# All Questions

1k views

### Is it possible to increase refractive index at lower densities?

Are there any materials for which the refractive index decreases with density? It seems that for most materials we would expect there to be a positive correlation between density and refractive index, ...
27 views

### Time resulting of a thermodynamics effect?

I heard in a documentary that : Irreversibility of time is linked to Heat Dissipation in thermodynamics. The theorist says "passage of time" is only a macro sensation, the underlying effect ...
15k views

### Refractive index of air in dependence of temperature

What is the exact dependence of the refractive index of air and the temperature? Is there a theoretical derivation of it?
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### Diagonal lorentz invariant matrix

Is there any general way to show that diagonal Lorentz invariant matrix must be proportional to $g^{\mu \nu}$? I can show it is true, but my proof is not clean enough for such a simple problem.
2k views

### Does a “capacitor” for light exist?

If I have a light that is flickering at a frequency low enough to be perceived by the human eye, is there any type of material that exists that will smooth out the appearance of flickering? Similar to ...
44 views

### gauss law and electric flux

Can you please explain me what is electric flux. I know it is $\oint\mathbf{E}\cdot\mathrm{d}\mathbf{S}$, but in many place its given flux means to flow. But how does electric field "flow"? Also if we ...
43 views

### Compensating Parallax problem of a washing machine's drum [on hold]

We would know how it is possible inside a washing machine, when the drum is full with garments, so it is very heavy, to compensate, when the drum is rotating at high speed for the parallax of rotation,...
90 views

### What is the decay width and why is it given in energy units?

I'm reading Thomson, Modern Particle Physics, and in chapter 16 author says that the decay width of the Z boson is $\Gamma_Z =2.452 \pm 0.0023 \,\mathrm{GeV}$. He also says the total width of the ...
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### Does charge of capacitor at constant voltage change after dielectric material is inserted?

I have a planar geometry capacitor, connected to a battery that supplies V volts. Initially there is vacuum/air in between both plates. Afterwards, some dielectric material is inserted in between both ...
76 views

### Sliding sphere wear shape

Please refer to the figure attached. Consider a normal force is acting on the top of sphere. A constant coefficient of friction causes frictional force throughout the sliding. I want to know after ...
21 views

### Range of voltmeter [closed]

I have understood how the range of an ammeter cannot be decreased. But I don't seem to understand how the range of a voltmeter can be decreased. Pls help.
84 views

### Why does hot water freezes faster than normal water? [duplicate]

What is the reason behind hot water gets Freezes faster than normal water when we keep both the hot and normal water together in freezer ?
35 views

### Invariance of Mass in SR: Comparing Taylor and Wheeler's Spacetime Physics with a Comment made in Purcell and Morin's Electricity and Magnetism

Purcell and Morin's Electricity and Magnetism mentions on pp. 241: Mass is not invariant in the same way (in a relativistic theory). We know that the energy of a paritcle is changed by its motion, ...
1k views

### The speed of light upon reflection [duplicate]

(NOTE: I am an 8th grader, so I may not be capable of perfectly elaborating my point by scientific measures. Also, English is not my first language) I have read that the speed of light is constant, ...
431 views

### Why does solid turn to liquid deep in the Earth? [duplicate]

Usually pressure decreases inter-molecular movement thereby keeping the substance in solid state. Given this, why do the earth's crust and mantle, which are both solid, turn into a liquid outer core, ...
22 views

### Can I get hit positions in a plate from a normal distribution of a radiation does at that plane? [on hold]

I know this might sound confusing, but I was wondering if I had a distribution that defines the radiation does at a plane at a certain position (lets say $z=783~\mathrm{cm}$) here are the images of ...
28 views

### Is there a verified physical balance in the Maxwell source terms derived from the QM wavefunctions? [on hold]

In QM one construct the charge and current source distributions for Maxwell equations, seen in standard textbooks on quantum mechanics, so that the external EM field can be calculated. A nice feature ...
14k views

### Inertia Vs Momentum

At my recent lesson on kinematics, my teacher taught about inertia and momentum. This is what she said. Inertia: a characteristic of an object that resists changes to its state of motion. Momentum: ...
71 views

### Does angular momentum of hydrogen atom imply motion of electron around the nucleus?

Does the non-zero orbital angular momentum (or z-component of angular momentum) of a stationary state of hydrogen atom imply motion of electron (or at least the probability density $|\Psi|^2$) around ...
1k views

### Does a pivot exert an additional force, normal to the centripetal force, on an object rotating about it?

A thin plank of mass $M$ and length $l$ is pivoted at one end. The plank is released at 60$^{\circ}$ from the vertical. What is the magnitude and direction of the force on the pivot when the plank is ...
106 views

### Length Contraction in Relativity

Let's suppose we have a one dimensional rod made of elementary point particles, in contact with each other placed along the x-axis.If the rod is moving along x-axis then we know (because of relativity)...
253 views

### Dielectric boundary

I am trying to determine why electric field may be confined to a certain region if there is a large difference in the permitivity for example if electric field flows through water and then reaches a ...
44 views

### What conditions are necessary for Birkeland currents to form?

If charged particles flow through non-magnetised space will they form Birkeland current? Or can such currents only form along existing magnetic field lines? Part of the reason I ask is this: http://...
43 views

### Is there a delay in the effect of the gravitational or electromagnetic force if a secondary body suddenly appears? [on hold]

If the sun were to suddenly vanish in our solar system, it would take about 8 minutes for the Earth to exit its orbit. This is because the gravitational field of the sun would vanish at a rate of $c$. ...
64 views

### Is earth continuously changing its orbit size i.e, circumference of the orbit? [on hold]

One year have 365 days, but in leap year we have 366 days. Does that mean earth is continuously change its orbit size for every four years. like . 365.0 -- 365.1 -- 365.2 -- 365.3 -- 365.4 -- 365.5 -- ...
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?
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### Why would speed of light be directional if spacetime is discrete?

