# All Questions

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### If you lift something does the internal energy increase or remain unchanged?

So change in internal energy $\Delta U = q + w$; so in lifting an object you do work on it, thus increasing its internal energy. Is this correct? Sorry if this is a stupid question.
2answers
150 views

### electrostatics and magnetism

Let a square loop be placed in a uniform electric field in a position of stable equilibrium. Then, because of the electric field, if the electric field is in the rightward direction then a positive ...
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375 views

### Viscous Burgers equation physical meaning

The viscous Burgers' equation: $$q_{t}+q\:q_{x}~=~\nu\:q_{xx}, \mbox{ where } \:\:\nu >0,$$ combines the nonlinear propagation of $q(x,t)$ and the diffusion. What is this equation for? (in ...
2answers
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### What causes Potassium to decay into Argon the way it does

From evolutionwiki: "Potassium 40 decays into argon 40 through a process known as electron capture. In electron capture, an electron from the innermost electron shell "falls" into the nucleus, ...
0answers
22 views

### Measuring a capacitor accurately

I want to accurately measure the value of an electrolytic capacitor at DC voltages, and accurately predict its voltage based on current drawn from it, or put the other way, accurately measure the ...
0answers
45 views

Suppose I have a simple functional $$F=\int{dx\;\phi^{*}(x)\phi(x)}\tag{1}.$$ Assuming $\phi(x)$ and $\phi^{*}(x)$ are independent and I take a functional differential with respect to $\phi(x)$ and $\... 2answers 166 views ### Otto and Carnot efficiency comparison Lets say both engines operate between same temperature limits. Carnot eff: $$e_\text{Carnot} = 1-\frac{T(L)}{T(H)}$$ Otto eff: $$e_\text{Otto} = 1-\frac{T(4)-T(1)}{T(3)-T(2)}$$ ... 1answer 107 views ### Boomerang angular momentum So I'm building a boomerang to fly 200 feet in order to do that I was told to increase the moment of inertia to increase the resistance from change in state, and I was told that it would increase the ... 0answers 38 views ### Twin paradox without gravity Suppose we have 3 people Person1 was already born on a spaceship that travels in the speed of 0.5c toward person2 Person2 was born in a point in point in the space also in the exact time of person1 ... 2answers 56 views ### Free fall vs throw with air resistance If I let a stone drop from a certain height and throw a stone from a certain height, with air resistance, which will reach the ground first? When I draw a free body diagram the air resistance in the ... 1answer 233 views ### Significance of the exception to Gleason's Theorem when n = 2 Gleason's Theorem famously asserts that (appropriately defined) measures on the lattice of a complex Hilbert space can be implemented by density operators via the trace operation, except in the case ... 1answer 36 views ### Vacuum persistance amplitude E. Fradkin's Field Theories in Condensed Matter Physics formulas 3.57 and 3.58: I feel really sad about it, but all my tries of getting from formula $$Z = \operatorname{tr} \hat{T} \prod_{j=1}^{... 3answers 433 views ### Distinguishable, Indistinguishable Paramagnetic Ideal Gas In the canonical ensemble, the partition function for an ideal gas is given by:$$\frac{Z}{N!}$$The factor N! accounts for the indistinguishability of the particles of the ideal gas. What ... 1answer 41 views ### Clarifications on Ampere's Law I have just learnt Ampere's Law, useful for calculating the magnetic field in situations having a high degree of symmetry. However, I have some conceptual doubts regarding it: Before I begin, I would ... 0answers 9 views ### What can be the result of interaction of H- ion (2.1 MeV) and stationary H2+ ion? I believe there is Coulomb scattering. However, are there any other channels? Could the products of the reaction be H and H2, and what would be the cross section of this reaction? 0answers 11 views ### Projectile on inclined plane [on hold] Two inclined planes OA and OB as shown . A particle is projected from point P with velocity u along a direction perpendicular to plane OA . If the particle strikes plane OB perpendicularly at Q , ... 5answers 2k views ### Measuring the speed of light and defining the metre - absolute or relative? If the metre is now defined as the distance light travels in vacuum in 1⁄299,792,458th of a second and the speed of light is accepted to be ... 2answers 45 views ### Could a pocket universe be birthed out of the negative pressure in our universe? I have just recently started learning about inflationary cosmology so I don't know much about it. But from my understanding, the theory says the big bang was started by the expansion of a small region ... 2answers 914 views +50 ### What's the relationship between weakly hypercharged bosons and Higgs bosons? In this video, Leonard Susskind does a good job trying to explain succinctly the Higgs field and exactly how it gives elementary fermions mass, except for one point he seems to skip a few things. The ... 0answers 41 views ### Intuition behind deriving the FRW metric Consider a 3D sphere embedded in a 4D “hyperspace”:$$(x^1)^2 + (x^2)^2 + (x^3)^2 + (x^4)^2 = a^2,\tag{74}$$where a is the radius of the 3D sphere. The distance between two points in 4D space ... 1answer 104 views ### 1-D Fick's first law - partial derivative? I've recently been reviewing some concepts, including diffusion. Fick's 1st law:$$J = -D\frac{\partial C(x,t)}{\partial x}$$as I understand it, applies to the steady state. For 1-D diffusion and ... 0answers 22 views ### Is mass relative? [duplicate] This question has to do with the relativistic mass of an object. I have been reading that as energy increases, so does mass to a much lesser extent, and that this applies to kinetic energy as well. ... 0answers 4 views ### Liquid metallurgy [on hold] I want to know more about liquid metallurgy? all i know it is a technique used for fabrication of material. Please help me if you know about this technique. Thank you. 0answers 26 views ### How is a irreversible process (conventionally) represented on a T-S plane and why cannot it be (really) represented? A reversible process can be represented on a T-S plane, and the area under the curve is the heat exchanged by the system. On P-V plane a irreversible process is conventionally represented with a ... 