0
votes
0answers
93 views

Time between two collisions in a 2D gas

I'm trying to code a simulation of a 2D gas in a box. The molecules are represented by circles of radius $R$ having elastic collisions with the walls only (for now). I read that the average time step ...
1
vote
0answers
33 views

Does chemical formula of matter influence on how would it be charged? [closed]

Suppose we rub glass ($\mathrm{SiO_{2}}$) against paper(which is made from cellulose ($\mathrm{C_6H_{10}O{5})n}$). We receive positively charged glass and negatively charged paper. Is that because of ...
1
vote
1answer
43 views

Clarification about first spherical harmonic

If the quantum numer $l$ is equal to zero, each components of L has eigenvalues $m=0$. So the states that have $l=0$ are simulaneous eigenvectors of each components of L. But, my textbook says, it is ...
0
votes
0answers
39 views

What is ferromagnetism?

Simple question. As far as I've understood it; a ferromagnetic material is one where all the electron spins are alligned parallely - that is, the wavefunction has an antisymmetric spacial part and a ...
1
vote
0answers
24 views

Properties of materials _not_ dependent on fermi surface?

So I'm studying a second solid state physics course where we've covered calculating things like magnetic susceptibility, specific heat and resistivity by considering excitations of electrons around ...
0
votes
0answers
65 views

Quantum harmonic oscillator, clarification about the period

We can write the general state |A> at time $t=0$ as $|A, 0>=\sum a_n |n>$ where |n> are the eigenvectors of the oscillator. In my textbook there is written that if the $a_n=0$ for every n=even ...
0
votes
0answers
16 views

Distance needed to double the moment of inertia of a cylinder

The moment of inertia formula for a solid cylinder is $0.5MR^2$. How would I need to change the radius in order to double the moment of inertia of the solid cylinder?
1
vote
1answer
73 views

Modeling a pool ball rolling and slipping given initial velocity

I am trying to model a pool game. Currently my physics is faked by giving it an initial velocity and just subtracting from it every second. To make it more realistic I want to use physics to determine ...
1
vote
1answer
142 views

How can we calculate the maximum height of a fountain? [closed]

There are two limiters in this question. First is drag of air. Second is the viscous forces of pipe.(belong to the tube where water hitts up) The question is how can we calculate the maximum height ...
0
votes
1answer
71 views

Why do we need theorems like **Parallel Axis Theorem**

In rigid body pure rotation, quantities like $\omega, \tau, L, I, r_i$ (symbol with usual meaning) are axis dependent. Assume rigid body to be sphere rotating about the axis passing trough the center. ...
0
votes
1answer
27 views

How drag force relates to velocity in the air?

As you know in the formula of drag $$F=0.5 *d*A*C*v^2$$ in different velocities we have different Reynolds number. and the Reynolds number relates to coefficient of drag. my question is how can we ...
2
votes
1answer
82 views

When are energy, mechanical energy, momentum, and angular momentum conserved?

I am in AP Physics and my only real hangup is knowing when the said quantities are conserved. Please define what is the SYSTEM in your answer. I kind of have the basic idea. For example, if there ...
2
votes
0answers
72 views

Two dimensional spacetime and the Gauss Bonnet theorem

Generally two dimensional spacetimes are deemed to be static, as the Gauss Bonnet theorem implies that the Einstein Hilbert action would be a constant independent of $g$. But as far as I can tell, ...
0
votes
0answers
18 views

Interferometer based on atoms and gravitational field detection

I know that ultra cold atoms can be used to measure a gravitational field, but how does this work exactly. More specifically, I know that an interferometer based on atoms can be used to make very ...
1
vote
3answers
124 views

Why does Torricelli's law seems to fail when water speeds up after I put my finger in a hose?

We are taught that the speed of the fluid through a hole in a water filled tank obeys Torricelli law: $v=\sqrt{2gh}$, thus the speed is independent of the size of the hole. Why is it then that making ...
0
votes
1answer
126 views

Can entropy be regarded as energy dispersal?

