pendulum swinging in a plane, and the angle between the vertical direction and the pendulum is $\phi(t) = \phi_0 \cos\omega_0t$. The length of the pendulum is $l$, its ...
1
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1answer
35 views

Probabilities of pure states and density operators

According to my skript: A pure state is a ray: $\quad$ $\{λψ\}$, where $ψ ∈ \mathcal H$, $||ψ|| =1$ fixed and $λ ∈ \mathbb C$, $|λ| = 1$. Pure states are uniquely given by 1-dimensional orthogonal ...
1
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2answers
33 views

Heating of two identical balls

You are given two identical steel balls of radius (say) 5 cm. One ball is resting on a table, the other ball is hanging from a string. Both balls are heated (with a blow torch) until their radii have ...
0
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1answer
30 views

Obtaining momentum operator $P^\mu$ from Lagrangian and energy-momentum tensor $T^{\mu\nu}$

I am pretty new to quantum field theory. Given the Lagrangian density, $$ \mathcal{L} = \frac{1}{2} ( \partial_\mu \phi ) ( \partial^\mu \phi ) - \frac{1}{2} m^2 \phi^2 $$ and its energy-momentum ...
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0answers
20 views

Forces acting on a body on a slope, while the slope is accelerating forward [on hold]

I have the following problem: I have to determine the initial acceleration a body with mass $m$ experiences while it is on a slope and the slope is pushed forward with acceleration $a_0$ and the ...
-1
votes
0answers
21 views

Conservation of angular momentum -Two small balls A and B, each of mass m, are joined rigidly to the ends of a light rod of length L [on hold]

It is clamped at the centre in such a way that it can rotate freely about a horizontal axis through the clamp. The system is kept at rest in the horizontal position. A particle P of the same mass m is ...
0
votes
1answer
45 views

How can I write the anderson hamiltonian as a matrix? [on hold]

How can I write this Hamiltonian: $$ H = \sum E_d \hat{n}_d + \sum_k \epsilon_k\hat{n}_k + \sum_k V_{kd} (\hat{a}^\dagger_k \hat{a}_d + \hat{a}^\dagger_d \hat{a}_k) $$ in matrix form using its ...
0
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0answers
19 views

Confused about question and related concept [on hold]

Dust particles are uniformly distributed in space with a mass per unit volume 'd' . A circular plate of area A is moving in this space at velocity v. The dust particles that come into contact with ...
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votes
1answer
35 views

Torque and force from friction upon a rotating 2D disc [on hold]

I am developing a 2D physics engine, and therefore I got stuck with this problem: A disc (2D) is sliding over a surface. It is both rotating and translating. How will friction act upon it? What ...
1
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1answer
29 views

Boundary Conditions in a Step Potential

I'm trying to solve problem 2.35 in Griffith's Introduction to Quantum Mechanics (2nd edition), but it left me rather confused, so I hope you can help me to understand this a little bit better. The ...
2
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0answers
27 views

Why is the resolution of an electron microscope on the order of the electron wavelength?

A problem (Problem 1.4) in Zettili's Intro to Quantum Mechanics is: Estimate the energy of the electrons that we need to use in an electron microscope to resolve a separation of 0.27 nm. I was not ...
0
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1answer
20 views

Length of Focal Spot Along the Optic Axis

I am currently attempting a physics question on optics. A Fresnel Zone Plate has overall radius a and contains N half-period zones. The odd-numbered zones are open but the even -numbered zones are ...
0
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0answers
15 views

Forces on inclined cube [on hold]

A 30kg cubic block, of side length 300mm, slides down a lubricated plane with a slope of 30. The lubricant has an absolute viscosity of 3 2 2.3 10 N s m.the speed of the cubic block down the plane ...
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0answers
19 views

Determining Mars' mass: what am I doing wrong? [on hold]

