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.

learn more… | top users | synonyms (1)

0
votes
0answers
15 views

Dry Ice Sublimation time, practical calculations

I would like to send a parcel to a family abroad with frozen (to the temperature of -18 ° C.) products.The products are small balls having a diameter of 4 cm. Balls are made from pizza dough. I'm ...
-1
votes
0answers
13 views

Circuit Diagram

Three resistor are put into play, two in parallel and another in series to them. One of the resistors in parallel is decreased. what will happen to the voltage and current.
1
vote
1answer
44 views

Loading a capacitor in an RC circuit

I have an RC circuit to which I am going to apply voltages of 5V every 100ns. So every 100ns, the capacitor will change its "state" from charging up to uncharging. I have been trying to find the a ...
0
votes
0answers
15 views

Electric Field due to volume charge [on hold]

Electric Field at point P due to volume charge Since,the Electric field at point (0,0,z) is dE = ρvdvaR/4πε0R2 aR = cosα az + sinα aρ can anyone derive the above given expression of aR.How it ...
1
vote
1answer
176 views

Question about superconductivity

A long cylinder of radius $R$ is made from two different material. Its radius $r<r_0$ $(r_0<R)$ part is a material with superconducting transition temperature $T_1$, and its $r_0<r<R$ ...
-1
votes
0answers
29 views

An inclined pulley system [on hold]

M1 and M2 both are 400g. There is no friction and the pulleys are massless. I am trying to work out the acceleration on M2. I am unsure whether the acceleration should be: $a\space =Mg\space -\space ...
-1
votes
1answer
40 views

The “classical radius” of the electron [on hold]

According to one theory, the mass m of the electron arises simply as a consequence of the electrostatic energy of its electric field, through Einstein's famous expression ε=mc^2, where ε is the field ...
0
votes
1answer
46 views

Which moves faster in a time $t$: $A$ or $B$? [on hold]

Suppose we have two objects, $A$ and $B$, both made of the same substance. They also have the same shape (e.g. two spheres), but $A$ is larger and therefore more massive than $B$. Both $A$ and $B$ ...
1
vote
1answer
27 views

entropy of a long molecule chain with respect to its length

Consider a (very long) one-dimensional chain of $N$ moleculs, which can be in either of the energy states $\alpha$ or $\beta$. The configurations have length $a$ or $b$ respectively. Show ...
2
votes
1answer
47 views

Landau's Problem - Poisson bracks of a spherical symmetry function and angular momuntum in z axis

In landau's Mechanics, there's a problem: I think, if the function has the property spherical symmetry, or: $\phi(r,p)=\phi(-r,-p)$ The form suggested by Landau follows this property, but I can't ...
1
vote
0answers
55 views

Schrodinger Wave Functional (quantum fields) - Solving Functional Gaussian Integrals

Okay, So i'm doing some research that involves the Schrodinger representation in quantum field theory. The ground state wave functional for the Klein Gordon field is a generalized gaussian in position ...
3
votes
2answers
98 views

Solution of dynamics of density matrix

Given the dynamics of the density matrix: $ \frac{d}{d t}\begin{pmatrix} \rho_{00} & \rho_{01} \\ \rho_{10} & \rho_{11} \end{pmatrix} = \begin{pmatrix} \lambda ...
0
votes
0answers
8 views

What is the power of combination of an erect and an inverted halved convex lens?

So I have this question from a competitive exam. And I don't know to solve for the second part (I have a guess at the answer, but don't quite understand it). For the first part, focal length ...
1
vote
1answer
32 views

Moment of inertia help [on hold]

Okay so lets say I have a lever with 2 objects on it, object one and object 2. Object 1 has a mass of $m_1$ and is $r_1$ away from the fulcrum; object 2 has a mass of $m_2$ and is $r_2$ away from the ...
0
votes
0answers
24 views

Nonlinear pendulum with periodic torque: Can it intersect itself?

For a nonlinear pendulum, if a third direction in phase space is achieved by adding periodic torque, can the orbit of the pendulum intersect itself? Why or why not?
1
vote
1answer
183 views

Heisenberg XXX time evolution operator for three qubits

I've a problem to reproduce the result in equation (4) on page three of this paper: http://arxiv.org/abs/0802.2588. So far I've understood that they apply a Heisenberg XXX interaction between ...
2
votes
0answers
44 views

Sum of energy for 2 solids in rotation

I would like to compute the sum of energy of the following case: Two solids are turning (disks). Yellow solid is turning at $w1 rd/s$ around its center of gravity and blue solid is turning at ...
0
votes
1answer
33 views

How to find how far away could a binary system be? [on hold]

Human eye resolution is about one arcminute. Two stars in a binary system are separated by 10 AU. How far away can they be such that your eye could still distinguish them as two separate stars rather ...
5
votes
1answer
864 views

Adiabatic expansion in van der Waals gas

Given a Van der Waals gas with state equation: $$\left( P+\frac{N^2 a}{V^2}\right)\left( V-Nb \right)=NkT,$$ show that the equation of an adiabatic process is: $$\left( ...
3
votes
1answer
63 views

How to deal with $\nabla(\delta\Psi)$ in functional derivatives?