In several answers here the claim has been made that thermodynamic entropy can be regarded as energy dispersion. See, in particular here, and here and here. This is apparently the pet theory of a ...
1
vote
0answers
48 views

Calculus/Vector Calculus and so on in special relativity book recommandation

I want to learn calculus 1,2,3 vector calculus, analysis and so on in special relativity. But I never found a good and clear book/source on it. Can someone recommend an easy book or internet source on ...
1
vote
1answer
89 views

Wave reflection, boundary conditions?

When a wave-pulse is reflected such that initially it was moving in the denser medium, and towards the other side of the interface lay the rarer medium, will there have been a phase change post ...
1
vote
0answers
48 views

Are these two equations correct? [closed]

I know this is a pretty simple question, but like there should be room for those as well. I have the following two formulas describing how long it'll take for an object to decelerate. For example if ...
1
vote
0answers
51 views

Is there a theoretical maximum speed for wind?

What we call wind is actually molecules moving into an area of lower particle density due to their thermal motion. A lower particle density offers less collisions (resistance), so more molecules move ...
2
votes
1answer
37 views

Where did the gamma matricies disappear to?

In the discussion of external bremsstrahlung, the following amplitude is used: $$M=i\bar{u}_e(k')e \left(\gamma^\nu\epsilon_\nu \left[ \frac{i \gamma^\nu(k'_\nu+\omega_\nu) + ...
1
vote
0answers
136 views

1/m Laughlin state and $U(1)_M$ chiral CFT

I am a little confused that people claim that the edge theory of a 1/m Laughlin state corresponds to a $U(1)_m$ chiral CFT. This indicates there should be $m$ primary field operators in $U(1)_m$ ...
1
vote
0answers
34 views

Right lens for my laser [closed]

I want to know if there is a way to find out size of lens(radius, it has to be sphere), when i know focal distance(120cm) and refractive index(1.7), for 2cm(diameter) laser ligting on it?
1
vote
0answers
60 views

Which property do the word Pressure refer to in General Theory of Relativity?

In this course by MIT Alan Guth while delivering the lecture stated " Both Pressure and Energy densities can produce gravitational fields. Negative pressure creates repulsive gravity and positive ...
2
votes
5answers
228 views

Is boundary well defined if variation of metric don't vanish on the boundary?

Suppose that you want to calculate the variation $\delta S$ of an action induced by some arbitrary variation $\delta g_{\mu \nu}$ of the spacetime metric : \begin{equation} S = \int_{\Omega} ...
8
votes
2answers
225 views

Why isn't a meter defined from a kilogram of water?

Why are there different official definitions for a kilogram and for a meter when a meter can be defined by the volume of a kilogram of water? For instance, using the triple point or some other state ...
1
vote
0answers
20 views

Drawing tangents on space [closed]

Can I draw tangents on space? If yes/no, why? A co-ordinate system is needed to draw tangents over curves. Is there any curve on space theoretically?
3
votes
1answer
167 views

Tadpole diagram in QED

In $\phi^4$ theory with the $\lambda \phi^4 / 4!$ interaction term gives rise to “tadpole” diagrams like this: If I have a standard QED interaction with $e \bar\psi \gamma^\mu \psi A_\mu$, can I ...
1
vote
1answer
147 views

Can acoustic waves travel through human body?

If I stand inside a perfect anechoic chamber in front of a unidirectional acoustic waves emitter, I would hear the tone played either using my ears or using a microphone for ultrasonic waves. But what ...
3
votes
0answers
61 views

Bottle cap burst phenomenon [closed]

There is a trick you can do with a simple plastic water bottle: After putting the cap on as tight as you can, you twist the body of the bottle a few times so that it compresses the air in the bottle. ...
0
votes
0answers
18 views

Do equalized Fermi Levels imply zero concentration gradient?

The explanation of a P-N junction says that the Built in Voltage opposes the continual diffusion current leading to a net current of 0. What i don't understand is how there can be continual ...
1
vote
0answers
144 views

Is Wikipedia wrong about Huygens-Fresnel Principle?

Someone posted a question about why backward waves don't exist according to the Huygens-Fresnel principle: Why doesn't a backward wave exist? In following up on this question I read the Wikipedia ...
0
votes
2answers
52 views

Why do we need Diffusion currents to explain semiconductor current flow?

