Recent Questions - Physics Stack Exchange most recent 30 from physics.stackexchange.com 2019-10-20T09:10:41Z https://physics.stackexchange.com/feeds https://creativecommons.org/licenses/by-sa/4.0/rdf https://physics.stackexchange.com/q/509149 0 Global Hidden Variables Yamar69 https://physics.stackexchange.com/users/227483 2019-10-20T08:58:17Z 2019-10-20T08:58:17Z <p>Bell's theorem seems to disprove the existence of local hidden variables. My question is: how can we prove/disprove the existence of global hidden variables? Thank you.</p> https://physics.stackexchange.com/q/509146 0 Derive Columb's Law from Gauss's Law by solving PDEs? Ma Joad https://physics.stackexchange.com/users/182971 2019-10-20T08:49:39Z 2019-10-20T08:49:39Z <p>The standard way of deriving Columb's law is given in <a href="https://physics.stackexchange.com/questions/299719/deriving-coulombs-law-from-gausss-law">this post</a>. I am just wondering if I can do it in another way. Let a point charge <span class="math-container">$q$</span> be located at the origin. The problem is then essentially solving the Laplace equation <span class="math-container">$$\nabla\cdot \mathbf E=0\Rightarrow \nabla^2 \phi=0$$</span> with boundary condition <span class="math-container">$\phi=\frac{q}{4\pi\epsilon_0}\delta(0)$</span> at origin. I try to solve it using Fourier–Legendre Series, but find it really strange to evaluate the Fourier coefficients with <span class="math-container">$\delta$</span> in it.</p> <p>The question is: can we derive Columb's law by computing the series solution to the equation?</p> https://physics.stackexchange.com/q/509142 0 What will happen if we Boil Water in the Absence of Gravity? Intellex https://physics.stackexchange.com/users/238167 2019-10-20T07:39:38Z 2019-10-20T08:17:54Z <p>We know that, <a href="https://en.wikipedia.org/wiki/Convection" rel="nofollow noreferrer">Convection</a> is the mode of heat transfer due to the motion of individual molecules of a fluid (liquid or gas). The motion of the constituent particles arises due to difference in densities.</p> <p>In the absence of gravity, water takes a spherical shape in order to minimize the energy due to surface tension, simply it wants to minimize its surface area for greater stability. Let us assume, we are able to manage to heat the centre of the water sphere by some kind of mechanism, how will the heat be transferred from the centre of the water sphere to its surface? Do convection currents, which are the major modes of heat transfer in the presence of gravity, take place in the absence of gravity? I think it will not take place. Is the heat transfer is due to conduction? </p> <p>Further, it would be great if you could explain what will happen as time progress? Will the bubbles of dissolved gases formed inside stay where they are formed or move to the surface or they move towards the centre? How will the water boil (or it just explodes)?</p> <p>For reference, this is how water looks in the absence of gravity (in the International Space Station):</p> <p><a href="https://i.stack.imgur.com/U7eHA.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/U7eHA.png" alt="enter image description here"></a></p> <p>Please note: Water is coloured to enhance visibility </p> https://physics.stackexchange.com/q/509141 0 What is the layer of ice formed? chittaranjan rout https://physics.stackexchange.com/users/245292 2019-10-20T07:24:44Z 2019-10-20T08:04:39Z <p>For several days, the temperature at the bottom of a pond is 40 C and the air temperature just above the top surface of the pond is – 10 C. Depth of the pond is L. The Thermal conductivity of ice is 3 times that of water. Thickness of frozen layer of the ice formed must be</p> https://physics.stackexchange.com/q/509140 0 Which weight would be necessary to power a computer with this system? [on hold] Basj https://physics.stackexchange.com/users/74588 2019-10-20T07:02:27Z 2019-10-20T08:19:57Z <p>Let's say I install such a system:</p> <p><a href="https://i.stack.imgur.com/Ki7WD.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Ki7WD.jpg" alt="enter image description here"></a></p> <p>in a corner of my room, that produces energy in a similar way than <a href="https://www.maison-pendule.fr/pendules_images/produits/mp_28132_grande.jpg" rel="nofollow noreferrer">grandmother clocks</a>, connected to an alternator (or any other system that transforms mechanical energy to electricity) :</p> <ul> <li>height: 2,5 meters</li> <li>speed: 2 meters / 15 minutes, so that I have to take a short break in my work <em>every 15 minutes</em> to lift the weight </li> <li>desired output: 60 W (for a standard laptop)</li> </ul> <p><strong>Which formula should I use to estimate the weight that would be necessary to do that?