0answers 63 views ### Lorentz transformation: Can distance between two observers depend on the observers? [on hold] Consider reference frames S and S'. S' is moving in positive x direction at speed v relative to S. Time measured by S is t and that measured by S' is t'. When t = 0, then t'= 0. Coordinate systems ... 1answer 70 views ### Proof of two Lorentz-algebra identities I am currently working through the QFT introduction text by Peskin and Schroeder and try to fill in two identities that I wasn't able to prove (it should be fairly simple, but my experience with this ... 1answer 19 views ### Charge density on two hemispheres, enclosing a conducting sphere [on hold] Two uniformly charged non conducting hemispherical shells each having uniform charge density \sigma and radius R form a complete sphere (not stuck together) and surround a concentric spherical ... 5answers 6k views ### If dark matter only interacts with gravity, why doesn't it all clump together in a single point? I'm a complete layperson. As I understand, dark matter theoretically only interacts with the gravitational force, and doesn't interact with the other three fundamental forces: weak nuclear force, ... 1answer 399 views ### Force of a solenoid on a ferromagnetic material As I was trying to find an answer to another problem, I was informed that the equation I was using would not work. The equation I was using was$$F = (NI)^2\mu_0\frac{\text{Area}}{2g^2}$$If this ... 3answers 221 views ### Are there Gauge fields that are not 4-vectors? In my understanding Gauge fields are fields that have some kind of redundancy, i.e. a transformation that does not change the physical state. As far as I can see all the Gauge fields in the Standard ... 2answers 41 views ### How to resolve “bottleneck” of encoding input of quantum computer? [on hold] As far as I understand, all quantum computing purpose is to accelerate exponentially (upon input length) computation time of given task. But here user faces bottleneck to serialize input, and ... 1answer 27 views ### Electric Field of Bar [on hold] Problem: Positive charge +Q is distributed uniformly along the positive x axis from x = 0 to x = a. Negative charge -Q is distributed along the -x axis from x = 0 to x = -a. A point ... 0answers 6 views ### Charpy problem, Impact velocity [on hold] Calculate the velocity of the hammer of an impact testing device at its lowest position if the hammer starts moving at an angle 30° to the horizontal (mass of rod=0.5kg, length of rod 400mm. mass of ... 0answers 8 views ### I can't get the right solution to this thermal expansion problem [on hold] The problem : A glass flask is filled to 'the mark' with 60 cm3 of mercury at 20 ºC. If the flask and its contents are heated to 40 C ºhow much mercury will be above the mark? \alpha for glass : 9.... 1answer 246 views ### Closed timelike curves in the spin-2 gravity formalism Let's say we take some topologically trivial CTC spacetime, like the Gödel metric:$$ds^2 = -dt^2 - 2e^{\sqrt{2}\Omega y} dt dx - \frac{1}{2}e^{\sqrt{2}\Omega y} dx^2 + dy^2 + dz^2$$And then I ... 0answers 16 views ### Physics problem: stage of rocket [on hold] A manned rocket is rising with a velocity of 3 km/s. At exactly 2000m, it disconnects the first rocket stage. How long does it take the stage to hit the ground, and what is the velocity of impact.(... 0answers 18 views ### Conditions to find standing waves harmonics I came up with a doubt on standing waves conditions. The type of question I find difficult to answer is of the following type. Consider a rope. I do not know if the rope is fixed at both end or at ... 8answers 320 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 ... 3answers 52 views ### Tension in a string with pulley and two objects at opposite ends Tension is confusing me. According to my understanding of tension the tension in a string is uniform but while going through the solution of a problem I read that tge tension on the left part of the ... 2answers 15k views ### Would a light or a heavy ball roll fastest down a slope? A small, light ball and a larger, heavier ball are released from the top of a slope. Which will move further? which will come down faster? 1answer 48 views ### Can we derive Biot Savart law from Coulomb's law? Can we derive Biot Savart law from Coulomb's law by setting a current source at an instant in place of one of the charges? Let's say$dl$is the length of the conductor having$dQ$charge which ... 1answer 69 views ### Point charge inside a electrically neutral cavity in conductor placed eccentrically, and effect of external electric field if switched on bear with me, but i would like a definite answer, now, starting off the external charge density on the outer surface of sphere WILL be uniform by unique solution of Laplace equation and letting the ... 1answer 228 views ### Why does amplitude of a mechanical wave increase and then decrease with increasing driving frequency? I was doing an experiment where I explore how the frequency of the oscillation of a vibrating water hose (the end of the water hose), affects the amplitude created by the water path (a wave-like ... 0answers 29 views ### how to work out the probability of walking through a wall [on hold] I was in science on Friday and I asked my teacher how to calculate the probability of walking through a wall.I just was curious because I like learning quantum mechanics 1answer 23 views ### Energy loss of photons crossing a dielectric medium How much energy do photons lose when crossing an "isotropic" dielectric medium? What would be possible mechanisms. I assume the straight forward answer would be that the photon loses zero energy and ... 1answer 152 views ### S-matrix in Weinberg QFT I'm a bit confused by Weinberg's discussion of scattering. He defined the in and out states$|\Psi^{\pm}_\alpha\rangle$with particle content$\alpha\$ as states that transform under the Poincare group ...

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