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.
0
votes
1answer
212 views
Expressions for canonical partition function and probabilities $p(E_i)$
Given an atom with 4 allowed states corresponding to the energy levels
$E_1 = 0$, $E_2 = E$, and $E_3 = 2E$ with degeneracies 1, 1, and 2 respectively.
How do I find the expressions for the ...
2
votes
1answer
100 views
Is there a closed form solution to the Esdale river problem?
This is probably not well known problem but it looks like open problem. What kind of methods there are to find a closed form solution to the physical situation?
Can you solve this problem?
You're ...
4
votes
0answers
58 views
I am trying to calculate the branching ration of higgs goes to 2 photons using the standard model [closed]
I need to use the three lowest order feynman diagrams to first calculate the squared matrix element to put into fermis golden rule formula and then from there get the branching ratio of higgs decays ...
6
votes
2answers
143 views
Radioactive Decay
Problem:Nuclei of a radioactive element $\Bbb X$ having decay constant $\lambda$ , ( decays into another stable nuclei $\Bbb Y$ ) is being produced by some external process at a constant rate ...
1
vote
1answer
54 views
Energy required for ionizing Helium
The energy required to remove both electrons from the helium atom in its ground state is 79.0 eV. How much energy is required to ionize helium (i.e., to remove one electron)?
...
3
votes
1answer
38 views
inductance value of a single turn thin circular coil
the question may seem simple but I haven't found any fitting formula yet. The problem is the following:
consider a single-turn, circular coil made of reasonably thin wire (diameter of the wire much ...
-3
votes
0answers
36 views
A space vehicle travelling at a velocity of 1296 (km/h) separates by a controlled explosion into two sections of mass 7276 (kg) and 236 (kg) [closed]
A space vehicle traveling at a velocity of 1296 (km/h) separated by a controlled explosion into two sections of mass 7276 (kg) and 236 (kg). the two parts continue in the same direction with the ...
1
vote
0answers
36 views
tricky electric potential [closed]
I have this question: $q_1$ is a Uniformly Charged long Wire(inf in z axis) with $\lambda$ (charge per unit length).
In (0,0,0) I have A very long conducting cylinder of radius a (also inf in z axis). ...
0
votes
0answers
66 views
The electric field inside a uniformly polarized cylinder
Given a uniformly polarized cylinder with some polarization $P\vec{z}$ which has radius $R \gg d$ where $d$ is the height and $R$ is the radius. The cylinder's center is on $\vec{z}$ and it's on the ...
1
vote
3answers
65 views
Relation between the time, velocity and acceleration
This is question from I.E. Iredov's General Physics:
$1.22$ : The velocity of a particle moving in the positive direction of the $x-axis$ varies as $v = α \sqrt x$, where $α$ is a positive ...
-1
votes
1answer
73 views
electrical forces of two charges repelling each other
If I place two positive charges of different magnitudes on the x-axis (one at the origin, one at the some position x), as the two charges repel each other are the forces that they impart on each other ...
-4
votes
0answers
35 views
How to calculate average speed when time is greater than distance? [closed]
I understand that average speed is calculated as $\frac{distance}{time}$ however this question says the time is greater than speed. Here's the question:
A bicycle travels 15 Km (kilometers) in 30 ...
1
vote
1answer
44 views
Conversion of energy to mass with radioactive decay
Consider the following problem:
The nuclei of Am-241 decay by the emission of $\alpha$-particles with a kinetic speed of $8.8 \cdot 10^{-19} J$. In a certain source of Am-241 there are $ 4.0 \cdot ...
-1
votes
1answer
31 views
Can someone help with this hydrostatic force question? [closed]
A gate is in the form of a trapezoid 8 ft wide at the top, 5 ft wide at the bottom and is 3 ft high. If water reaches the top of the gate, what is the total hydrostatic force on the face of the gate?
...
0
votes
0answers
54 views
The gauge-invariance of the probability current
It is simple to show that under the gauge transformation $$\begin{cases}\vec A\to\vec A+\nabla\chi\\
\phi\to\phi-\frac{\partial \chi}{\partial t}\\
\psi\to \psi ...
1
vote
3answers
123 views
How can I solve for time without knowing the vertical velocity?
A guy posted this problem on a forum:
There is a bird sitting on a pole of height h. you throw a rock at it and the moment the rock leaves your hand the bird starts flying horizontally away from ...
0
votes
0answers
27 views
Differential BJT Amplifier [closed]
I am trying to solve this assignment
Consider this schematic (the BJT's are in the effective area, with
$\beta=200$ so as to $i_e\approx i_c$).
Define the common and differential gains, ...