Problem: Given that the acceleration of gravity at the surface of Mars is 0.38 of what it is on Earth, and that Mars' radius is 3400 km, determine the mass of Mars. Solution: $0.38 \mathbf{g} = ...
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0answers
15 views

collision rates [on hold]

You have a container with $n_A$ gas molecules of type $A$ and $n_B$ gas molecules of type $B$. Consider bi-molecular collisions. Denote by $C_{XY}$ the total number of collisions of $X$ and $Y$ type ...
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0answers
11 views

Wave optics - Interference [on hold]

A lens with refractive index=1.5 is coated with a thin film of refractive index of 1.2 in order to reduce the the reflection from its surface at wavelength= 4800 Angstrom . Find the minimum thickness ...
0
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0answers
14 views

Kinematics Numrical [on hold]

Two small stones are tied at the ends of a rope of length 5 m. Imagine you are holding them, one in each hand, with the rope hanging loosely on the top floor balcony of a tall building. You release ...
1
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1answer
14 views

The concept of an electrically short, medium and long transmission line is primarily based on? [on hold]

The concept of an electrically short, medium and long transmission line is primarily based on ? A. nominal voltage of the line B. physical length of line C. wave length of line D. power transmitted ...
2
votes
1answer
81 views

Trying to show that the current is conserved

$ \newcommand{\p}{\partial} $ I am trying to show that the current $J^{\mu} = (\gamma_{\nu}\partial^{\nu} \phi - m\phi)\gamma^{\mu}\psi$ is conserved for all fields that satisfy the Klein-Gordon and ...
-1
votes
0answers
9 views

Calculation of the Lowest Energy of the trial wavefunction [on hold]

After a calculation of the lowest energy using two variational parameters a and b it is found that: E(a,b) = (3a+b)^2 - ab What is the optimal (minimum) value of E If I expand the bracket and ...
1
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1answer
37 views

Combination of errors question

A resistance R is in series with an inductance L. At angular frequency ω the magnitude of the complex impedance Z of this combination is given by |Z|^2 = R^2 + (ωL)^2 . Find |Z|, and the error in |Z|, ...
0
votes
1answer
55 views

Exact formula in simplest terms for Magnetic field at a point outside a solenoid [on hold]

I need the COMPLETE formula for magnetic field outside the solenoid. So the situation I am stuck in is I have to solve this question: The magnetic field at the centre of coil of $n$ turns, bent in ...
0
votes
1answer
26 views

Gauss law and capacitance [on hold]

In a parallel plate capacitor of capacitance $C$, the plates have $q$ and $-q$ charge on each other. I am curious to know if charge 'h' is given to +ve plate and 'k' to -ve plate how the potential btn ...
0
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0answers
19 views

Expectation value calculation using Hermite polynomials [on hold]

I am facing this problem of not being able to correctly determine $x^2$ expectation value using the eigenfunction for harmonic oscillator involving Hermite polynomials as derived in the analytic ...
0
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0answers
24 views

When do 3 particles on the vertices of an Equilateral triangle meet? [migrated]

I am having a problem solving the following question. It says that "3 particles A,B and C are situated at the vertices of an equilateral triangle ABC of side d at t=0. Each particle moves with a ...
0
votes
0answers
27 views

Nuclear Physics Homework [on hold]

Can one help to solve this questions, I solved question 1 and my answer was 8.6MeV, but I want to make sure that my answer is correct ! In Colomb scattering of 10 MeV protons by a target of $^7$Li, ...
0
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0answers
37 views

A few questions related to Magnetic fields [on hold]

Before starting the question, I would like to say that these questions might be easy to solve, the concepts required may be simple, but I did not understand how to do these questions, and so I am ...
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0answers
16 views

Statics of rigid body [on hold]

A uniform rod of length 2a metre and mass M kg is smoothly hinged at A.It is maintained in equilibrium by a horizontal force of magnitude P acting at B.The rod is inclined at 30 degree to the ...
0
votes
3answers
73 views

Expected value of $xp$ in harmonic oscillator [on hold]

