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.
3
votes
0answers
170 views
Shape of a string/chain/cable/rope?
The height of a string in a gravitational field in 2-dimensions is bounded by $h(x_0)=h(x_l)=0$ (nails in the wall) and also $\int_0^l ds= l$. ($h(0)=h(l)=0$, if you take $h$ as a function of arc ...
3
votes
1answer
104 views
Is it possible to determine particles' velocities from their crossings on a ring?
2 particles are constrained to move in a ring. Both particles begin moving at $t=0$ from $p=0$, each particle moving in the opposite direction of the other. It is known that they are moving at ...
3
votes
2answers
454 views
How do I figure out the probability of finding a particle between two barriers?
Given a delta function $\alpha\delta(x+a)$ and an infinite energy potential barrier at $[0,\infty)$, calculate the scattered state, calculate the probability of reflection as a function of ...
3
votes
0answers
140 views
How to build a lab at home/school? (general purpose, sophisticated, reliable and cheap) [closed]
The title includes the main objective and all the criteria; namely:
A. A lab is to be built.
B. It must adhere to the following criteria:
Reasonably cheap for a limited-fund ...
3
votes
0answers
295 views
Goldstone modes and Heisenberg model
The ideia is to show that, because of Goldstone modes, 2d systems are quite different from 3d ones. So, considering the Heisenberg model, I'll post here what I'm asked to and my current thoughts on ...
3
votes
1answer
123 views
Equation of Motion in a Non-Inertial (Rotating) Frame
Let me preface my question by informing you this is for an assignment, so I would rather not have explicit answers but rather be given guidance in arriving at the correct solution.
The question is ...
3
votes
0answers
114 views
Matter-wave interference from free falling cold atoms
and another exam question, this is about current research:
Interference of matter waves has been studied using ultra-cold atoms. The phase of a matter wave for free-falling cold-atoms at time $t$ ...
3
votes
0answers
451 views
How do I derive the critical temperature for bose condensation in two dimensions?
In class we derived the 3D case, but there's a step I don't understand:
$$ N = g \cdot {V \over (2 \pi \hbar)^3} \cdot \int\limits_{0}^{\infty}{1 \over{e^{\left( E_p \over{K_B T}\right)}-1}} d^3 p = ...
3
votes
2answers
551 views
The Energy-Momentum Tensor and the Ward Identity
I have a question regarding a homework problem for my quantum field theory assignment.
For the purposes of the question, we can just assume the Lagrangian is that of a real scalar field:
...
3
votes
4answers
924 views
How can I explain why the weak nuclear interaction between individual nucleons is 'weak'?
By considering the energy-time uncertainty principle, estimate the range of the weak nuclear interaction at low energies. Compare this range to the size of a typical nucleon (for example, a proton) ...
2
votes
4answers
268 views
How are we able to view an object in a room with bulb..?
This is a very basic question on optics. How are we able to view an object kept in a room with a bulb?
From what I understand, light rays from bulb will hit the object and some colour will be ...
2
votes
7answers
325 views
My math says this is wrong, but what part of energy conservation am I violating?
Basically energy of conservation says it doesn't matter what path I take, energy will be the same from before as it is after.
So I took the block at the top's energy equal to the energy when it ...
2
votes
2answers
276 views
Plotting $\psi$ for finite square well potential
Lets say we have a finite square potential well like below:
This well has a $\psi$ which we can combine with $\psi_I$, $\psi_{II}$ and $\psi_{III}$. I have been playing around and got expressions ...
2
votes
3answers
276 views
Delta Dirac Charge Density question
I have to write an expression for the charge density $\rho(\vec{r})$ of a point charge $q$ at $\vec{r}^{\prime}$, ensuring that the volume integral equals $q$.
The only place any charge exists is at ...
2
votes
3answers
814 views
Proving angular momentum is conserved for a particle moving in a central force field $\vec F =\phi(r) \vec r$
A problem I am trying to work out is as follows:
A particle moves in a force field given by
$\vec F =\phi(r) \vec r$. Prove that the angular momentum of the particle about the origin is constant.
...
2
votes
2answers
342 views
How will you fix the direction of the lines of force due to circular current?
I have tried to learn it myself but I was unable to understand. Can you please explain it in detaails with simple example.
Thanks
2
votes
4answers
3k views
How to calculate displacement of an object
I am having a little bit of trouble with this following problem -
Suppose a particle has initial velocity and is moving with a constant acceleration. After t seconds its velocity is v. What will its ...