0
votes
1answer
679 views
Beginner Thermal Dynamics Question [closed]
A syringe is set up filled with air (mainly N2 and O2) as shown in the diagram below. The surface area of the syringe is 15.3 cm2. The initial pressure inside the syringe is Pi = 114 kPa and the ...
0
votes
1answer
41 views
Calculating the moment inertia for a circle with a point mass on its perimeter
I want to calculate the tensor of the moment of inertia. Consider this situation:
The dot represents a points mass, in size equal to $\frac{5}{4}m$. $m$ is the mass of the homogenous circle. I'm ...
1
vote
1answer
45 views
Energy eigenvalues of a Q.H.Oscillator with $[\hat{H},\hat{a}] = -\hbar \omega \hat{a}$ and $[\hat{H},\hat{a}^\dagger] = \hbar \omega \hat{a}^\dagger$
I just finished deriving the commutators:
\begin{align}
[\hat{H}, \hat{a}] &= -\hbar \omega \hat{a}\\
[\hat{H}, \hat{a}^\dagger] &= \hbar \omega \hat{a}^\dagger\\
\end{align}
On the ...
0
votes
1answer
27 views
Paraxial rays and marginal rays question
Paraxial rays are found to focus far away the lens whereas marginal rays focus near to the lens, why? I didn't find satisfactory answer in my academic book..
4
votes
2answers
109 views
Vector potential
I have difficulty understanding the following vector calculus example. Text can be found here. It is the 5th Q&A -- starting with equation (31.1035).It concerns finding the vector potential of a ...
0
votes
0answers
13 views
Minimum thickness of bubble ensuring max reflectance
A soap bubble has index of refraction of 1.33. What minimum thickness of this bubble will ensure max reflectance of normally incident 530 nm light?
Ans is 99.6, but how do I get that?
I am ...
0
votes
2answers
119 views
How can I add an acceleration vector to a velocity with a different direction?
I am in my last year of high school and am struggling with some homework. I'm sorry if this question is incredibly stupid, but I simply can't find the answer in my notes.
If I have an object with a ...
5
votes
3answers
108 views
Horizontal $E$-field for a charged conducting disk
For part of a simulation I am writing, I need to know the electric field emitted from a charged conducting disk. If the disk was laid out in the $x$-$y$ plane, I am interested in the field in that ...
1
vote
1answer
262 views
( Legendre Generating Function) Off axis Electric Potential from an insulated disk
An insulated disk, uniform surface charge density $\sigma$, of radius $R$ is laid on the $x,y$ plane. Deduce the electric potential $V(z)$ along the z-axis. Next consider an off axis point $p'$, ...
0
votes
0answers
36 views
Work required to pull capacitor plates apart when connected to battery [closed]
For (a), I just use $F = k \frac{Q_1 Q_2}{r^2}$, then (b) $W = - \Delta U = - qE \Delta d$, correct so far? But how do I do (c)?
0
votes
0answers
62 views
What does this notation mean in terms of quantic numbers, and how to imagine the electrons in this quantic system? (Helium $2^1$ $P$ and $2^3$ $P$)
Helium atom in the $2^1$ $P$ and $2^3$ $P$ excited states
Now I'm guessing that 1 electron should be considered in the 1s state, but what about the other?
Should I consider the other as simply ...
1
vote
2answers
172 views
Preventing a block from sliding on a plane (with friction)
Assume a small square block $m$ is sitting on a larger wedge-shaped block of mass $M$ at an upper angle $\theta$ such that the little block will slide on the big block if both are started from rest ...
0
votes
0answers
39 views
Find mass of Plutonium in alpha decay [closed]
I did:
$$\Delta mc^2 = (m_{Pu}+m_{U}+m_{He})c^2$$
$$\frac{5.24 \times 10^6 eV}{(3 \times 10^8)^2} = m_{Pu} + 235.043924 + 4.002603$$
$$m_{Pu} = 239.0465$$
But answer is 239.052157, slightly ...
0
votes
0answers
30 views
Max wavelength of radiation to remove electron from $He^{+}$ [closed]
I am thinking, I use the formula
$$E_n = -13.6 \frac{Z^2}{n^2}$$
$$E_n = -13.6 \frac{2^2}{1^2}$$
But this gives $54.4$, correct answer is $22.8$. Whats wrong?
0
votes
0answers
29 views
State of (orbit of) electron given wavelength [closed]
I was thinking I use the formula
$$\frac{1}{\lambda} = R(\frac{1}{2^2} - \frac{1}{n^2})$$
But what I got looks something like:
$$\frac{1}{1.99\times 10^{-9}} = 1.097\times 10^7 ...
1
vote
1answer
51 views
Eigenfunctions in a harmonic oscillator
This assignment is about the one dimensional harmonic oscillator (HO).
The hamiltonian is just as you know from the HO, same goes for the energies, but I get that the wavefunction of the particle, at ...
