# All Questions

4 views

### Working on reflection 1 [on hold]

If a number of images are seen in a thick mirror then which image will be brightest? first second last
30 views

### Change of basis in quantum mechanics-spin vectors

Suppose ${x,y,z}$ are my spin vectors, such as spin 3/2. If I define an operator to be $W = e^y$, I can write it as $\sum_i e^{y_i} \lvert y_i\rangle\langle y_i\rvert$ where $y_i$ are the eigenvalues ...
19 views

### How can I redraw this circuit where I can apply Kirchoff's Voltage and Current Law to get correct Voltage at V?

I thought I can draw the -5V as a source but it did not turn out to be right, unfortunately.
19 views

### Density Functional Theory (DFT) tutorial guide

I am going to start learning about DFT calculations. Could anyone advise me the best starting point for that? Simple example guiding tutorial with explanations would be great. Any input would be ...
30 views

### Potential Energy in a System and its Relation to the Total Energy

In this answer it was stated that potential energy is a property of a system and not an individual particle. If we have two particles (1 and 2) interacting via a conservative force, we can write an ...
13 views

### How does a change in birefringence correspond to a change in intensity?

In the optical Kerr effect, a pump beam causes birefringence in a sample, while a probe beam measures the return of the sample to equilibrium. My question is, how does the change in birefringence ...
49 views

### How do radio signals contribute to gravity?

First, the inspiration for this question: I just read that it takes one hour to send a picture from the New Horizons space probe, to Earth. It also takes around 5 hours for that picture to reach ...
27 views

### Exertion of force due to spin of Earth

If Earth were to spin at a different speed would I be experiencing a different weight?
17 views

### Struggling with a conversion. Showing number of atoms per cubic meter using pressure in Torr

I have been trying to show that the following is true: N = 9.65 x 10^24 x (P/T) Where N is the number of atoms per cubic meter of an ideal gas at pressure P (in Torr) and temperature T (in Kelvin). ...
73 views

### Clustered-Entanglement thermodynamics

I want to understand the tools required to do calculations in entanglement thermodynamics in general, but I'm considering first how to model entanglement mathematically in terms of many-body systems ...
26 views

### Is work energy theorem $W=\Delta K$ or $W=\Delta E$?

I know that work is the change in kinetic energy. I saw on an equation sheet that work is equal to the change in total energy. Is this true? If so, how?
96 views

### Why can't we see virtual photons? [on hold]

If photons are the carriers for the electromagnetic force, then why can we not see electromagnetic fields, given that photons are involved in this interaction?
121 views

### Is there a simple expression for $[x,e^{ixp}]$?

I'm sure this exists somewhere, but somewhat surprisingly it is not that easy to google.* The commutators $$\left[x,e^{i(ax^2+b(xp+px)+cp^2)}\right]$$ of position and the exponential of a quadratic ...
28 views

### A derivation in Schwinger's proper time approach

I have a question in derivation of Schwinger's proper time method in chapter 2.1 of http://link.springer.com/book/10.1007%2F3-540-45585-X from Eq.(2.20)-Eq.(2.23) to the classical action expression ...
20 views

### Spinor field normalisation from poles in the propagator

In the theory of free scalar bosons (KG field) it is a basic result that the propagator $\Delta(p)$ has poles at $p^2=m^2$, with residue $1$ (or any other constant, depending on conventions). Thinking ...
42 views

### Why does all the current flow through a short circuit if its voltage drop is considered zero?

Path of least resistance vs. short circuit I know the path of least reisistance has been clarified already However, to derive the equations you need to assume that the voltage of each parallel ...
44 views

### How to bring a stick to the ground using many people? [on hold]

The game is called "Helium Stick." The game involves a group of people facing each other on opposite sides of a long stick. People are holding the stick with one of their fingers from below. The ...
21 views

### Noether's theorem functionals [duplicate]

I'm working through a derivation of Noether's theorem, and at this line $\delta$$L = \frac{\partial L}{\partial \phi}$$\delta \phi$ + $\Pi^\mu \delta (\partial _ \mu \phi)$ The next step is to use ...
36 views

### How to measure the initial velocity of a potato gun?

I have made a 2m potato gun (It's like a cannon - the noise makes your ears buzz for some second :) ) Its uses alcohol as fuel. an igniter fires. then the potato will be shot out. anyway. as it's ...
14 views

### A conceptual question regarding suspended ring inside vertical inductor coil

Suppose a metal ring is suspended inside a vertical inductor (with an elastic string ) when the circuit is open. When the circuit is closed, does the metal ring tend to jump up or fall down? Why? I ...
27 views

### Bet about the Precession of Book [duplicate]

Say you spin a book and throw it to someone. If you spin it around it's spin while throwing it perpendicular to the spine, it doesn't precess. You spin it around the length (longer dimension) and it ...
23 views

