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122745 | Asymmetry of induced e.m.f due to a bar magnet falling through a coil | It is well known that as a bar magnet falls freely through a coil, an electromotive force is induced in the coil by Lenz's Law. However, it appears that the peak e.m.f induced (ignoring the direction) is higher after the passing through a coil, than before it passes through the coil. Why so? My explanation was that aft... | What is the relationship between the acceleration of a falling bar magnet after it passes through a coil and the induced electromotive force (e.m.f.) in the coil according to Faraday's Law, considering the effects of both gravity and magnetic flux linkage? | 29,788 | cqadupstack-physics |
22439 | Increasing mass' effect on the balance between centripetal force and centrifugal force | Okay, this is nothing more than a thought experiment which popped into my head while driving home from work today. Take the case of a single body orbiting another, larger body, as in a planet and a star. The planet is in a stable, non-decaying orbit. Now assume that the mass of the planet was noticeably increased insta... | What would be the effect on a planet's stable orbit if its mass were to suddenly increase without any external forces acting on it, and how would this relate to gravitational and centripetal forces? | 14,839 | cqadupstack-physics |
68067 | How general relativity gets to an inverse-square law | I understand that a general interpretation of the $1/r^2$ interactions is that virtual particles are exchanged, and to conserve their flux through spheres of different radii, one must assume the inverse-square law. This fundamentally relies on the 3D nature of space. General relativity does not suppose that zero-mass p... | What is the physical interpretation of the $1/r^2$ law for gravity in the context of General Relativity, and how does it relate to the conservation of flux? Additionally, is Newton's Law of Gravity consistent with General Relativity, and if so, under what conditions? | 16,299 | cqadupstack-physics |
8530 | How to measure the spin of a neutral particle? | If a charged particle with charge $q$ and mass $m$ has spin $s \neq 0$ we can measure an intrinsic magnetic moment $\mu = g \frac{q}{2m}\hbar \sqrt{s(s+1)}$. This is how spin was discovered in the first place in the Stern-Gerlach Experiment. But for a neutral particle $\mu = 0$, so we cannot measure the spin of the par... | How can the spin of a neutral particle, such as the neutron or neutrino, be experimentally measured given that it has an intrinsic magnetic moment of zero? | 8,711 | cqadupstack-physics |
47982 | When is the FAIR accelerator supposed to be finished? | The FAIR accelerator is a planned facility for antiproton and ion research. Ground water wells are being put in, the forest is being cleared... But when is it supposed to be completed? Does anyone know? | What is the estimated completion date for the FAIR accelerator facility dedicated to antiproton and ion research? | 704 | cqadupstack-physics |
126741 | What kind of free energy do we use for a superconductor in a magnetic field? | My reasoning is as follows (using Gaussian units): Start from the second law: $$dU=TdS+dW,$$ where $dW$ is the work done by the magnetic field. To derive $dW$, we consider a solenoid with current $I$ and voltage $V$: $$dW=I\cdot V\cdot dt,$$ with $$\begin{align} V&=\frac{N}{c}\cdot\frac{d(B\cdot A)}{dt} \qquad &&\text{... | Given the expressions for Gibbs free energy \( G \) and Helmholtz free energy \( F \) as follows:
1. \( F = U - TS \Rightarrow dF = -SdT + HdM \)
2. \( G = F - HM \Rightarrow dG = -SdT - MdH \)
If a superconductor is in a magnetic field where temperature \( T \) and magnetic field strength \( H \) are controlled, exp... | 36,728 | cqadupstack-physics |
30870 | Dimensional analysis for gravitational radiation expression | on this paper, please refer to equation 2.117 for the power emitted for a rotating mass system: $$ P = - \frac{128}{5} G M^2 R^4 \Omega^6 $$ power in cgs should be (g is grams, m is meter, s is seconds): $$ g m^2 s^{-3} $$ now, $G$ is in $m^3 g^{-1} s^{-2}$ and $\Omega$ is in $s^{-1}$ so 2.117 right hand side is $$m^7 ... | Is the dimensional analysis in the modified expression for power, $$ P = - \frac{128}{5 c^5} G M^2 R^4 \Omega^6 $$, accurate? Specifically, can you verify if the dimensions of the right-hand side correctly align with the expected units of power in the cgs system, which are $$ g m^2 s^{-3} $$? | 23,146 | cqadupstack-physics |
107880 | Is spectrum of Discrete-Time Fourier Transform (DTFT) periodic or not | I can't think of any better title. Here is the content that I got question http://cnx.org/content/m10247/2.31/ As it state the nature of DTFT's spectrum is periodic as it show in figure 1   spectrum inherently periodic for all discrete-time signals, or does its periodicity depend on the specific characteristics of the time-domain signal \( s(t) \)? Provide an example of a discrete-time signal where the DTFT spectrum is periodic and another example where it is ... | 24,851 | cqadupstack-physics |
18631 | Effect of introducing magnetic charge on use of vector potential | It is well known that Maxwell equations can be made symmetric w.r.t. $E$ and $B$ by introducing non-zero magnetic charge density/flux. In this case we have $div B = \rho_m$, where $\rho_m$ is a magnetic charge density. But this means that $B$ cannot be expressed as the curl of vector potential anymore. Does it mean tha... | Is it possible to formulate a consistent theory of electrodynamics that incorporates both electric and magnetic charge densities using vector and scalar potentials, and if so, how would one maintain \( U(1) \) gauge invariance in such a framework? | 5,322 | cqadupstack-physics |
86844 | If time travel to the past is not possible why is this situation considered in Feynman diagrams? | I recently read R.P Feynman's _QED:A Strange Theory of Light and Matter._ It is believed that time travel to the past is not possible. Then why is particles going backward in time considered in the book while drawing Feynman diagrams? | In Feynman diagrams, how are virtual particles that appear to travel backward in time represented mathematically, and what does this imply about the conservation of energy and momentum in quantum field theory? | 37,761 | cqadupstack-physics |
6457 | Calculate the electric field of a moving infinite magnet, without boosting | Consider a rectangular slab of permanently magnetized material. The slab's dimensions are $L_x$, $L_y$, and $L_z$, and the slab is uniformly magnetized in the $\hat{x}$-direction. The slab is not accelerating or spinning. Does the slab generate an electric field? In a frame where the magnet is stationary, we know $\mat... | In a frame where a long, thin slab of permanently magnetized material is moving in the \(\hat{z}\)-direction, what is the electric field \(\mathbf{E}\) outside the slab near its center when \(L_z\) approaches infinity? Can you explain how Maxwell's equations account for this scenario where \(d\mathbf{M}/dt = 0\), and w... | 6,858 | cqadupstack-physics |
90810 | Stress-energy Trace of Massless Klein Gordon Field | I've calculated the trace of the stress-energy for a massless KG field and I keep getting $T = - (\partial \phi)^2$ in 3+1 dimensions. I'm using $$T_{\mu\nu} = \partial_\mu \phi \partial_\nu \phi - \frac{1}{2} g_{\mu\nu} (\partial \phi)^2$$ Since $g_{\mu\nu}g^{\mu\nu} = 4$, the second term is double the first. On the o... | What is the trace of the stress-energy tensor for a massless Klein-Gordon field in 3+1 dimensions, and how does it reconcile with the claim that such a theory is conformal, given that David Tong's notes imply a trace of zero for conformal field theories? | 37,988 | cqadupstack-physics |