I am trying to compute the functional derivative of the following functional $$F[\Psi]=\int{}d^nx\Psi{}e^{(\nabla\Psi)^2}.$$ What I have tried up till now is the following ...
0
votes
0answers
12 views

Electric Field Resulting From a Conductor - 3 Situations [on hold]

really simple question here that I can't seem to visualize for whatever reason. If you have a spherical conductor with radius R, what is the strength of the electric field when: The net charge on ...
0
votes
1answer
35 views

What is the relative speed of the galaxy with respect to the Earth [on hold]

Consider a distant galaxy. Though this is a bit contrived, suppose that an astronomer measures the fourth transition of the Brackett series of transitions from n > 4 to n = 4 in Hydrogen at a ...
0
votes
1answer
221 views

Apply Gauss' Law to find Electric Field around nonconducting plastic sheets

The question: Two very large, nonconducting plastic sheets, each 10.0 cm thick, carry uniform charge densities $\sigma_1$,$\sigma_2$,$\sigma_3$ and $\sigma_4$ on their surfaces, as shown in the ...
0
votes
3answers
2k views

Normal force of loop-the-loop at the side of the circle

In the loop-the-loop ride a car goes around a vertical, circular loop at a constant speed. The car has the mass of 230 kg and moves with the speed of 300 m/s. The loop-the-loop has a radius R=20 m. ...
1
vote
1answer
46 views

One step in deriving the Einstein-Hilbert action

In this amazing first principles derivation of the Einstein-Hilbert action there is one small manipulation needed to show $$c g_{ab,cd}\left(\eta^{ac}\eta^{bd} - \eta^{ab}\eta^{cd}\right)$$ is ...
0
votes
1answer
25 views

Net Electric Field of Multiple Charges [on hold]

Question: Four charges, each of magnitude +Q are placed at the corners of a square of side a. Obtain an expression for the electric potential at the centre of the square and at the mid point of one ...
0
votes
1answer
46 views

Finding the spectrum of a curious hamiltonian

I wish to analyse the following hamiltonian, i.e. find its eigenvalues and eigenstates. $$H = \frac{1}{2}\epsilon(\sigma _z \otimes \mathbb{1} + 1\otimes \sigma _z) - \Delta (\sigma _x \otimes \sigma ...
4
votes
1answer
63 views
+50

Solving the Liouville equation for classic harmonic oscillator via method of characteristics

I'm interested in solving Liouville's equation $$\frac{\partial W}{\partial t} + \{ H,W\}=0$$ with $$H=\frac{p^2}{2m} + \frac{1}{2}m\omega^2x^2$$ using the method of characteristics. However I ...
0
votes
0answers
19 views

How to calculate the atomic form factor of Lithium? [on hold]

I have a density function, $\rho=e^{\frac{-2r}{\pi a^3}}$, and I have also two numbers, but I don't know what they mean. They are: $a_k=0.2 \mathring A$ angstrom, $a_l=1.6 \mathring A$ . And teacher ...
0
votes
0answers
22 views

Calculating time of being in a lift only by weight & travel distance

If someone is in a lift and normally weighs 90kg, when the lift starts accelerating the person weighs 100kg. After the lift has maintained a constant speed for a while it starts slowing down. While ...
0
votes
0answers
15 views

A car collides with a net [on hold]

A little car of 500 grams with a speed of 2 m/s collides with a net with a spring constant of 400 N/m. If the force is bigger then 40 N, the net breaks. 1) Prove that the net brakes 2) What is the ...
0
votes
1answer
116 views

Calculating the magnetic field from rectangular conductor?