2
votes
3answers
220 views
Is this interpretation of $\psi=\frac{1}{\sqrt{\pi a^{3}}}e^{-r/a}$ correct?
Apologies if this is stating the obvious, but I'm a non-physicist trying to understand Griffiths' discussion of the hydrogen atom in chapter 4 of Introduction to Quantum Mechanics. The wave equation ...
2
votes
2answers
592 views
Why might the normal force on a box not be equal to its weight?
Very simple homework question which I managed to get wrong:
"The weight of a box sitting on the floor points directly down. The normal force of the floor on the box points directly up. Need these ...
2
votes
3answers
206 views
How do I integrate $\frac{1}{\Psi}\frac{\partial \Psi}{\partial x} = Cx$
How do I integrate the following?
$$\frac{1}{\Psi}\frac{\partial \Psi}{\partial x} = Cx$$
where $C$ is a constant.
I'm supposed to get a Gaussian function out of the above by integrating but don't ...
2
votes
2answers
472 views
How to apply a Hadamard gate?
How to apply a Hadamard gate to 3 qubits? by example how to apply $H$ to $(1/\sqrt{2})(\left|000\right> + \left|111\right>)$?
2
votes
1answer
157 views
Find the Hamiltonian given $\dot p$ and $\dot q$
I have these equations:
$$\dot p=ap+bq,$$
$$\dot q=cp+dq,$$
and I have to find the conditions such as the equations are canonical. Then, I have to find the Hamiltonian $H$.
To answer to the first ...
2
votes
3answers
202 views
waves on water generated by a falling object
Let an object of mass $m$ and volume $v$ be dropped in water from height $h$, and $a$ be the amplitude of the wave generated. What is the relation between $a$ and $h$. How many waves are generated? ...
2
votes
3answers
125 views
Vector space of $\mathbb{C}^4$ and its basis, the Pauli matrices
How do I write an arbitrary $2\times 2$ matrix as a linear combination of the three Pauli Matrices and the $2\times 2$ unit matrix?
Any example for the same might help ?
2
votes
2answers
312 views
Gauss' law giving zero field where field is not zero?
Two plastic sheets with charged densities as shown:
I'm trying to find the field at $B$. I obtained the correct answer by adding up the fields created by each charge density. But I realized that ...
2
votes
3answers
2k views
Convert acceleration as a function of position to acceleration as a function of time?
Suppose I have acceleration defined as a function of position, "a(x)". How to convert it into a function of time "a(t)"? Please give an example for the case a(x)= x/s²
2
votes
2answers
247 views
Nuclear physics Radioactivity
90Sr has a half life of 28.5y. It is chemically similar to Ca and enters the body through the food chain and collects in bones. It is a serious health hazard. How long in (years) will it take for ...
2
votes
6answers
336 views
Confused about unit of kilowatt hours
So I am a little confused on how to deal with the Kilowatt hours unit of power, I have only ever used Kilowatts and I have to design a residential fuel cell used as a backup generator for one day.
...
2
votes
2answers
1k views
Minimum possible Kinetic Energy of a confined electron
The problem is this:
Consider an electron confined in a region of nuclear
dimensions (about 5 fm). Find its minimum possible
kinetic energy in MeV. Treat this problem as
one-dimensional, and ...
2
votes
3answers
295 views
Jumping on earth versus jumping on the moon
Given the following problem:
On the moon the acceleration due to gravity is $g_m = 1.62 m/s^2$. On earth, a person of mass $m = 80 kg$ manages to jump $1.4 m$. Find the height this person will ...
2
votes
2answers
362 views
Galilean transformation of wave equation
I have this general wave equation:
\begin{equation}
\dfrac{\partial^2 \psi}{\partial x^2}+\dfrac{\partial^2 \psi}{\partial y^2}-\dfrac{1}{c^2}\dfrac{\partial^2 \psi}{\partial t^2}=0
\end{equation}
...
2
votes
1answer
270 views
How do I find average temperature given a temperature distribution?
I was told to find the temperature distribution of a wire with a current going through it. So I found $$T(x)=T_{\infty}-\frac{\dot{q}}{km^{2}}[\frac{cosh(mx)}{cosh(mL)}-1]$$
I need to find the ...
2
votes
3answers
2k views
why do we test electric fields with positive charges? not negative ones?