81326 | What is the physical meaning of group velocity for elastic waves through crystals? | In a chapter about phonons they define group speeds for elastic waves in crystals as the derivative of the dispersion relation: $$v_g = \frac{d{\omega}}{d{k}} $$ I wonder though how they come to this equation, i.e. what is the physical meaning of $v_g$? I find on the internet information about the physical meaning abou... | What is the physical significance of the group velocity (\( v_g \)) for elastic waves in crystals, and how can it be related to the propagation of wave packets within a crystal lattice? Additionally, can you provide a quantitative example showing how to calculate the group velocity given a specific dispersion relation? | 9,365 | cqadupstack-physics |
25687 | Polar ice deposits on the Moon - dispersed in permafrost loose rubble, or also massive/layered? | Water has been discovered on the Moon with remote sensing methods. Read the press release _Diviner results indicate presence of widespread ice on the Moon_ on the "Diviner" infrared instrument on the Lunar Reconnaissance Orbiter (LRO) spacecraft that is orbiting the Moon. Independently, H2O was also found in the spectr... | What are the specific locations and characteristics of identified ice deposits on the Moon, particularly in relation to the presence of polar caps, regolith coverage, and any crystal-water bound in minerals? | 16,281 | cqadupstack-physics |
77313 | How to name different approaches to relativistic quantum theory | In the introductory chapter of the QFT book by Mark Srednicki the author notes that > [p. 26] So now we have two different approaches to relativistic quantum > theory [...] Which [one of those two] we use is a matter of convenience and > taste. And _quantum field theory_ , the formalism in which position and time > are... | What is the specific name for the approach in relativistic quantum mechanics that promotes time to an operator, as mentioned in Mark Srednicki's book on quantum field theory? | 22,022 | cqadupstack-physics |
101683 | Gravitational redshift of temperature and electrostatic potential | Consider a charged black hole in four-dimensional Minkowski spacetime, with charge $Q$, mass $M>Q$: $ds^2=-f(r)dt^2+\frac{1}{f(r)}dr^2+r^2d\Omega_2^2$, with $f(r)=1-\frac{2M}{r}+\frac{Q^2}{r^2}$. When an observer at radial coordinate $r_1$ emits a photon, an observer at radial coordinate $r_2>r_1$ will perceive the pho... | Given a charged black hole with mass \( M > Q \) and a surface gravity \( \kappa \), the Hawking temperature is \( T_H = \frac{\kappa}{2\pi} \). If the surface gravity at the outer event horizon \( r_+ \) is known, how would you calculate the local temperature \( T_{loc}(r) \) for an observer at a radial coordinate \( ... | 22,504 | cqadupstack-physics |
19684 | Modification of de Donder gauge | The de Donder gauge is often used to simplify the linearised equations of motion of general relativity. If the metric is linearised as $g_{ab} = \bar g_{ab} + \gamma_{ab}$, then the de Donder gauge reads $\nabla^a(\gamma_{ab} - \frac{1}{2}\bar g_{ab}\gamma) = 0$. The partial differential equation for the gauge transf... | In the context of general relativity and the de Donder gauge, given that we reformulate the gauge condition as $\nabla^a(\gamma_{ab} - n \bar g_{ab}\gamma) = 0$, can you determine whether the modified partial differential equation for $n=1$ has a solution in an anti-de Sitter (AdS) background? What conditions would be ... | 18,475 | cqadupstack-physics |
87539 | Some hints for special case of metric tensor in GR | Let's have metric $$ ds^2 = dt^2 - dx^2 - dy^2 - dz^2 - 2f(t - z, x, y)(dt - dz)^2. $$ I need to prove that it is an exact solution for Einstein equations in vacuum for $\partial_{x}^{2}f + \partial_{y}^{2}f = 0$. The straightforward method is obvious, but does some method especially for this metric exist? Edit. As I s... | Given the metric
$$ ds^2 = dt^2 - dx^2 - dy^2 - dz^2 - 2f(t - z, x, y)(dt - dz)^2 $$
and the condition that \( \partial_{x}^{2}f + \partial_{y}^{2}f = 0 \), can you demonstrate that this metric satisfies the Einstein equations in vacuum? Additionally, how does the substitution \( u = (t - z) \) and \( v = (t + z) \... | 20,078 | cqadupstack-physics |
110902 | Anodized aluminium reflectivity at 10.6 um | I wonder how I can model anodized aluminium reflectivity in ray tracing-based optical simulation. I know that the parts my company is using are anodized to get covered with a ~20 um (as specified) thick corundum layer. I would like to be able to properly model angular dependence of such a surface reflectivity. I am usi... | What are the reflectivity values of anodized aluminum (corundum layer, ~20 µm thick) at different incidence angles, and how can these values be used in ray tracing simulations to accurately model angular dependence in optical simulations? | 3,460 | cqadupstack-physics |
28660 | Is the electric field zero inside an ideal conductor carrying a current? | By an ideal conductor, I mean one with zero resistance. Inside an ideal conductor with no current, the electric field is zero, but is the electric field still zero with the ideal conductor carrying a current? | If an ideal conductor with zero resistance is carrying a steady current, what is the electric field within the conductor, and how does it relate to the current flowing through it? | 13,675 | cqadupstack-physics |
122092 | Which is more efficient Water heating method : Electric heating coil OR Induction stove? | Which is more efficient Water heating method : Electric heating coil Vs Induction stove | What is the energy efficiency percentage of water heating using an electric heating coil compared to an induction stove when heating 1 liter of water from 20°C to 100°C? | 26,966 | cqadupstack-physics |
135317 | Finding a angular acceleration of the rod | A rod of mass 6kg and length 4m is hinged at its lowest end. A disc of mass 2kg is attached to the upper end of the rod. A light string is wrapped on the disc and its free end is pulled by applying a horizontal force F=16N. initially the rod is vertical and the rod and the disc are at rest. Find the initial angular acc... | What is the initial angular acceleration (in rad/sec²) of a 4m-long rod, hinged at its lowest end, with a mass of 6kg, when a 2kg disc is attached to its upper end and a horizontal force of 16N is applied to a string wrapped around the disc? Assume no friction at the hinge and between the disc and string. | 32,231 | cqadupstack-physics |
7576 | Will Earth Hour do damage to power supply system? | There is always a debate around Earth Hour every year, and the opposite side of Earth Hour usually claims that > The (sudden) decrease and increase of the power usage in the start and end > of Earth Hour will cause much more power loss (than the save of power), and > even do damage to the power supply system. Is this s... | What is the estimated impact of the sudden changes in power consumption during Earth Hour on the stability and efficiency of power supply systems, and how does this compare to the amount of energy saved during that hour? | 11,456 | cqadupstack-physics |
21325 | Polarization rotation: Jones Matrix that maps Horizontal to right circular | I am looking at the Poincaré sphere and I am trying to compute a Jones matrix for a particular rotation. Specifically, I would like it to perform the following maps: $O :|H \rangle \rightarrow |R \rangle$ $O :|V \rangle \rightarrow |L \rangle$ $O :|L \rangle \rightarrow |H \rangle$ $O :|R \rangle \rightarrow |V \rangle... | Can you determine a Jones matrix that maps the following polarization states for light: \( O : |H\rangle \rightarrow |R\rangle \), \( O : |V\rangle \rightarrow |L\rangle \), \( O : |L\rangle \rightarrow |H\rangle \), and \( O : |R\rangle \rightarrow |V\rangle \)? Additionally, could this same transformation be achieved... | 37,587 | cqadupstack-physics |
104395 | How does series circuit differ from parallel circuit? | Why does same current $I$ flow in the series circuit but it isn't the case with resistors in parallels? | In a series circuit with resistors R1, R2, and R3, if the total current I is 2 A, what is the voltage across each resistor if R1 = 4 Ω, R2 = 6 Ω, and R3 = 10 Ω? Additionally, explain why the same current flows through each resistor in this series configuration compared to resistors connected in parallel. | 8,705 | cqadupstack-physics |