I want to apply the Biot-Savart law to calculate the magnetic field at a point created by current flowing through a square/rectangular conductor. More specifically, a trace on a printed circuit board. ...
1
vote
1answer
207 views

Choosing the right point when calculating moments

A body is composed of two straight pins that are joined at a right angle. They have lengths $a$ and $b$ and the mass per unit length is $\rho$. When the body is balanced on a flat surface, as shown, ...
1
vote
0answers
23 views

Accelerating disk [on hold]

A disk of radius $R$ is accelerating to the right over a plane. At $t=0$ the speed and the acceleration of the axle are $v_1$ and $a_1$ what is the velocity and the acceleration of each point (1-4 ...
1
vote
1answer
35 views

Problem with formula for parabolic motus

I know that the path made by an object thrown in air with velocity $v_0$ with an angle $\alpha$ is: $$y=x \cdot\tan \alpha - \frac {gx^2}{2v_0^2\cdot \cos^2 \alpha} $$ At school we had to use this ...
1
vote
1answer
135 views

Two spherical cavities hollowed out from the interior of a conducting sphere. At the center of each cavity a point charge is placed

From Griffiths: Two spherical cavities, of radii a and b, are hollowed out from the interior of a (neutral) conducting sphere of radius R. At the center of each cavity a point charge is placed – ...
0
votes
1answer
29 views

Example of a state which is positive but its partial transpose is not positive

Could any one give me an example of a state whose density matrix is positive semidefinnite but partial transpose is not positive semidefinnite?
0
votes
1answer
59 views

Figuring out the time period of an Atwood system

SO I was working on Atwoods, when I came up with the following problem, It is more complicated but i should understand this simpler scenario first. As shown in the diagram above, (Atwood machine), ...
0
votes
1answer
34 views

Catapult projectile with different mass [on hold]

We have a physics law. We made catapult and we have to launch a water balloon. However before that we have to know where the water balloon will fall. We don't have radars or stuff like that to ...
-3
votes
1answer
42 views

How to calculate magnitute of average acceleration? [on hold]

Given: Starting from rest the acceleration is 500mph, in 1 second. How do I calculate the magnitude of acceleration for this in terms of $g$ as 9.8m/s^2? Starting off 500mph is about 220 m/s ...
1
vote
1answer
30 views

Show the equivalence of two electrostatic energy

$$U = \frac{1}{2}\iint_{all space}^{ } \frac{\rho(1)\rho(2)}{4\pi\epsilon_{0}r_{12}}dV_{1}dV_{2} $$ and $$U = \frac{\epsilon_{0}}{2}\int_{allspace}^{ } \vec{E} \cdot \vec{E} dV.$$ In the left ...
0
votes
0answers
15 views

Finding voltage drop in series parallel [on hold]

In this problem, I am asked to find the voltage drop for $R_3$. We are given that the current for one of the branches is 0.55 mA. I am somewhat familiar with Kirchoff's Law for currents and Kirchoff's ...
-3
votes
0answers
29 views

How to calcuate uniform circular motion? [on hold]

Given: Ideally, I maintain a speed of 265km/hr and 285 km/hr. Roughly, 73.6 m/s and 79m/s. I make a turn and experience around 4.5g's to 5.5g's, roughly 45 and 55 m/s^2, for about 7 seconds. I ...
1
vote
1answer
105 views

Infinite Resistive lattice problem

Ok so while there have been many discussions regarding the solution(say using the Fourier transform or the lattice Green's function approach) to the two-point resistance of an infinite-square ...
2
votes
1answer
140 views

Massless Dirac equation is Weyl covariant

Does somebody know how to show that the following equation is Weyl invariant? $$\gamma^ae_a^\mu D_\mu \Psi=0$$ where: $D_\mu \Psi=\partial_\mu\Psi+A_\mu^{ab}\Sigma_{ab}\Psi$ is the spin-covariant ...
0
votes
0answers
49 views

How to describe the motion of a box initially at rest on a table rotating with an increasing angular speed? [on hold]

Consider the following diagram. A box is on a table that can move circularly and the box is connected by a string attached to the fixed bearing. At $t=0$, the box is at rest, the string of length ...
-1
votes
0answers
29 views

Here is a question I am trying to solve. Sorry for the highlighting, it is not relevant [on hold]

Thermodynamics initial pression,= 5 bar final pression= 2 bar The steam expand adiabatically PV^(1.120)=C providen initial volume= 3.64x10^(-4metres cubes) diameter piston= 50 mm= 0.05 m a) ...
1
vote
0answers
29 views

Scintillator Length Calculations

This is sorta homeworkish, yet it still relates to my understanding of the physics behind it as well. Does anyone know how you determine t_98 (aka the length to contain 98% of the energy) for a ...
0
votes
0answers
11 views

Teeter Todder force of uplift [on hold]

What is the force of a 200lb man jumping from a three foot high elevation onto a four foot long teeter-todder with a four inch pivot point with a one hundred and thirty pound ma on the opposite ...
7
votes
2answers
120 views

Stable muon density inside a white dwarf star?

It occurs to me (though I'm hardly the first) that the decay $$ \mu^- \to e^- + \bar \nu_e + \nu_\mu $$ should be forbidden in electron-degenerate matter, since there must be an empty state available ...