1
vote
1answer
57 views
How long does it take a warm object to cool in air?
This is a work-related question. A warm steel torus of a given diameter & thickness is left in a room held at a controlled temperature, how long does it take to reach equilibrium? Assume the air ...
1
vote
1answer
76 views
Pendulum in an elevator
Suppose we have a pendulum tied to the ceiling of an elevator which is at rest. The pendulum is oscillating with a time period $T$, and it has an angular amplitude, say $\beta$. Now at some time ...
0
votes
1answer
45 views
Rotation System Dynamics - Unit conversion issues
I'm having issues with unit conversions when for rotational systems. All of my components (spring, mass, damper) come in Imperial units, so I did all my calculations in Imperial. When I convert to ...
0
votes
1answer
86 views
Shear Flow corresponding to Eccentric Shear Force of a Closed-section Beam (Structural Analysis - Mechanics)
Been stumped with this question for way too long... its a beam with a thin-walled rectangular cross-section, and a shear force is acting at a distance from the shear center. I know my decomposition of ...
-1
votes
0answers
60 views
Lifting house with balloons [closed]
I have an assignment about the movie Up. Here is the question:
Critique the brief floating scene. Did the animators use enough balloons during this scene? What important variables, (which affect ...
-1
votes
3answers
204 views
What's the right way to calculate charge on a capacitor?
How much charge is on each plate of a 4.00-F capacitor
when it is connected to a 12.0-V battery?
I said 2.4 x 10^-5 C because there are two plates of a parallel plate capacitor. But the key said only ...
1
vote
1answer
42 views
Apparent and real depth object in water [closed]
Did I get my formula right? Seems like the correct answer is $d_o = 1.33 \times d_i$ but I thought the formula I should use is $d_i = - \frac{n_2}{n_1} d_o$
0
votes
3answers
455 views
When I connect two charged capacitors side by side, what will be the voltage across them?
Say, I have two charged capacitors, one 3mF and one 2mF. The voltage across them are 20V and 30V respectively. Now if I connect the two capacitors side by side as shown below, what will be the voltage ...
2
votes
1answer
61 views
Time evolution operator to find expectation value
I have a state $\Psi (x,0) = \sum_{n=0}^{\infty} c_{n}u_n(x)$ and want to find the expectation value of any observable A at time t, $\langle \Psi(t)|\hat{A}|\Psi(t)\rangle$.
I know that I should ...
0
votes
3answers
291 views
charge density in a wire
if we have a infinitely thin and infinitely long straight wire on the z-Axis with given current I(t), how can i compute the charge density?
I figured out that the current-density is given by ...
0
votes
2answers
111 views
How do you calculate the time to emission of an electron from a metal given the incident radiation?
Here's the question:
A monochromatic point source of light radiates 25 W at a wavelength of 5000 angstroms. A plate of metal is placed 100 cm from the source. Atoms in the metal have a radius of 1 ...
2
votes
2answers
409 views
Gravitational field with cavity removed
I'm just struggling a little with this question:
A uniform sphere, of radius $R$, contains a spherical cavity of radius $R/4$, whose centre is $3R/8$ from the surface. The diameter passing through ...
4
votes
2answers
92 views
What's the physical difference between the quantities $\langle v_{i}v_{j}\rangle$ and $\langle v_{i}\rangle\langle v_{j}\rangle$?
What's the physical difference between the quantities $\left\langle v_{i}v_{j}\right\rangle $ and $\left\langle v_{i}\right\rangle \left\langle v_{j}\right\rangle $?
Where $\left\langle ...
4
votes
2answers
93 views
Proof for commutator relation $[\hat{H},\hat{a}] = - \hbar \omega \hat{a}$
I know how to derive below equations found on wikipedia and have done it myselt too:
\begin{align}
\hat{H} &= \hbar \omega \left(\hat{a}^\dagger\hat{a} + \frac{1}{2}\right)\\
\hat{H} &= ...
0
votes
1answer
70 views
Momentum change in colisions (Drude Model)
A particle suffers elastic colisions with scattering centers with a probability of colision per unit time $\lambda$. After a colision the particle is in a direction caracterized by a solid angle ...
0
votes
1answer
79 views
Reachable area of cannonball given fixed initial speed [closed]
The trajectory of a cannonball fired from the origin with initial speed $v_0$ at an angle $\theta$ is given by
$$y = x\tan\theta - \frac{g}{2v_0^2 \cos^2\theta}x^2.$$
For fixed $v_0$, at what angle ...
0
votes
0answers
18 views
Solving a simple equation [migrated]
I am just starting to study physics and I found this equation:
$$\ x = 8 - {6t} + t^2$$
If possible please explain in a step by step.
Sorry if it's too simple.