13978 | Tying knots in power cables to prevent power surge from lightning strikes | There are plenty of references to this claim on the internet that tying knots in power cables will prevent a piece of equipment e.g. television or computer from a power surge. How can this be debunked (or proven) using mathematics? I stumbled across this which seems reasonable to me, but is there some way this can be p... | How would you calculate the change in surge impedance caused by tying a knot in a power cable, and what effect would this have on the magnitude of a power surge passing through the knot? | 37,737 | cqadupstack-physics |
6782 | Is it possible to define a "it went through two slits" observable? | This concerns the famous two-slit experiment. Electrons or photons or your favorite particle, doesn't matter. As we all know, the attempt to detect which slit the quanta pass through leads to loss of the diffraction pattern. The interesting part of the experiment is describing what happens when neither slit has a parti... | In the context of the two-slit experiment, is it possible to construct a Hermitian operator that accurately represents the number of slits accessed by a quantum particle, allowing for a verifiable distinction between cases where the particle goes through one slit versus multiple slits, without providing information on ... | 5,854 | cqadupstack-physics |
113417 | Do all vacua in the string theory landscape have a different cosmological constant? | Or can two vacua with the same energy differ in other ways? | Can two vacua with the same energy have different properties, such as density or particle content, and if so, how would you quantify those differences? | 12,832 | cqadupstack-physics |
43860 | If wave packets spread, why don't objects disappear? | If you have an electron moving in empty space, it will be represented by a wave packet. But packets can spread over time, that is, their width increases, with it's uncertainty in position increasing. Now, if I throw a basketball, why doesn't the basketball's packet spread as well? Wouldn't that cause its uncertainty in... | How does the concept of wave packets apply to macroscopic objects like a basketball, and what factors limit the spread of its wave packet compared to a subatomic particle like an electron? | 27,543 | cqadupstack-physics |
76080 | Ice cube in a pool | Imagine this situation: Large ice cube placed at the bottom of an empty pool starts to dissolve. I was wondering if it is possible for cube to start float in the water? If yes, what fraction of the cube needs to dissolve? | If a large ice cube is placed in an empty pool and starts to dissolve, under what conditions can it start to float in the water, and what fraction of the cube must dissolve for it to achieve buoyancy? | 33,854 | cqadupstack-physics |
4690 | Why aren't string compactifications by generic conformal field theories considered more often? | Most string theory compactifications analyzed so far have as backgrounds a conformal field theory corresponding to a nonlinear sigma model with a Calabi- Yau target space, or some relatively classical background, possibly with orbifolding. But as long as the central charge of the Virasoro algebra is right, and worldshe... | What are the necessary conditions for a superconformal field theory to be considered as a valid background in string theory compactifications, and how does the central charge of the Virasoro algebra influence this? | 35,027 | cqadupstack-physics |
23385 | Zero energy universe | Regarding this thread: http://www.physicsforums.com/showthread.php?t=506985 Why is the idea that the total energy in the universe "zero" so popular (re: Laurence Krauss) and why is the flatness of the universe used to back this up when, according to that post, an open universe would not conserve energy so the total ene... | In the context of cosmology, how is the concept of "zero total energy" for the universe defined, and what implications does this have for the curvature of the universe (open, closed, or flat) and the conservation of energy? Additionally, what specific conditions would lead to a prediction of zero total energy in a flat... | 32,331 | cqadupstack-physics |
16976 | Thermodynamic System: Windmill | There is a closed system that is just a windmill attached to a rotating shaft that can mechanically power something. I know the system produces work on the surroundings from the rotating shaft and this is positive work, but is there also a negative work input from the wind pushing the blades of the windmill? Also, woul... | In a closed system consisting of a windmill attached to a rotating shaft, if the wind provides a force of 50 N acting on the blades and the radius of the windmill is 3 meters, what is the positive work done by the wind on the blades per revolution? Additionally, does the system experience heat transfer due to friction ... | 13,345 | cqadupstack-physics |
24319 | Will free-fall object into black hole exceed speed of light $c$ before hitting black hole surface? | In Newtonian mechanics, if we throw an object in against direction of gravity with speed $v$ and it achieve max height of $h$. Now if we allow object to fall from that height $h$, it will eventually attain speed $v$ when it reach position where we launch it. Now applying same idea to a black hole in general relativity.... | How does the curvature of spacetime around a black hole prevent an object falling into it from exceeding the speed of light, despite being thrown in with a velocity close to the speed of light? | 28,096 | cqadupstack-physics |
105870 | VdW Gas Entropy/Enthalpy/Gibbs Free Energy of Mixing in 2 Chamber Experiment | **What are the equations for $\Delta S_{mix}$, $\Delta G_{mix}$, and $\Delta H_{mix}$ for a real gas?** * * * Explanation: In one of the most well known experiments in thermodynamics, two chambers are placed beside each other with a membrane in between. The thought experiment is used to teach thermodynamic principles b... | What are the equations for calculating the changes in entropy ($\Delta S_{mix}$), Gibbs free energy ($\Delta G_{mix}$), and enthalpy ($\Delta H_{mix}$) when two different real gases are mixed, considering their Van der Waals parameters ($a$ and $b$)? Additionally, provide an example using specific values for the gases ... | 12,875 | cqadupstack-physics |
31665 | Can wireless power transfer harm organic life by anyway? | Now, humanity can beam 50-60 watts power through 10 centimeter-thick concrete block. Can such wireless power transfer harm us by anyways? | What is the threshold distance from a wireless power transfer source beaming 50-60 watts through a 10 centimeter-thick concrete block at which human exposure to electromagnetic fields may pose a health risk? | 9,776 | cqadupstack-physics |
65366 | Derivative of $\epsilon$ in Lennard-Jones equation | I just want to calculate the derivative of epsilon for the following Lennard- Jones equation: LJ = $4\sqrt{\epsilon_{ii} \epsilon_{jj}}$ $[(\sigma/ r)^{12}-(\sigma/r)^{6}]$ with respect to $\epsilon_{ij}$ I am very glad if anyone explains this. | What is the expression for the derivative of the Lennard-Jones potential \( LJ = 4\sqrt{\epsilon_{ii} \epsilon_{jj}}\left[(\sigma/r)^{12} - (\sigma/r)^{6}\right] \) with respect to \( \epsilon_{ij} \)? | 14,350 | cqadupstack-physics |
25909 | Missing radio source coordinates in G/T measurement technique ITU-R S.733.2 | Section 3 ‘Suitable radio stars’ of this recommendation lists the different stars/nebula and their corresponding flux-densities that can be used in the G/T measurement. There is no reference to the coordinates these flux densities were measured at. Some of the Nebula have large angular extension and I’m not sure where ... | What are the celestial coordinates of the radio stars and nebulae listed in Section 3 of the ITU-R recommendation S.733-2, and how do their flux densities relate to their angular extensions? | 13,396 | cqadupstack-physics |
