# All Questions

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### Heat Transfer and Heat added to a system

Let's say the temperature of an object is constant and equal to 20 °C. If we put this object to a surrounding which has a temperature of 25 °C, than there's heat transfer from the surrounding to ...
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### What is an awkward constant and is there a relation to ad-hoc hypotheses?

[I'm not entirely sure whether this is the right board, since it is not a technical but a soft question]. I'm reading the textbook "Spin Dynamics - Basics of Nuclear Magnetic Resonance", 2nd ed., ...
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### How does localization affect interferometry?

A classic setup (I suppose it's classic) in introductory quantum mechanics has a single photon passing through a double slitted grating. Directly across each slit of the grating is a device that can ...
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### Steps in a proof in kinetic equation of motion

This is a part of a kinetic equation of motion, \begin{align} x(t) - x_0 &= \frac{1}{2}a \left[ \frac{v(t) - v_0}{a} \right]^2 + v_0 \left[ \frac{v(t) - v_0}{a} \right] \tag{1.17} \\ &= ...
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### Gliders in a trailer

Everybody has heard I the old thought expirement of the birds in the trailer where the man hits the trailer the birds take flight and the trailer gets lighter. This has been disproven by the ...
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### What's a good mnemonic for intensive and extensive properties? [closed]

For some reason, I have always had a hard time remembering which is which. Intensive properties don't depend on the amount. Extensive do. What's a good mnemonic to help remember this?
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### Why is kinetic energy calculated using speed?

Why is kinetic energy calculated using speed? Since speed is distanced covered, and if a car moves 100 meters around a circular track -- stoping after moving half of the distance of the track -- the ...
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### Demonstration for the existence of longitudinal electrostatic oscillations

How could I demonstrate that in a linear, homogeneous and isotropic medium without losses but electrically charged, Maxwell's equation admit as solutions longitudinal electromagnetic waves, beyond ...
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### Ray optics : Circular mirror with reflecting surface on the inside

A light ray enters through an infintely small hole and gets reflected as shown. After how many reflections will it emerge out again? (It can come out in any orientation). Will it emerge out for any ...
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### Gravitational potential and intensity

Why don't we calculate gravitational field/intensity of a point due to a disc directly? We first calculate the potential then calculate the field by differentiating with respect to $dr$. Why don't we ...
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### Is it possible to consider loops that cut through charges when proving electric field is conservative?

My goal is to prove that the integral of E.dl along the blue curve is zero. My problem is shouldn't E field approach a discontinuity when approaching the plate? Even if we consider say a point ...
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### How does one magnetic field effect another

Do the fields cancel or are they additive or something else. Can we direct and focus them and or amplify then with similar fields? Can we eliminate the electric portion of an electromagnetic field?
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### Electric Field Inside non conducting sphere

Let we have a spherical, nonconducting, and thin shell of uniformly distributed positive charge Q and the radius is d. Will we get any electric field inside the sphere whereas we know the electric ...
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### What is general strategy to find the resistance of medium?

What is general strategy to find the resistance of medium? These are some of the examples Two metal balls of the same radius $a$ are located in a homogeneous poorly conducting medium with ...
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### How can travelling at light speed affect light itself? [duplicate]

~Hypothetical Scenario: Your driving in a car (with the headlights on high beam) and you gradually get faster until you reach the speed of light. Raised Assumptions: You can see in and out of the ...
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### Why in this problem I had to take the average of the electric field?

On an electrically charged soap bubble, acts an electric force outwards. We are given the total charge of the bubble, $Q$, and its radius, $R$. What is the value of the total force, acting to ...
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### Paper Napkin Folded

Why folded napkins are better absorbing water than ordinary napkin. I think there must be related to cohesion of water element. Water are captivated between layers because they appeals each other or ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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?
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### How does thin film interference work?

So thin film interference is when light is half-reflected half-refracted then the reflected and refracted wave interfere with each other to produce another color. What I don't understand is that the ...
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### Relation between accelerations of block and tangential acceleration of the pulley in pulley block system

I was doing some pulley block problems in rotational mechanics and almost everywhere when the pulley is rotating and some masses are connected by a light rope(for example I am taking a two block and 1 ...
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### Why does a vortex have so much suction power?

A vortex that's on the ground(tornado) can rip tiles off a roof of a house and basically sucks in the roof tiles. On an aircraft that develops vortex lift the vortex sucks in air molecules and reduces ...
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### How to develop the static and kinetic friction formula? [duplicate]

How do you end up with the formula for Amontons' first law of friction? ($F_s = \mu_s \times N$) or ($F_k = \mu_k \times N$). I know how to use the formula. Now I need to know how to get to it.
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### Educational benefit of putting units outside of variables? [closed]

When I studied physics (both in high-school and university), in all of the worked examples, variables would be used to denote physical quantities, and thus include dimensional units. For example, ...
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### Static vs. kinetic friction and the “normal-force” considered

Consider two cases. First, we have a box that a person applies a force to (say to the right). The force of friction (static) is then directed to the left. Second, say that the box is moving at ...
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### Permanent flying balloon

We all know balloons which are filled with gas that's less dense than air. After a few hours, the balloon drops to the ground. The reason for this phenomenon, as far as i understand, is that the ...
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### Why can iron and dryer both make clothes with less wrinkles?

