# All Questions

3answers
3k views

### How is a potential difference created between two points?

I would like to know how potential difference is created between two points?Is potential difference created by adding more electrons at one side?If that's the case how to add more electrons at one ...
0answers
23 views

### A few questions related to bernoullis principle & fluid dynamics in a pipe

I've been struggling with a few things as i've been introduced to bernoulli's principle, hopefully someone can help me clear things up(in somewhat plain, easy to understand language) In the examples ...
0answers
12 views

### Expectation value of an Observable and Eigenstates

I am learning about Quantum Mechanics at the moment and I was wondering about Eigenfunctions and Observables. The question I would like to ask is, If a wavefunction is not an eigenstate of an ...
0answers
7 views

### How can we look back at our Universe 13Billion years ago?

I read this article on Space.com and I quote "As far as we know, that's as far [back] as we can see — we get an image of the universe as it was when it was about 389,000 years old," So what I wanna ...
3answers
2k views

### How is the number of electrons in an atom found?

I was wondering, what type of experiments were held to identify the number of electrons in an atom? (For example, how do we say that carbon has 6 electrons and magnesium 12.) I would like someone to ...
2answers
361 views

### Relationship between Mars and Earth rotation

Is it by pure random chance that Mars and the Earth have nearly the same day duration (Mars day is barely 40 minutes longer, which is just 3% difference), or there is some causal relationship between ...
0answers
6 views

### This might make no sense light is time

I've done a lot of research on time I think when light/color is absorbed it is giving off as time and when it is going fast the light can't keep up
13answers
369 views
+50

### How to pour water from a bottle as fast as possible

When one pours water out of a bottle, it first flows smoothly but then a pressure 'blockage' develops and the pouring becomes interrupted and turbulent, so that the water comes out in splashes. This ...
1answer
152 views
+100

### Electric Potential Field of Parallel Electrodes within Grounded Shell

Any help would be greatly appreciated I think I need to solve this using boundary potential conditions and Laplace equations. Here is the system I'm attempting to model: The red rods are metal ...
0answers
6 views

### Expansion Of Universe : On the Basis Of Aether Theory

I would like to ask that in the recent experiment of discovering Gravitational Waves scientists have also found or believe that they have found the evidence of Dark Matter. So if dark matter is made ...
0answers
6 views

### Understanding stress-strain calculations

I'm trying to compute calculate the stress-strain curve for an elastic material with cylindrical geometry subject to an increasing uniaxial load. I understand that this requires: stress = $F/A_0$ ...
2answers
2k views

### Why does time dilation cause you to age slower? And is time considered relative to the observer?

I understand that the higher your velocity the slower light will move. But how does time itself slow down while you are moving faster?
3answers
22 views

### Charged Particles: How is energy conserved?

I can seem to understand where the energy comes from when two particles accelerate. I tried searching through the forums and it seems like the answer is always potential energy was the source of ...
0answers
45 views

### How to plot numerically the wave functions according to the Hamiltonian?

It is often difficult to analytically solve the Schrodinger equation, and so we need to obtain a solution numerically. An example plot is shown below. Here, the wave functions for a three junction ...
4answers
6k views

### Why does resistance increase with length?

This is for the case of wires only (pipes are easy). I see that if I place a 9-Volt (constant) battery across any wire, the current is proportional to resistance. I don't understand why, for a ...
3answers
45 views

### Does curved spacetime change the volume of the space?

Mass (which can here be considered equivalent to energy) curves spacetime, so a body with mass makes the spacetime around it curved. But we live in 3 spatial dimensions, so this curving could only be ...
1answer
102 views

### Quantum Dimer Model (QDM) hamiltonian

The Hamiltonian given by Rokhsar-Kivelson QDM is based on tensor products of the state vectors. Why is this the case?, is it because the lattice model is a mixed state? It'd be great if someone could ...
0answers
11 views

### Resistance increase with length because of more scattering?

I am referring to one of the answers to this old question. I can't add comment to it because I don't have enough reputation. So here I rephrase the question: does resistance increase with length ...
1answer
17 views

### Can large bodies spin both ways

Can a planet, stars, moons, solar systems or black holes rotate on 2 axis? and if so is it possible to have 3 or 4 axis? Two axes for rotational motion does not cover if planets can hove 2 axis in a ...
1answer
57 views

### Does a scale weigh a bee flying inside a box?

This question is very similar to this one or even this one except some minor differences... The box is much larger than the bee. The box has no top cover The bee is flying still in the middle of the ...
0answers
12 views

### At what direction does a ray reflect at a 90 degrees corner?

Say we pointed a laser that is projected into a $90^{\circ}$corner of a mirror made by perpedicular walls. I know that you cant use tangent since a derivative of boundary of mirror would not exist ...
1answer
26 views

### Cosmological Significance of Uncertainty

Is it fair to say that quantum uncertainty is the most fundamental and primordial characteristic of the universe? Wouldn't uncertainty have predated the initial expansion of spacetime, and not be ...
1answer
13 views

### Find the initial velocity's angle of a projectile knowing only the ratio between the horizontal distance traveled and max height

The ratio between the horizontal distance traveled and the launch height is 3 to 1. I think it is possible to find the angle given this information, but how exactly do you do it? I think it's arctan(2/...
2answers
160 views

### Quantum probability of entangled spin-1/2 particles?

Let us say that you have two entangled spin-1/2 particles (entangled in such a away that angular momentum is conserved). Let us say particle 1 moves to the left and particle 2 to the right. We measure ...
0answers
10 views

### How are the charges on the plates of a capacitor distributed?

Given a specific overall amount of energy a rectangular parallel plate capacitor stores what number of atoms on a specific plate would have any given specific charge?
2answers
102 views
+50