86593 | How to measure speed of a ceiling fan? | I thought for fun I might measure how my ceiling fan slows down when turned off, and maybe find out what forces act on it when it slows down. My problem is that for that I need to measure its rotational speed when it's turned on. When it's in the lowest speeds it's easy because you can see it rotate, but for higher spe... | What DIY methods can I use to measure the rotational speed of a ceiling fan at high speeds without a strobe light? | 18,738 | cqadupstack-physics |
22974 | How can a Higgs decay to heavier products than its mass? | How is it possible that a higgs at ~125 GeV can decay into 2 W bosons @ ~ 80 GeV a piece (for example)? Shouldn't a particle only be allowed to decay to lighter particles + energy? Diagram copied from this question  | If a Higgs boson with a mass of approximately 125 GeV can decay into two W bosons, each with a mass of about 80 GeV, how does this decay process comply with the conservation of energy and mass in particle physics? What is the total energy available during this decay, and how does it account for the masses of the result... | 16,164 | cqadupstack-physics |
111194 | What is the precise definition of state of a quantum system? | On studying quantum states of the hydrogen atom, I came with a question: what is the precise definition of a quantum state? Particularly, when solving the Schroedinger eq for the H using separation of variables, the quantum numbers and wavefunctions for the states are clear. However, mathematically speaking, any combin... | What is the relationship between the linear combinations of wavefunctions for the hydrogen atom and their corresponding quantum numbers, and how can one determine the quantum numbers for a linear combination of these states, given that such combinations yield more than the standard three quantum numbers? | 38,061 | cqadupstack-physics |
64467 | Einsteins train though experiment. What if the train is moving at the speed of light? (layman Q) | I understand einstein's train paradox. Where one man on a platform is passed by a man in a traincar, at the moment they meet a flash of light is given off in the middle of the train car. To the man on the platform the light hits the ends of the car at opposite moments in time. To the man in the train car they hit at th... | In the hypothetical scenario where a train is moving at the speed of light, what would be the observations of the man on the platform regarding the flash of light emitted from the center of the train car? Specifically, would he perceive the light hitting the back of the train at the moment the flash occurs, and would h... | 12,624 | cqadupstack-physics |
26157 | Is there any way to judge whether a bolide might produce a meteorite? | If you see a bolide, is there any way to judge whether it's worthwhile to try to find an impact? (I mean, short of a big "bang" and a debris cloud rising from the remains of your car...) I imagine it's a once-in-a-lifetime thing at best, but it seems like the meteor hounds have a few leads per year. | What is the approximate impact energy of a bolide with a mass of 1 kg and a velocity of 20 km/s upon entering Earth's atmosphere, and how would this energy compare to the energy produced by a typical chemical explosive like TNT? | 7,694 | cqadupstack-physics |
12440 | Waves and information | I'm looking to transfer very simple information using audio waves. One of the approaches I'm looking into is using different frequencies. For example, "command 1" will be transmitted using 500Hz wave, where "command 2" using 700Hz wave. On the other side, the decoder will look if we have a wave with that frequency and ... | What techniques can be used to decode and interpret multiple audio wave frequencies simultaneously transmitted in a wave packet, and how would you determine the original frequencies and associated commands from the resulting superimposed waveform? | 9,204 | cqadupstack-physics |
46772 | Lagrangian of 2D square lattice of point masses connected by springs | Zee's QFT book mentions the Lagrangian of a square 2D horizontal lattice of point masses, connected by springs, and considering only vertical displacements $q_{i}$, as $ L = \frac{1}{2} \sum\limits_{a}(m \dot{q}_{a}^{2}) - \frac{1}{2} \sum\limits_{ab}(k_{ab}q_{a}q_{b}) - \frac{1}{2} \sum\limits_{abc}(g_{abc}q_{a}q_{b}q... | Given a square 2D horizontal lattice of point masses connected by springs, where the potential energy of any two masses connected by a spring is described by the expression \( \frac{1}{2} k (l - l_0)^2 \) (with \( l \) being the distance between the two masses), and considering the vertical displacements \( q_i \) of e... | 32,383 | cqadupstack-physics |
110578 | Dropping a ball in a train moving close to the speed of light? | Suppose a train is moving very close to the speed of light, say 0.999c relative to a stationary observer on Earth. Now a stationary observer on Earth will observe clocks on the train to tick slower than usual. Now suppose a boy within the train drops a ball. The stationary observer measures how long it takes for the ba... | If a boy drops a ball from a height \( s \) inside a train moving at a speed of \( 0.999c \), how long does it take for the ball to hit the ground as measured by an observer on Earth, taking into account the effects of time dilation? Provide the formula for the time measured by the observer on Earth in terms of \( s \)... | 21,873 | cqadupstack-physics |
132974 | Axioms behind entropy! | The concept of entropy is very ubiquitous, we learn about its uses starting from Information Theory (Shannon entropy) up to its basic definition in statistical mechanics in terms of number of micro-states. Limiting the discussion to physics, when studying a physical system, can be a box filled with an ideal gas, a melt... | What are the necessary axioms that a function must satisfy to qualify as an entropic term in statistical mechanics, ensuring both physical and mathematical consistency? | 21,731 | cqadupstack-physics |
20003 | Why don't electrons crash into the nuclei they "orbit"? | I'm having trouble understanding the simple "planetary" model of the atom that I'm being taught in my basic chemistry course. In particular, 1. I can't see how a negatively charged electron can stay in "orbit" around a positively charged nucleus. Even if the electron actually orbits the nucleus, wouldn't that orbit e... | In the planetary model of the atom, if an electron is considered to be in orbit around a positively charged nucleus, what is the energy of the electron in terms of its distance from the nucleus, and how does this energy relate to the stability of the orbit according to classical physics? | 22,749 | cqadupstack-physics |
99229 | Commutator of parity and Hamiltonian operators under even potential function | I need to show what is $[H,P]$ where $H$ is the Hamiltonian and $P$ the parity operator. $V(\underset{\sim}x) = V(-\underset{\sim}x)$ in this case. I start off with $$ \langle \underset{\sim}x|HP|\psi\rangle = \langle \underset{\sim}x|(\frac{p^2}{2m}+V)P|\psi\rangle = \langle \underset{\sim}x|(\frac{p^2}{2m}+V)P|\psi\r... | What is the commutator \([H, P]\) where \(H\) is the Hamiltonian operator given by \(H = \frac{p^2}{2m} + V\) and \(P\) is the parity operator, under the condition that the potential \(V(x)\) satisfies \(V(-x) = V(x)\)? Please provide a detailed evaluation of the commutator including the steps you follow to derive the ... | 11,520 | cqadupstack-physics |
108690 | A small technical explanation of nuclear energy | I don't know if this is the place to ask this kind of questions but I'm sure you guys can help me. I'm looking for a small (4 pages max.) technical explanation of nuclear energy, preferably of an academic source. It would be best if the physics and mathematics involved is understandable by last-year high school student... | What are the equations that govern the process of nuclear fission, and how do they relate to the energy release in a typical nuclear reactor, explained at a level comprehensible for last-year high school students? | 33,083 | cqadupstack-physics |