Why can iron and dryer both make clothes with less wrinkles? Is there some physical or chemical process that smooths out the clothes? Which factor in a dryer that smooths out the clothes, rotation, ...
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### Question on Electromagnetic Induction

If a magnet is moving through a solenoid, why is there no electromotive force being induced while the magnet is at the centre of the solenoid? (Is it because emf is induced in both directions?) Can a ...
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### Hydrogen anion system [closed]

For the H- system,how we calculate the energies correct to the first order for the ground state and first excited state. Is it possible for a single proton to hold two electrons and still be stable?
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### Interferences in a superposition of two energy states of a particle at rest

In Feynman's volume III, chapter 7, he describes a particle which is in a superposition of the energy states $E_1$ and $E_2$ as $$e^{-i(E_1/\hbar)t} |E_1\rangle + e^{-i(E_2/\hbar)t} |E_2\rangle$$ ...
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### Calculating trajectory of particle moving in a potential (SHM)

I have been given the potential of a simple harmonic oscillator: $$V=\frac{1}{2}kx^{2}$$ I want to calculate the value $x(t)$ of a particle moving in this potential, with initial conditions ...
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### Pitch and loudness relation

Using an Oscillator in a program, I noticed that the lower and the higher frequencies are less loud than the middle ones. I suspect there is a relation between pitch and loudness but can it be ...
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### What physical process is the source of blackbody radiation? [duplicate]

Blackbody radiation is the radiation given off an object solely as a result of its absolute temperature. The (continuous) spectrum is given solely by the temperature. How can this be? Why should a ...
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### Does chemical energy contribute to mass? [duplicate]

Does chemical energy contribute to the mass of an object? I don't mean the bond energy, but the possible energy that could be released (i.e. Does an atom of oxygen and a molecule of hydrogen (H2) have ...
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### Reverse spinning wind generator [closed]

I broke a blade on my wind generator. The maker says they have new blades that rotate the opposite direction now. At first that sounds like a no-go, but then after thinking about it, the wires are ...
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### What happens if locally manifold is seen as an Euclidean space? [closed]

I have been trying to understand the definition of a manifold and I have found out that the most common definition can be paraphrased as: A manifold is a space that has a complicated "topology" ...
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### Why is the charge of a proton positive? [duplicate]

Is there a reason that a proton has a positive charge while an electron has a negative charge? Are these just names that were given to the charges or was there a reason for making a proton have a ...
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### Mimicking Free Fall

I was thinking about this situation with a friend of mine, and we had opposing views: Suppose you have a frictionless table with mass M resting on top. You attach a chain to the mass M, a chain with ...
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### OPE of parity even theories in CFT.

If I consider an OPE of some operators, which belong to a theory where parity is not violated, will I have a constraint on the kind of operators appearing in the right hand side ? For example, I ...
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### Induced emf vs Potential difference

Let a square conductor frame length l, resistance R,be pulled out with a constant velocity v from a magnetic field B perpendicular to plane of the frame .Then an emf e=Blv is produced across the ...
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### Adiabatic Invariance of 2 mass system

I am trying to solve the second part of Problem 10 from David Tongs CM notes. Specifically we have The neutron star is in a non-circular orbit with $E < 0$. Give an expression for the adiabatic ...
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### Why is sum of Boltzmann factors equal to $1 + \exp(-\epsilon / k T)$?

I am bit confused about how they write $$Z = \sum e^{- \epsilon_i / kT} = 1 + e^{-\epsilon/kT}$$ Where did the subscript $i$ go and where does the $1+e^{-\epsilon/kT}$ come from?
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### Physical meaning of eigenvectors of mass matrix

What is the physical meaning of the eigenvectors of the mass matrix? If I consider a 2-dof system with one mass linked to two orthogonal springs and I write the equations in any orthogonal system of ...
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### Coma Cluster X-Ray Mass

Does anyone have a reliable reference for the x-ray mass of the Coma Cluster? I've got a value of 3e13 M⊙ from Gursky's 1971 report. Anything more recent would be appreciated.
I learned about the Einstein Field Equations and also I know they depend on energy-momentum tensors. The solution of the field equations is a space-time metric. $$G_{\mu \nu}=8\pi GT_{\mu \nu}$$ ...
Consider an observer B is moving with speed $0.8c$ relative to another observer A in standard-configuration (I think it is called that in english: that the system B is only moving along the $x$-axis ...