### Static vs Dynamic pressure

According to Bernoulli's law, the narrower the tube where the fluid is flowing the lower its static pressure and the higher its dynamic pressure. Does this make any difference? If the dynamic pressure ...
1answer
331 views

### Gas compressed and cooled

If a certain amount of a gas is compressed into a cylinder by the action of a piston that reduces its volume (lets say at a low speed), the kynetic energy of the gas molecules increases, what is ...
0answers
26 views

### Books for someone interested in physics/space [duplicate]

As an avid reader of more realistic SF and someone who has always been interested in physics and astrophysics, I was wondering if someone could suggest both some introductory books on the subject, and ...
0answers
5 views

### What does gravitational lensing equal in time diolation to gravity?

How close does a person need to get in a black hole in orbit to get 2x speed in time and what would that equal in g force?
3answers
3k views

### What is this shadow being cast upon?

The picture below is from on top of Mt. Shasta at sunrise. The sun was directly behind me when I took the picture and it appears to be casting a shadow on.... the sky itself? Can anyone explain ...
0answers
23 views

### What is the minimum math level required to comprehend general relativity? [duplicate]

I am currently working on a B.S. In chemistry and only need to go up to Calculus II, though I like to dabble in physics. What other math classes could I take to gain greater understanding of higher ...
0answers
7 views

### how to get the matrices in partial trace

Good day, I want to ask the matrix that obtained from this link How to take partial trace?. answer from @Nontriviality below In principle what you do is to multiply the square matrix by rectangular ...
1answer
115 views

### Reduced density matrix

During a course on quantum mechanics we've been talking about density matrices. Now I came across the following exercise. Consider a two spin $\frac{1}{2}$ systems, labeled 1 and 2. Calculate: ...
0answers
19 views

### How would you go about solving this graphical mass problem?

I've tried finding the x and y components of the vectors created by each mass to find the resulting vector's magnitude. However, my answer of 0.354 meters keeps getting marked wrong. Any ideas on how ...
0answers
35 views

### Confusion in Maxwell's derivation of Ampere's Force Law - Part III [on hold]

I am reading Maxwell's "A Treatise on Electricity and Magnetism" and I have some confusion in the following pages: The element ds is resolved into its components $\alpha$ and $\beta$;and the ...
0answers
17 views

### How much potential energy stored by a dam?

Considering a dam of height $h$ which holds back a rectangular based reservoir of water with base area $A$. The reservoir contains a mass $m$ of water (up to $h$) of density $\rho$. How much potential ...
0answers
10 views

### How To Arrive At Ground State Metric of Kaluza-Klein Theory

The ground state metric, after an extra dimension of space is compactified (to a circle) in Einsteinian gravity, is the metric which corresponds to the R_4 × S_1 geometry of the separated dimensions. ...
0answers
21 views

### Determination of nature frequency and differential equation of vibration of Hartnell governor?

I found this solution for the nature frequency but here it does not include the Ball weight and centrifugal force in the moment balance equation about the pivot (O), it is wrong answer...is not it? ...
3answers
68 views

### Significance of wave number?

Till now all I know about the wave number is its formula i.e. ${\frac{2\pi}{\lambda}}$. I always wanted to know what it really means. So can anyone please, explain me its physical significance?
0answers
15 views

### Is it safe to use the USB ports of a 110v surge protector connected to a 220v socket? [on hold]

During my visit to the US, I have bought a surge protector with USB ports: Monster Power EXP 350 I'd like to know if it is safe to use while connected to a 220V european power socket, using a ...
1answer
122 views

### On Thermodynamics of Spontaneity of the Reduction-Oxidation Chemical Reactions

According to the first and second law for a closed system containing different chemicals we have \begin{align} &\delta Q - \delta W = dU = T dS - p dV +\sum_i \mu_i d N_i\\ &\Rightarrow\;\...
0answers
32 views

### How to start teaching? [on hold]

Twenty years ago I was professor of mathematics and mechanics in Russia. Here, in the USA, I've never had experience in professorship. Today I have maybe a strange idea to try to educate people ...
1answer
31 views

### Derivation of continuum expression of the first law of thermodynamics

Continuum expression of first law of thermodynamics: $$\frac{D E_t}{D t}=\nabla\cdot({\bf \sigma\cdot v}) - \nabla\cdot{\bf q}$$ (I've seen it in my physics book) How this equation is derived? ...
3answers
48 views

### Size and density of neutron stars

Most of the books which I looked at give approximately 10 km as the radius of a neutron star. Just yesterday I looked at a book by Dave Goldberg titled The Universe In the Rearview Mirror (2013) which ...
0answers
19 views

### Antenna measurements and parameters calculation

Lets suppose we have some data collected from antenna measurement. For every theta and phi point in any coordinate system ( here refering to Ludiwg coordinate systems II and III) the data can be ...
1answer
19 views

### The expression of the density in terms of molecular mass and the distribution function

I am reading a book about the boltzmann equation, the author gives the expression of the fluid density $\rho$ as follows: \rho(\mathbf r,t) = \int {M\,f(\mathbf r,\mathbf c,t) \, \mathrm d\mathbf c}\$...
7answers
1k views

### Prove that an electron in a hydrogen atom doesn't emit radiation

According to electrodynamics, accelerating charged particles emit electromagnetic radiation. I'm asking myself if the electron in an hydrogen atom emits such radiation. In How can one describe ...
3answers
51 views

### Can astronomers observe neutron stars optically?

Are there any neutron stars near enough for astronomers to observe them optically? If not, then how close are we to having the technology to do so?

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