110629 | Calculate shadow path&size + object size based on distance | What are the variables I need to take in account if I want to calculate shadow path and size? I will try to simplify my questions. I have a $2D$ circle (radius: $28$) which means the size of the circle (in box) is $56\times 56$ at specific point (lets say $(300,300)$). I tell the $2D$ circle to go in specific path, l... | Given a sphere with a radius of 28 units, moving along a vertical path from the point (300, 300) to (300, 200), calculate the size and path of its shadow on a flat surface located 10 units below the sphere. Assume the light source (the sun) is directly above the sphere at a height of 10 units. What will be the dimensio... | 37,809 | cqadupstack-physics |
14869 | What are nuclear isomers? What is isomeric energy? | Can someone explain nuclear isomers to me, and in particular what the energy involved is? I understand generally that we're talking about moving from a less to more stable configuration of nuclear particles, but that's about as far as my understanding goes. | What is the energy difference in MeV between the ground state and the first excited state of the nuclear isomer 经验原子 (if applicable), and how does this energy relate to the stability of its configuration? | 14,274 | cqadupstack-physics |
87265 | Eigenvectors of the angular momentum operator $S_x$ | For a spin of $\frac{1}{2}$ the angular momentum operator can be written as $\vec{S} = \frac{\hbar}{2} \vec{\sigma}$ in matrix form. Find the eigenvalues and eigenvectors of $S_x$ where $\sigma_x = \begin{pmatrix} 0 & 1 \\\ 1 & 0 \\\\\end{pmatrix}$ **Attempt:** * _**Finding the eigenvalues:_** $|S_x - \lambda I|=det\... | Determine the eigenvalues and normalized eigenvectors of the operator \( S_x = \frac{\hbar}{2} \sigma_x \), where \( \sigma_x = \begin{pmatrix} 0 & 1 \\ 1 & 0 \end{pmatrix} \). Provide your final answer with the eigenvalues clearly stated and the corresponding normalized eigenvectors in their simplest form. | 27,122 | cqadupstack-physics |
41022 | Can the Euler-Lagrange equations be derived from an infinitesimal Principle of Least Action? | The Euler-Lagrange equations can be derived from the Principle of Least Action using integration by parts and the fact that the variation is zero at the end points. This has a mystical air about it, with the system somehow "sniffing" out all possible paths in the distant future and choosing the one that minimizes the a... | Can you show how the Euler-Lagrange equations can be derived using an infinitesimal principle of least action, and what assumptions or conditions need to be considered in this derivation? | 21,085 | cqadupstack-physics |
107554 | Applying Ehrenfest's theorem to Hamiltonian | It maybe a stupid question, but from the Ehrenfest's theorem, we have \begin{eqnarray*} \frac{d\langle A\rangle}{dt} &=& \left\langle\frac{\partial A}{\partial t}\right\rangle + \frac{1}{i\hbar}\left\langle[A,H]\right\rangle \end{eqnarray*} The if we apply it to the Hamiltonian, \begin{eqnarray*} \frac{d\langle H\rangl... | Can you explain the implications of Ehrenfest's theorem when applied to the Hamiltonian of a particle in an infinite potential well, and clarify if it is valid to conclude that the time derivative of the expectation value of the Hamiltonian is non-zero in this case? | 31,768 | cqadupstack-physics |
19291 | Why doesn’t gravity break down in a large black hole? | By popular theory gravity didn’t exist at the start of the Big Bang, but came into existence some moments later. I think the other forces came into existence a little latter. When a black hole crushes matter to a singularity (infinite density), at some point shouldn’t the forces cease to exist including gravity? Kent | At what approximate time after the Big Bang are the fundamental forces, including gravity, believed to have emerged, and how do these theories relate to the behavior of forces near a singularity in a black hole? | 31,145 | cqadupstack-physics |
18491 | Watercooling performance and amount of water or liquid in the loop | Imagine a watercooling system. It consist of a reservoir, pump, tubes and a radiator. We all know how it works. My question is, only inreasing the reservoir's volum, say from 1 liter to 2, will actually increase the performance? I do remember some thermodynamic laws from school, Delta T of water to ambient after a give... | If a water cooling system originally has a 1-liter reservoir and is upgraded to a 2-liter reservoir, how will this change affect the system's cooling performance in terms of the time it takes for the water temperature to reach ambient temperature? Consider the principles of thermodynamics in your analysis. | 38,195 | cqadupstack-physics |
29362 | Why Liouville's theorem is obvious? | In Florian Scheck's _Mechanics_ , he stated the local form of Liouville's theorem as follows: Let $\Phi_{t,s}(x)$ be the flow of the differential equation $-J\frac{d}{dt}x=H_{x}$. Then for all $x,t,s$ for which the flow is defined, we have $$D\Phi_{t,s}(x)\in Sp_{2f}.$$ In his proof he claimed that $$\frac{d}{dt}[D\Phi... | Why does the equation \( D\Phi_{s,s}(x)^{T} J D\Phi_{s,s}(x) = J \) hold at \( t = s \) within the context of Hamiltonian mechanics, and what conditions guarantee that the determinant of \( D\Phi_{s,s} \) is equal to 1? | 17,443 | cqadupstack-physics |
126396 | The effect of A/C and global warming | I had been thinking about the way an air conditioning system moves heat from one place to another. The unit runs and drops the temp. in the building and raises the temp. outside. Also there is mechanical energy that converts to heat (the compressor and fan motors) and is also put outside. If this is the case, is there ... | How much heat is typically expelled by an air conditioning system in a residential building, and how does this cumulative heat release from multiple systems in a city potentially affect local and global temperatures? | 25,072 | cqadupstack-physics |
116640 | How could one measure the necessary CP violation needed to account in the SM? | The Standard Model contains at least three sources of CP violation. The first of these, involving the Cabibbo–Kobayashi–Maskawa matrix in the quark sector, has been observed experimentally and can only account for a small portion of the CP violation required to explain the matter-antimatter asymmetry. The strong intera... | What is the necessary amount of CP violation, measured in degrees or another quantitative unit, required to explain the matter-antimatter asymmetry in the early universe? | 25,458 | cqadupstack-physics |
41243 | What came first, Rice Crispy or "Snap," "Crackle," and "Pop"? | The fourth, fifth, and sixth derivatives of position are called "Snap" "Crackle" and "Pop". What came first, the rice crispy characters, or the physics units? | What is the relationship between the timing of the creation of the Rice Krispies characters and the formal definition of the fourth, fifth, and sixth derivatives of position in physics? Specifically, in what year were the Snap, Crackle, and Pop characters introduced compared to when the terms "snap," "crackle," and "po... | 12,309 | cqadupstack-physics |
89994 | How can one calculate the gamma-ray energy spectrum for proton-proton collisions? | How does one calculate the the energy spectrum of gamma-rays produced by a proton-proton collision? I'm at a complete loss. | What is the expected energy spectrum of gamma-rays produced in a proton-proton collision at an energy of 10 TeV, and how can this be calculated using conservation laws and particle interaction models? | 1,066 | cqadupstack-physics |
2656 | How bright can we make a sun jar? | A sun jar is an object that stores solar energy in a battery and then releases it during dark hours through a led. Assume: * a $65cm^2$ solar panel * a 12h/12h light/dark cycle * insolation of $2.61kWh/m^2/day$ * perfectly efficient components (i.e. without violating Entropy laws or other theoretical limits) ... | What is the maximum luminous intensity in lumens that a sun jar, equipped with a $65cm^2$ solar panel and powered by a battery charged to store the daily insolation of $2.61kWh/m^2/day$, can sustain for 12 hours of nighttime use? | 5,760 | cqadupstack-physics |