### Why can we treat (most) metals classically at room temperature?

When we consider how light and metals interact with each other at room temperature, the Drude model, which is a classical theory, is usually and successfully used. My lecturer explained something ...
37 views

### Determining the phase delay between H and E fields

I want to determine the phase delay between H and E fields in a medium with losses and not electrically charged. In this medium we also have $\sigma_c\approx \varepsilon \omega$. The enunciation of ...
41 views

### Can the mass of an object be changed by adding opposing magnetic fields

Apparently it does. Or is this vodoo physics. If not what is really happening in this case? https://www.youtube.com/watch?v=8N2TS3VReTA Boyd Bushman Changed the mass of a black box object, rock. He ...
11 views

### Shaving down length of a GRIN lens to a desired wavelength

I have a GRIN lens that collimates light from a point source at 670nm. I want to shave the thickness of the lens down to use for 530 nm. I can't find anything useful for equations online. None seem to ...
93 views

### Why does light bend?

I have just read about dispersion of light by a prism and the thing which i donnot understand is that WHY does the light bend at all? Through prisms and slabs? I came to know that red light has the ...
147 views

### Problem in understanding the concept of 'superposition' as explained by Dirac

The concept of 'superposition' has really made me insane, actually. What I thought it was just simple superposition of matter waves. For instance, let's take the Double-Slit experiment: take the ...
37 views

### infinitesimal and infinity [on hold]

In differentiation and integration we use infinitesimal to evaluate derivative and integral, talking about infinitesimal first this is the no. that we actually can't say how small it is still we are ...
23 views

### What do water stains make things appear dark? [duplicate]

If a piece of grey tarmac is damp after the rain, it appears darker. If I spill water on my carpet, the damp patch is darker than the rest of the carpet. Generally speaking, a lot of things appear ...
48 views

### Magnetic and Electric fields

Unified field theory says the Magnetic field and Electric field are proven to be two manifestations of the same thing just viewed from different perspectives. That is the magnetic field is essentially ...
37 views

### What is a simple calculation to figure out how many watts needed to maintain a hot piece of tungsten?

The specific heat of tungsten is $.13 (\text{kJ /(kg K)})$. 1 cubic cm of tungsten is .0193 kg And the melting point of stainless steel is $1900 \deg \text{K}$ conservatively (giving it plenty of ...
46 views

### Einstein's 1905 “Concerning an Heuristic…emission and transformation of light”

I'm currently attempting to read Einstein's annus mirabilis papers, starting with his introduction of the quantization of light in the paper: "Concerning an Heuristic Points of View Toward the ...
4 views

### Attracting atmospheric positive ions to carbon nano tubes for collection

Can nano magnets built into carbon nano tubes be used to attract moving positive ions from atmosphere to increase the resultant positively charged collecting mass?
36 views

### Help trying to make circuit! [on hold]

I tried to make circuit. I have battery of 9V and led light with Vf of 3.30. How can i find the cureent of circuit. Basically two wires are going from battery to led bulb making it light but can i ...
47 views

### We can move with speed of light [on hold]

We all know de-broglie equation $λ = \frac h {mv}$ but let us take it a little further. Let $λ$ be the wavelength associated to an object with mass $m$, $v$ its velocity because of its energy $E_G$ ...
43 views

### Evolution of a system - Thermodynamic potential

There is something I don't understand about thermodynamic potentials. We know that the necessary condition for a system to evolve is an increase in entropy. For a system kept at constant volume and ...
102 views

### Special Relativity and Gravity

As Einstein was seeking a relativistic theory of gravity, he thought that special relativity should be upgraded to general relativity thus promoting the Minkowski space to curved pseudo-Riemannian ...
73 views

### acceleration and General Theory of Relativity

Carlos I Calle is a prominent NASA scientist with extensive experience with Einstein's theories. In 2011-12 he wrote a book for the Wiley 'Dummies' edition--Einstein for Dummies.There he says that ...
41 views

### Why bremsstrahlung occurs only with the nuclei? Why not with the electrons?

In many books I read that bremsstrahlung effect (for e+) only occur when the electron goes near the atomic nuclei. Why is not possible when cross near an atomic electron? Thanks,
40 views

### If a body slides down a frictionless inclined plane what will be the net normal force?

If a body(m) slides down a frictionless inclined plane (M), will the net normal force between the ground and the inclined plane be (M+m)g ? I feel it should be less than (M+m)g. This is because one ...
16 views

### Entangled or Separable-State measurement

This image is from a Nature paper. A question about the caption: how is Victor's data used for sorting Alice and Bob's measurements into subsets? I would really appreciate it if you could walk me ...
839 views

107 views

### What would be the dose rate of 109 grams of uranium per day/year?

I know that with a Pancake probe that can read alpha you can get up to 30K CPM on a small plate. I ordered a 6" red plate off ebay and measured outside the box when it came in. It registered around ...