Is there any difference between using a positive vs a negative charge to test an electric field? If so, please explain.
2
votes
2answers
126 views
Hot Air Balloon and Buoyancy
This is a conceptual question in a solution I am trying to understand.
Problem statement:
I have a balloon with a volume of V $m^3$. The outside air temp is $K$ kelvin
and mass to lift is $m$ kg.
I ...
2
votes
2answers
239 views
Are Newton's law of Cooling and Stefan's law related?
Many of Indian school textbooks claim a proof of Newton's law of cooling from Stefan's law of black-body radiation.
As far as I am aware of, Newton's law is based on cooling from convection currents ...
2
votes
2answers
170 views
What is the current radius of cosmological event horizon?
Doing some crude calculations (using the value of $H_0$ at this point of time only, since it is time dependent but not distance dependent thanks to Johannes answer) what is the radius of cosmological ...
2
votes
1answer
103 views
Perturbation method & eigenvalues
I have a problem but I don't understand the question. It says:
"Show that, to first order in energy, the eigenvalues are unchanged."
What does it mean?
It means that if the Hamiltonian has the ...
2
votes
3answers
328 views
Find Inductance in a Coil
Question: An inductive coil has an internal resistance of 20 $\Omega$. When an AC Voltage source with a frequency of 100 Hz is connected to the coil, the current lags the voltage by 30 degrees. What ...
2
votes
1answer
304 views
Solving time dependent Schrodinger equation in matrix form
If we have a Hilbert space of $\mathbb{C}^3$ so that a wave function is a 3-component column vector
$$\psi_t=(\psi_1(t),\psi_2(t),\psi_3(t))$$
With Hamiltonian $H$ given by
$$H=\hbar\omega
...
2
votes
2answers
787 views
Calculating work done on an ideal gas
I am trying to calculate the work done on an ideal gas in a piston set up where temperature is kept constant. I am given the volume, pressure and temperature.
I know from Boyle's law that volume is ...
2
votes
3answers
210 views
How to get an integral formula for the flux time derivative
$$\frac{d}{dt}\int \limits_{A} \mathbf B d \mathbf A = \int \limits_{A} \left( \frac{\partial \mathbf B}{\partial t} + \mathbf v (\nabla \cdot \mathbf B ) + [\nabla \times [\mathbf v \times \mathbf B ...
2
votes
1answer
2k views
How do I relate the direction of a compass needle to the direction of a current?
A compass needle is placed above the wire points with its N-pole toward the east. In what direction is the current flowing?
If a compass is put underneath the wire, in which direction will ...
2
votes
2answers
285 views
Convert state Vectors to Bloch Sphere angles
I think this question is a bit low brow for the forum. I want to take a state vector $ \alpha |0\rangle + \beta |1\rangle $ to the two bloch angles. What's the best way? I tried to just factor out ...
2
votes
2answers
406 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 ...
2
votes
2answers
287 views
Why did they omit the action-reaction pair force in this problem?
Problem
!
Solution
!
My solution
!
Let me explain
$n_{bg}$ means the force exerted by the block on the ground. This is because the normal force of the slab exerts (or touches) a force on the ...
2
votes
2answers
696 views
What is the potential difference between point X and point Y?
Here is the problem:
In the above figure I want help on finding the potential difference between X and Y.
It is getting quite confusing due to the battery in the middle. I found the current in both ...
2
votes
1answer
560 views
infinite parallel conducting planes
two infinite parallel conducting planes grounded and are separated by a distance d. place a point charge "q" between the two planes, using the "green teoerma reciprocity" how I show that the total ...
2
votes
2answers
513 views
Calculating uncertainty in the final result (combining uncertainties)
I'm struggling to determine the uncertainty in $F$ so it would match the textbook answer.
The problem statement is: A force F is obtained using the equation: $F = \frac{mv^2}{2\pi(x_2 - x_1)}$.
The ...
2
votes
2answers
2k views
oscillations of blocks connected by a spring
Imagine two blocks of masses m1 and m2 joined together by a spring of spring constant K.
Now let the spring be stretched by a distance X and then the system is released. suppose during the stretching ...
2
votes
2answers
855 views
Michelson rotating mirror experiment
Could someone explain the calculation required to answer this question. It is from a text book and the answer is recorded as 585Hz but I cannot replicate the answer.
In 1931 Michelson used a ...