106734 | Does car have more kinetic energy when turning? | I asked this Phys.SE question Does car lose kinetic energy when turning? Assume a car turning without losing its speed by holding to a point by a rope. IMO, while the car is turning, its kinetic energy is constant $E=(1/2)mv^2$, but it has additional rotation energy as it changes it direction $E=(1/2)Iw^2$ (while turni... | If a car is turning in a circular path at a constant speed, how does the distribution of its kinetic energy change, and what is the total mechanical energy of the car considering both translational and rotational motion? Calculate the total mechanical energy of the car if its mass is 1000 kg, its velocity is 20 m/s, an... | 23,684 | cqadupstack-physics |
75271 | Electric field. Linear charge density | I was wondering if anyone could help me out in this exercise I've been struggling to solve. A straight, nonconducting plastic wire $ 8.50 cm $ long carries a charge density of 175 $ nC/m$ distributed uniformly along its length. It is lying on a horizontal tabletop. Find the magnitude and direction of the electric field... | Calculate the magnitude and direction of the electric field produced by an 8.50 cm long nonconducting plastic wire with a uniform linear charge density of 175 nC/m at a point 6.00 cm directly above its midpoint. | 21,562 | cqadupstack-physics |
65063 | When (Exactly) Does Bremsstrahlung Radiation Occur? | Regarding the Bremsstrahlung Radiation emitted when a charged particle curves inside a uniform magnetic field; Is there a way of calculating the angle through which the charged particle will precess before emitting the first quantum of radiation? Also, is there a way of determining the angle through which the charged p... | What is the formula to calculate the angle of precession of a charged particle moving in a uniform magnetic field before it emits the first quantum of Bremsstrahlung radiation, and how does this angle compare to the angle of precession between the first and second emissions? | 25,282 | cqadupstack-physics |
91639 | Does turning sharply on a bicycle conserve more energy than a wide turn? | I use a bike to commute, so I spend a lot of time thinking about how to get the most bang out of my momentum. Aside from the extra distance traveled in a wide turn, does making a sharp turn save you any energy? My guess is no, because these things tend to even out, but it definitely _feels_ like I'm going much faster. ... | When riding a bike, how does the radius of a turn affect the energy expenditure during the turn, and can we quantify the difference in energy used between a sharp turn with a radius of 1 meter versus a wide turn with a radius of 5 meters, assuming constant speed? | 5,475 | cqadupstack-physics |
7757 | Weak equivalence principle tests | on the wikipedia article about the equivalence principle there is a mention about testing the EP against parity-violating masses; > "The equivalence principle is untested against opposite geometric parity > (chirality in all directions) mass distributions. A parity Eötvös experiment > contrasting solid single crystal s... | How do the properties of solid single crystal spheres in different enantiomorphic space groups relate to testing the equivalence principle against parity-violating mass distributions, and why is their geometric arrangement significant in this context? | 448 | cqadupstack-physics |
134466 | If you spin a magnet in space, far from any magnet field, would it spin forever, being able to produce electricity forever? | This is of course a purely theoretical question and it would take energy to get the magnet moving in the first place but once it's moving in the vacuum of space, with no gravity or magnetic field nearby, could it spin nearly forever (as in billions of years) producing a magnetic field from which you could get electrici... | If a magnet is set into motion in a vacuum, where it experiences no gravitational or magnetic influences, what factors would determine the duration of its spin, and how efficient would it be in generating electricity over a period of billions of years? | 18,396 | cqadupstack-physics |
33695 | What is the diameter of the sun as a function of wavelength/frequency (around 10GHz)? | I have this vague recollection of being told that the diameter of the apparent surface of the sun is a function of what band you observe it in. I'm looking for a model of this that works for bands in the 1-100GHz range. If the "surface of the sun" has no formal definition, then it might as well be something that will b... | What is the effective diameter of the Sun's apparent surface at 10 GHz, considering a tangential radio transmission link experiences a 3 dB reduction in signal-to-noise ratio due to solar radiation and signal attenuation? | 10,030 | cqadupstack-physics |
104475 | Translation invariance in crystals | **Context** : solid state physics - dynamics of atoms in crystals. Coupling constants are defined as: \begin{align*} \frac{\partial^2\Phi}{\partial r_{nai}\partial r_{mbj}} = \Phi_{nai}^{mbj} \end{align*} where $\Phi$ is the total energy of the crystal expressed as a function of nuclei coordinates. $r_{nai}$ is the pos... | What is the significance of the zero-th cell in the expression \(\Phi_{0ai}^{(m-n)bj}\) when discussing translation invariance in the context of coupling constants in solid state physics? | 5,154 | cqadupstack-physics |
88108 | How to describe hadron-hadron strong interactions from QCD? | I'm trying to set up the problem of deriving the thermodynamics of hadronic matter. I know how to proceed in the case of an effective description such as mean field (Walecka/linear sigma model) but I'm trying to start from QCD. I do not wish to actually obtain the partition function, but I want to at least set up the c... | Given the framework of quantum chromodynamics (QCD), how can one formulate the Lagrangian that describes hadronic matter without resorting to effective field theories, while ensuring to include the interactions among baryons, mesons, and the strong and electromagnetic forces? | 15,743 | cqadupstack-physics |
57266 | Quantum Mechanics Notation for BRA KET | I've been given this homework problem, but I do not understand its notation. > Please perform the following where the wavefunctions are the normalized > eigenfunctions of the harmonic oscillator Hamiltonian. > > i) Compute $\langle x\rangle, \langle p \rangle, \langle x^2\rangle$ , and > $\langle p^2\rangle $ for $| 0 ... | Given the normalized eigenfunctions of the harmonic oscillator Hamiltonian, specifically for the first three quantum states $|0\rangle$, $|1\rangle$, and $|2\rangle$, calculate the expectation values: $\langle x \rangle$, $\langle p \rangle$, $\langle x^2 \rangle$, and $\langle p^2 \rangle$. Additionally, for the state... | 28,619 | cqadupstack-physics |
20674 | How does compression of carbon atoms work? | From what I understand, it's extremely difficult/ impossible to compress atoms closer together. I'm wondering how this applies in the case of graphite and diamond, where the carbon atoms adopt a different structure under different temperatures and pressure. | How does the structure of carbon atoms in graphite differ from that in diamond, and what specific conditions of temperature and pressure are required to transition from graphite to diamond? | 24,280 | cqadupstack-physics |
31909 | Why is Titan able to maintain an atmosphere, and not Luna? | I just read stability of hypothetical lunar atmosphere. From the correct answer, i understand, the low escape velocity from Luna is part of the reason it is unable to retain an atmosphere. Titan apparently has a comparable escape velocity * Titan = 2.65 km/sec * Luna = 2.4 km/sec ; yet Titan maintains an atmospher... | Given that the escape velocities for Titan and Luna are 2.65 km/sec and 2.4 km/sec respectively, what factors besides escape velocity could explain why Titan is able to maintain a dense atmosphere while Luna cannot? Consider aspects such as temperature, gravity, and composition in your response. | 5,785 | cqadupstack-physics |