In Feynman's Simulating Physics with Computers, Feynman states that "we might change the idea that space is continuous to the idea that space perhaps is a simple lattice and everything is ...
31 views

### Bose-Einstein condensate effect

Does every boson gas go through the condensate effect? Could there be a boson gas with high energy (photons for example) that would have enough energy to be at different quantum states even at 0 K?
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### Why can we approximate massive particles as massless or vice versa?

Our descriptions of massless and massive particles are very different. For example: Massless particles have only two polarizations, which we call helicities. Spin projection on axes different than ...
23 views

### Coleman-Mandula theorem and mass gap

I had a couple of naive questions about Coleman-Mandula theorem. One of the assumptions of the theorem is the non-existence of massless particles in the spectrum. Since we do have massless photons ...
42 views

### Hamilton's principle and virtual work by constraint forces

I have a question about the following pages(pg 47 and 48) from Goldstein's "Classical Mechanics" I do not understand how (2.34) shows that the virtual work done by forces of constraint is zero. ...
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### Use my example to explain why loop diagram will not occur in classical equation of motion?

We always say that tree levels are classical but loop diagrams are quantum. Let's talk about a concrete example： $$\mathcal{L}=\partial_a \phi\partial^a \phi-\frac{g}{4}\phi^4+\phi J$$ where $J$ is ...
26 views

### Parallel combination of cells [on hold]

In this circuit suppose that $E_1 >E_2$. This means that the positive terminal of $E_1$ will have a greater potential than the positive terminal of $E_2$. Then current should flow from the ...
3k views

### Scale factor and age of the universe

Why is the cosmological scale factor (expansion rate of the universe) not simply the time $t$, i.e. the age of the universe?
9k views

### What is entropy really?

On this site, change in entropy is defined as the amount of energy dispersed divided by the absolute temperature. But I want to know: What is the definition of entropy? Here, entropy is defined as ...
11 views

### Modeling Optical Properties of Doped Crystals

I am interested in modeling the optical properties of crystals doped with lanthanides to investigate their luminescent properties. What would be the best way to pursue this? EDIT (In response for a ...
3k views

### The electric field of a conductive sphere containing a charge - grounded vs not grounded

Let's suppose we have a sphere but unlike theoretical ones it'll have some thickness say $\Delta r$ and inner radius $R$. What I was wondering about is how will it behave if we place some charge $q$ ...
70 views

### Does a “capacitor” for light exist? [duplicate]

If I have a light that is flickering at a frequency low enough to be perceived by the human eye, is there any type of material that exists that will smooth out the appearance of flickering? Similar to ...
58 views

### Switching on a light bulb. Initial EM/electron pulse propagation

TLDR When switching on an electrical device (e.g. a light bulb or a heater), there is an initial wave (pulse) travelling with very high speed (close to the speed of light) and “carrying the ...
6k views

### What is a tensor?

I have a pretty good knowledge of physics but couldn't understand what a tensor is. I just couldn't understand it, and the wiki page is very hard to understand as well. Can someone refer me to a good ...
708 views

### Is the deformation of spacetime, elastic deformation or plastic deformation?

Is the deformation of spacetime, elastic deformation or plastic deformation?
198 views

### Meaning of Strongly and Weakly Degenrate

In ideal Bose and Fermi gases we often use Either Strongly Degenerate Ideal Bose/Fermi or Weakly Degenerate Ideal Bose/Fermi gas. As far as I know mathematically if the fugacity $z=e^{\beta\mu}$ close ...
59 views

### Question about the coefficient of restitution

I found out a very interesting way to model the coefficient of restitution (COR), which yields as a result the intrinsic COR of a body made out of certain matter (i.e it's COR with respect to a ...
33 views

### Why do we get a diffraction pattern with a slit whose width is not comparable to the wavelength of the incident light?

In single slit experiments with sodium light whose wavelength is of order micrometer and slit width of order of hundreds of micron, we still observe the diffraction pattern. How do we explain this?
8k views

### What causes a soccer ball to follow a curved path?

Soccer players kick the ball in a linear kick, though you find it to turn sideways, not even in one direction. Just mid air it changes that curve's direction, i.e., it swings, as footballers say. Is ...
18 views

### Calculating effective dielectic constant

I have a system of metallic nano-particles supported on glass where I have observed a difference in the in-plane dielectric constants of the metal ($\varepsilon_x$ is different from $\varepsilon_y$ ...
30 views

### Swirl velocity from circulation - Vortex

I have a vector field in cartesian coordinate $(x,y)$ with coordinate origin at vortex core. For calculating the swirl velocity $v_\theta$ I followed below steps, Step 1: Find the circle coordinates ...
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### How would I find the velocity of an object after it doesn't fully enter the gravitational pull of a planet and is kind sling shot out of its gravity?

Obviously that question doesn't have a lot of jargon having to do with math and physics. I am not a physicist, I'm not a mathematician, I'm trying to make a game and simulate some physics found in ...
There is a charge $q$ at a perpendicular distance $z = d$ from an infinite conducting plate $z=0$. We use the image method and place $-q$ on the other side of the plate and calculate the field. This ...
The figure above shows a pulley system consisting of 3 masses ($m_1$, $m_2$ and $m_3$), a homogeneous wheel (radius R, mass M) and 2 massless pulleyes which are connected by a massless rope. Mass $m_1$...