I wanna find out the expected value of the $xp$ operator for the $n$-th excited state of the harmonic oscillator, i.e. calculate the value $\langle n|xp|n \rangle$. I express the position and momentum ...
0
votes
1answer
48 views

How to determine pressure at the bottom of a reservoir? [on hold]

I am trying to solve a problem of fluid dynamics. This is not a school assignment or anything like that, just a small project for myself. Suppose we know: 1. h and L 2. v0 approximated at 0 since the ...
0
votes
1answer
39 views

London equation and Maxwell equation

I am reading Tinkham's Introduction to Superconductivity (2nd ed) (Amazon link). On pages 4-5, they state that:" The second London equation 1.4, when combined with the Maxwell equation ${\rm ...
-1
votes
0answers
35 views

A block of mass M moving on a frictionelss horizontal surface collides with a spring of spring constant k and compresses it by length L [on hold]

A block of mass M moving on a frictionelss horizontal surface collides with a spring of spring constant k and compresses it by length L. What is the maximum momentum of the block after collision? ...
0
votes
0answers
24 views

How to find charged distribution in three parallel plates? [on hold]

there are three equidistant identical parallel plates . plate-1 is isolated while plate-2 and plate-3 are connected with conducting wire. what will be the charge distribution on each plate
0
votes
2answers
74 views

If earth stopped spinning how much heavier would 1kg feel

My girlfriend seems to think if the earth stops spinning we will all fall off However, she did spark my imagination, due to centripetal force things must feel lighter then they otherwise would on an ...
1
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0answers
30 views

Weinberg Chapter 19.5, pion scattering amplitude derivations

I've been reading through Weinberg's Chapter 19, and am somewhat confused about how he manages to derive equations 19.5.25 and 19.5.29, which read: ...
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votes
1answer
41 views

Physics Riddle - Know Plant Earth [on hold]

I hope it's OK to ask here, because it's pretty much as basic as physics can get. So here it is: I have a hot air balloon that can rise to any altitude I'd like, a precise watch, a sandbag, a weigh ...
1
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0answers
37 views

Explicit demonstration of the relativistic invariance of the Weyl equation

It can be demonstrated explicitly that the Dirac equation is relativistically invariant. This is a proof (borrowed from Peskin & Schroder, see the unnumbered equation after the eqn. 3.31): ...
2
votes
1answer
70 views

Conservation Of Angular Momentum of a falling stone

I am currently stuck on the following question. A stone is dropped from a stationary helicopter 500m above the ground, at the equator. How far from the point vertically below the helicopter does it ...
2
votes
1answer
37 views

Subluminal speed when going in a circle around someone

Some time ago I thought about such situation: There are two people in the room. Both have synchronized watches on their wirsts. Then the person A starts running around person B with the speed of ...
0
votes
1answer
25 views

Newton's Second law of motion: Elevator problem [on hold]

this is a question about newton's second law of motion that I am a bit stuck on. One end of the cable of an elevator is attached to the elevator car,and the other end of the cable is attached to a ...
-3
votes
0answers
40 views

Path integral method for harmonic oscillator [on hold]

Use the path integral method to calculate the transition $\langle x(f),t(f) \rvert x(i),t(i) \rangle$ for a harmonic oscillator.
0
votes
1answer
41 views

3 pulley and 2 masses,relation between their accelerations [on hold]

I want to find acceleration of each mass in this system. lets take length of rope between three pulleys as $L$. then we can use this relations ...
0
votes
0answers
62 views

$e^{\mp i \pi \hbar S_z } S_{ \pm} e^{ \pm i \pi \hbar S_z} = -S_{\pm}.$ [on hold]

I am supposed to show that $$e^{\mp i \pi \hbar S_z } S_{ \pm} e^{ \pm i \pi \hbar S_z} = -S_{\pm}.$$ I tried things like the Hadamard equation, but did not really get close to this equation. Does ...