110092 | Application of Displacement Current | I'm reasonably happy with the derivation and results of displacement current, however, I'd like to be aware of a few practical applications of this idea. So far, the only one I'm aware of is when finding the magnetic field in between a capacitor using Ampere's Law. In such a case $I_c=0$ and $I_D$ has some definite non... | What are some practical applications of displacement current in physics, other than calculating the magnetic field between the plates of a capacitor using Ampere's Law? Please provide specific examples and relevant equations where applicable. | 8,029 | cqadupstack-physics |
127815 | What is the advantage of segmented particle traps? | I'm currently trying to familiarize myself with the physics of ion and particle traps, especially with linear Paul traps. Many scientific experiments I've come across use segmented electrodes (like in this image from a Nature article). My question is _what do you gain by segmenting the electrodes?_ I could imagine that... | What advantages do segmented electrodes in linear Paul traps provide for manipulating trapped ions or particles compared to using uniform electrodes, specifically in terms of control over their position and trapping stability? Quantitatively, how does the segmentation impact the frequency stability of the trap compared... | 18,851 | cqadupstack-physics |
33918 | Could dark energy be powered by force particles that obey quantum mechanics? | From what (little) I know about physics, I understand that the universe is expanding due to dark energy, and I understand that no one quite understands it yet. I also understand that the cosmic background radiation hints that there were quantum fluctuations in the universe's infancy. Could the background radiation fluc... | What is the significance of quantum fluctuations in the cosmic microwave background radiation, and how might these fluctuations be related to the properties of dark energy as force-carrying particles in the context of quantum mechanics? | 6,346 | cqadupstack-physics |
1445 | Behaviour of mass and momentum distributions under Newtonian Gravity | In the context of this question should mass distribution $\rho(r,t)$ and momentum distribution $p(r,t)$ be well behaved ? By 'well behaved' it is meant that derivatives of all orders exist everywhere. I don't see any reason why they should be well behaved. For the equations (given in the answer (by Mark Eichenlaub) to ... | Why is it essential for the mass distribution $\rho(r,t)$ and momentum distribution $\vec{p}(r,t)$ to be continuously differentiable, and what implications would it have on the physical equations governing their behavior if they are not? | 15,120 | cqadupstack-physics |
5299 | Where's the mass? | When you look a the Polyakov or Nambu-Goto action, you see a T for tension, but no mass. When you look the action of a relativistic particle, you see the m. See here. So here's the question. Where is the mass in the String actions? For a classical relativistic string I would expect that the equations of motion have m a... | How does the tension in a string relate to the mass of a relativistic string, and can you derive an expression that shows this relationship? | 34,856 | cqadupstack-physics |
131122 | Modeling a stool with three legs pushed on one leg | If a stool with three legs is pushed (horizontally) on one leg in the direction of the axis passing through the other two legs, it starts both moving and rotating. Is it correct to assume 1/3 of force on each of the legs? This comes a bit in contrast with intuition. If we imagine the legs flexible, then the pushed leg ... | When a stool with three legs is pushed in the direction of the axis through two of its legs, what is the distribution of the frictional force between the legs, and how does the deformation of the legs affect this distribution? Additionally, how should one account for rotational motion in modeling this scenario? | 6,522 | cqadupstack-physics |
119169 | How to distinguish between the spectrum of an atom in motion and the one of a scaled atom? | **Galaxies are moving** dragged by the space expansion. When atoms are in motion the doppler effect will shift the spectra of the emitted photons. The proton-to-electron mass ratio, $\frac{m_e}{m_p}$ has been measured constant along the history of the universe, but nothing was said about the constancy of the electron o... | How can one experimentally determine if the spectral lines of an atom in motion differ from those of a scaled atom, considering potential factors such as the isotope ratio's abundance or specific redshift measurements? | 10,086 | cqadupstack-physics |
116926 | Why does electric field remain constant in parallel plates when distance changes? | Given the formula E0 = ∆V/d, why does electric field stay constant when distance changes? If a property of parallel plates is that ∆V is constant, shouldn't E0 change as d changes? | If the voltage difference (∆V) between two parallel plates remains constant while the distance (d) between them changes, what is the relationship between the electric field (E0) and the distance (d)? Calculate E0 when ∆V is 12 V and d is changed from 0.02 m to 0.05 m. | 6,182 | cqadupstack-physics |
86561 | Fluid Virtual Mass | In this paper of Lighthill, the author studies the motion of a fish in a constant flow field $U$, modelling the movement of the fish as a deforming, rotationally symmetric, rigid surface in an inviscid fluid. The cross section of the solid volume delimited by the surface, is called $S_x$ and depends on $x$, along the b... | Given the context of Lighthill's study on the motion of a slender fish modelled as a circular cylinder in a fluid, calculate the virtual mass \( \rho A(x) \) per unit length of the cylinder when the cross-section \( S_x \) is circular with a radius \( r \). How does the area \( A(x) \) relate to the cross-sectional are... | 31,667 | cqadupstack-physics |
35752 | Heat Equation Equalities | While studying the heat equation, I ran into a few equalities that I cannot understand. For example, **Fourier's law of heat conduction** claims that $$\varphi(x,t)=-K_0\frac{\partial u}{\partial x},$$ where $\varphi$ is the heat flux (the amount of thermal energy per unit time flowing to the right per unit surface are... | Can you derive Fourier's law of heat conduction, $\varphi(x,t)=-K_0\frac{\partial u}{\partial x}$, starting from first principles, and explain the physical meaning of each term involved in the equation? Additionally, how does the relationship $e(x,t)=c(x)\rho(x)u(x,t)$ illustrate the connection between thermal energy d... | 12,568 | cqadupstack-physics |
107079 | What is wrong with this derivation of moment of inertia of a sphere? | I know the moment of inertia of a solid sphere is $\frac 2 5 MR^2$, but I keep getting $\frac 3 5 MR^2$ when deriving it: For a sphere of mass $M$ and radius $R$, $$\rho = \frac {M}{\frac 4 3\pi R^3}$$ $$I = \int_0 ^R (\rho 4 \pi r^2)(r^2) dr = (\rho 4\pi)(\frac {R^5} 5) = \frac 3 5 MR^2$$ What exactly is wrong with th... | What is the correct moment of inertia for a solid sphere of mass \( M \) and radius \( R \), and what is the error in the calculation that leads to the incorrect result of \( \frac{3}{5} MR^2 \) instead of \( \frac{2}{5} MR^2 \)? | 30,925 | cqadupstack-physics |
108178 | How can a pion with neutral charge exist? | A $\pi^0$ consists of an up and anti-up quark. However, I also learned that when a particle and its anti-particle meet, energy is produced. So, my question is that how can $\pi^0$ exist? Won't it turn to energy instantly if it is formed? | How does a neutral pion ($\pi^0$), which consists of an up quark and an anti-up quark, manage to exist without annihilating instantly into energy, given that particle-antiparticle pairs typically annihilate upon contact? | 1,454 | cqadupstack-physics |