Why do we need the idea of carrier concentrations to explain current flow? Can we simply not associate the disparity in carrier concentrations between two samples to a disparity in relative charge ...
0
votes
2answers
36 views

Can I see the edges of the visible spectrum at home?

So we can modify light outside of the visible spectrum via sophisticated lenses with the result being a new frequency inside the visible spectrum. What I would like to do is different. I would like ...
0
votes
2answers
113 views

Confusion with Blackbody Radiation

A blackbody is a theoretical object that perfectly absorbs all the light that falls on it. From what I understand this is an ideal situation and does not actually exist in reality. Certain objects are ...
3
votes
1answer
409 views

Why doesn't a backward wave exist? [duplicate]

Huygens principle says every point of wavefront emit wavelet in all directions. Then why does a back ward wave not exist? Can any expert tell real answer? On different sites I get different and ...
1
vote
1answer
26 views

One block of pulley system collides with the ground [closed]

Question:- My attempt: Since the P block is given a velocity v in the downward direction, the Q block will move in the opposite direction(upwards) with the same velocity v. When the P block ...
2
votes
0answers
88 views

What would be the abilities of a wave function collapsing oracle? [closed]

Although the Bell's theorem dismiss local hidden variables as an explanation of the quantum variability, there is nothing to prevent an observer to see the occurrence of a wave function collapse as ...
3
votes
0answers
148 views

Does Ashcroft and Mermin chapter 13 problem 4 have a misprint?

I've been working through Ashcroft and Mermin to review some aspects of solid state physics and I think problem 4 in chapter 13 might have a misprint. Since there don't seem to be errata available I ...
1
vote
1answer
27 views

How does one build a framework of sensors from nothing? [closed]

I would like to understand how you can go from nothing to a framework of sensors you can use to try out things in physics/thermodynamics. That is, everything has to be bottoms up, nothing premade ...
0
votes
1answer
68 views

Calculating root mean square velocity using current vs time graph

There was a question in my today's physics exam. Me and my friend had conflicting views about the question. Here is the graph and the question is "Calculate the value of Irms using following graph." ...
3
votes
3answers
66 views

How do we know that the rate at which a body loses heat is proportional to the difference between its temperature and that of its environment?

Did someone do an experiment, or was that fact derived from other ideas we had about how the world works?
1
vote
2answers
136 views

Motor Speed depends on voltage or number of turns

What limit the size of motor speed? Does the speed of motor depend on the voltage applied or magnetic field strenght or frequency or what?
2
votes
0answers
81 views

Drawbacks of single-field inflationary models with $\phi^2$ or $\phi^4$ potential?

I am aware that existing models of inflation, especially the single field chaotic inflation with quadratic or quartic potential, e.g. $$ V = \frac12 m^2 \phi^2 \qquad \text{or} \qquad \lambda \phi^4 ...
0
votes
1answer
41 views

Calculate the relativity of time of an event occurred in different frames

Imagine we have a train which is moving with a velocity and there's an observer inside of that train and there is an observer that is outside of the train observing the train moving. There's a ball ...
1
vote
3answers
82 views

Violation of newton's third law in case of fluids

If a steel ball hits an iron surface it bounces back due to the Newton's third law of motion. Then why does'nt it bounce back when it hits a water surface ? or for that matter even air ? The ...
0
votes
1answer
116 views

Confused regarding sign convention of silvered plano convex lens

I'm totally confused while finding the sign convention of a silvered plano convex lens. I know that equivalent power of such a system can be found by adding the power of the mirror and twice the power ...
2
votes
1answer
56 views

Do physicists subscribe to Occam's razor? If so, how do you define 'all things being equal'? [closed]

The question here is the title: Do physicists subscribe to Occam's razor? If so, how do you define 'all things being equal'?
0
votes
1answer
58 views

Back Emf in Motor

Why does back emf tend to decrease as the rate of doing work increases. When load increases it reduces the angular speed of motor as a result induce current due to back emf also decreases because the ...
1
vote
1answer
64 views

Calculating velocity of a spacecraft between moon and earth [closed]

I'm in high school, I'm not very good at physics, and I have this project (year long). I'm trying to determine the accuracy of a simulator (Universe Sandbox 2) with theoretical velocities vs. the data ...

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