118998 | Creating large aspheric lens profile for CNC cutting out of Acrylic plastic for huge Camera Obscura | I plan to use a CNC mill to cut a plano-convex aspheric lens from a large (300mm dia) thick(30mm) block of Acrylic (PMMA/Plexiglass). I need a focal length on the order of 800-2000mm The purpose of this large lens will be a lensed Camera Obscura that I hope will have a very bright image. I have built a lensed camera ob... | What mathematical principles and tools should I use to design a plano-convex aspheric lens profile with a focal length between 800mm and 2000mm, suitable for creating a bright image in a camera obscura? | 3,781 | cqadupstack-physics |
76886 | Please explain me this relativistic occurrence | So, for example, we have the planet, a spaceship (cylinder shaped) and a ball in it (spaceship is like transparent so we can see the ball). We are on the planet looking at the ship. The spaceship moves the speed of light and the ball with it. But what if the ball was moving, for example, 50000 km/h in the ship? It shou... | If a spaceship travels at the speed of light and contains a ball moving at a speed of 50,000 km/h relative to the spaceship, what is the speed of the ball relative to an observer on the planet outside the spaceship, and how does this relate to the principles of special relativity? | 25,195 | cqadupstack-physics |
11193 | How many electrons are displaced when combing hair? | Feynman talking about electricity: http://www.youtube.com/watch?v=qhh32JYkQPk When brushing your hair, Feynman mentioned that a "few" electrons were transferred to the brush (or vice versa, can't remember off the top of my head how the convention goes). Clearly this will vary depending on how much you comb your hair, s... | What is the minimum charge, in coulombs, required on a brush to produce an electrostatic effect strong enough to make lightweight particles, like pepper, move or jiggle a piece of paper at a close range? | 15,305 | cqadupstack-physics |
62228 | Transfer of electron energy to atoms (heating up of matter by absorption of photons) | If an electron absorbs a photon to get exited to a higher energy level, it should either come back to same state or any other lower state by emitting the required photon. How then can there be a net transfer of energy to the atom? Heating up means increase in kinetic (vibrational) energy of the atoms. If the energy abs... | How can atoms increase their kinetic energy from absorbing photons if the energy is typically re-emitted as lower-energy photons? | 5,387 | cqadupstack-physics |
79695 | Punching - Force or Momentum? | If I want to punch a person inflicting maximum damage, what do I need to care about? My force of punching, i.e, do I need more acceleration? Or do I need momentum, i.e my velocity for punching? | In order to maximize the damage inflicted by a punch, should I prioritize increasing the acceleration of my fist or the velocity at which I punch, and how do these factors relate to the momentum of the punch? | 22,069 | cqadupstack-physics |
79509 | How to determine the positions of two points in a radial line by an intensity level dB? | The following is the question from my school. **A source emits sound uniformly in all directions. A radial line is drawn from this source. On this line, determine the positions of two points, 1.00m apart, such that the intensity level at one point is 2.00dB greater than the intensity level at the other.** I have no ide... | A source emits sound uniformly in all directions. If the intensity level is measured in decibels (dB), how far from the source should two points be positioned to ensure that one point has an intensity level that is 2.00 dB greater than the other, given that the distance between the two points is 1.00 m? | 31,592 | cqadupstack-physics |
130826 | Grain boundary sliding in creep | To prevent grain boundary sliding so that creep is less likely to occur, usually engineers would design components of larger grains or have columnar grain structure to prevent grain-boundary sliding. Why can these two methods prevent grain boundary sliding? For columnar grains, would they be more easy to slide against ... | What is the relationship between grain size and grain boundary sliding in materials under creep conditions, and how does a columnar grain structure influence this behavior compared to equiaxed grains? | 16,584 | cqadupstack-physics |
13595 | Where does the excess energy emitted by a microwave go? | If there is nothing in the microwave, where does the excess radiation go? Why doesn't the radiation accumulate and blow it up? Should I cook two pieces of Canadian Bacon twice as long as I cook one? | When a microwave is operating without any food inside, what happens to the microwaves, and how does this prevent the microwave from accumulating energy to the point of causing an explosion? Additionally, if I cook two pieces of Canadian bacon, how should I adjust the cooking time compared to cooking one piece? | 5,413 | cqadupstack-physics |
76079 | Does the time to reach the highest point $P$ equal the time to reach the ground from $P$ in a trajectory, taking drag into account? | Assume a very simplified model without Corolis effect, the falloff of the local gravitational field and the like. My answer is _no_. It is sufficient to look at the vertical velocity of the projectile, because only that determines the time. The periods do not equal, because it is possible to vertically accelerate the p... | What is the effect of drag on the time of flight for a projectile launched vertically with an initial velocity greater than its terminal velocity, and how does this compare to the time of flight for a projectile launched with an initial velocity lower than its terminal velocity? | 34,524 | cqadupstack-physics |
105794 | astronomy & light pollution | If you were staring to the sky in a big city and electricity is turned off in a big area around you, would you be seeing the sky with little light pollution instantly? | If the electricity were turned off in a large urban area, how long would it take for the light pollution from that area to decrease significantly, allowing for a clearer view of the night sky? | 21,339 | cqadupstack-physics |
122110 | Totally antisymetric wavefunction: clarification about terminology | Pauli's Principle says: "The wavefunction of two identical fermions must be totally antisymmetric". I know that, for a antisymmetric wavefunction, $(-1)^L*(-1)^{S+1}*(-1)^{I+1}=-1$ "totally antisymmetric" means this relation or it means that these 3 relations: $(-1)^L=-1$ and $(-1)^{S+1}=-1$ and $(-1)^{I+1}=-1$ must be... | Given the conditions of Pauli's Principle, for a system of two identical fermions, if the orbital angular momentum quantum number \( L \) is 2, the total spin quantum number \( S \) is 1, and the total isospin quantum number \( I \) is 0, what is the overall symmetry of the wavefunction? Is it totally antisymmetric, an... | 22,693 | cqadupstack-physics |
68938 | What are endomorphism bundle valued $p$-forms and exterior covariant derivatives and their use in Chern-Simons theory? | Chern-Simons Forms appears in several places in physics for examples, * Fractional Quantum Hall Effect, * response of Topological Insulator, * invariant of knot, * electromagnetism in 2+1 space-time, and so on. I need to know about: * endomorphism bundle. * endomorphism bundle valued forms. * exterior... | What is the exterior covariant derivative of an endomorphism bundle-valued \( p \)-form, and how can it be computed in relation to Chern-Simons forms? | 1,217 | cqadupstack-physics |
93506 | Squashed 3-sphere? | What is a squashed 3-sphere? In context of quantum gravity. I stumbled upon a term 'squashed 7 sphere' but that's concerning supersymmetry. Is it just normal 3-sphere metric, that is just 'squashed' at the poles or is it something entirely different? Google gave no meaningful answer, other the hundreds of articles just... | What is the volume of a squashed 3-sphere, and how does its metric differ from the standard 3-sphere metric? | 13,191 | cqadupstack-physics |
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