</strong></p> https://physics.stackexchange.com/q/509138 2 Does an object in a v-shaped channel experience static friction? SarahL https://physics.stackexchange.com/users/226898 2019-10-20T06:39:03Z 2019-10-20T08:21:29Z <p>As the title states, I was wondering if an object that is at rest in a horizontal, v-shape channel like the one in this diagram would experience static friction:</p> <p><a href="https://i.stack.imgur.com/7jb86.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/7jb86.png" alt="ball in a v-shaped channel"></a></p> https://physics.stackexchange.com/q/509135 0 Drag torque on a rotating plate at small Re user245275 https://physics.stackexchange.com/users/245275 2019-10-20T05:29:13Z 2019-10-20T05:29:13Z <p>A flat, rectangular plate rotates through a initially stationary fluid: the long side is parallel to the Y-axis, and the short side is "hinged" on the x-axis (i.e. it rotates about the x-axis, similar to a trap door). <em>How do I calculate the torque due to friction between the plate and the surrounding fluid?</em></p> <p>I assume that I need to integrate the linear drag force (<span class="math-container">$F_d$</span>) along the surface of the plate, substituting linear velocity (<span class="math-container">$v$</span>) for the cross product of angular velocity (<span class="math-container">$\omega$</span>) and the distance from the axis of rotation (<span class="math-container">$r$</span>). <span class="math-container">$$\tau_d = \int_{r=0}^{r=r_0}\ r*F_d = \int_{r=0}^{r=r_0}\ r*1/2 \rho (r \times \omega)^2*C_d*A$$</span> Where <span class="math-container">$A$</span> is the area <span class="math-container">$C_d$</span> is the coefficient of drag and <span class="math-container">$\rho$</span> is the density of the fluid.</p> <p><strong>1)</strong> <em>Is this the right way to calculate friction torque?</em> </p> <p><strong>2)</strong> <em>Is friction drag (from the surrounding fluid) always normal to surface of the plate (so <span class="math-container">$r \times \omega = r\omega$</span>)?</em> It seems like it should be, since the friction is from a boundary layer developing on the surface of the plate due to rotation (I think)? Or is friction acting parallel to the axis of rotation (and thus accomplishing nothing)?</p> <p><strong>3)</strong> <em>What happens under low Reynolds number conditions (<span class="math-container">$Re &lt;&lt;1$</span>)?</em> What confuses me is that drag on a flat plate normal to flow is nearly 100% pressure drag (rather than drag from friction). But at low <span class="math-container">$Re$</span> the flow is laminar, so that there is no difference in pressure on either side of the plate. This makes it seem like a flat plate rotating at low <span class="math-container">$Re$</span> doesn't experience drag, which feels wrong. I assume that laminar flow conditions would be the only time that a flat plate normal to flow would have drag dominated by friction -but again, I'm not sure. </p> <p>Another complication: does the boundary layer have time to develop if the plate only rotates 90 degrees, starting in an initially stationary fluid?</p> https://physics.stackexchange.com/q/509134 0 Decay of spin-1/2 particle into two spin-1/2 particles suncup224 https://physics.stackexchange.com/users/47020 2019-10-20T05:29:07Z 2019-10-20T05:48:27Z <p>My question is: Does angular momentum conservation considerations alone forbid a spin-1/2 particle from decaying into two spin-1/2 particles?</p> <p>According to this <a href="https://physics.stackexchange.com/questions/266850/clebsch-gordan-coefficients-neutron-cant-decay-into-a-proton-and-electron?rq=1">stackexchange post</a>, the answer seems to argue that you cannot add two spin-1/2 particles and get a spin-1/2 particle. Yes, I understand that the tensor product of two spin-1/2 particles can be re-organised as a direct sum of a spin-1 space and spin-0 space. However, I disagree that this forbids the decay, because <strong>orbital</strong> angular momentum could change such that spin+orbital angular momentum ends up being conserved. Hence I believe that under angular momentum conservation considerations alone, it is still possible to for the decay to happen. Is this reasoning correct?</p> https://physics.stackexchange.com/q/509132 1 Partition function of one-dimensional harmonic oscillator of non-interacting fermions Siddharth Pandya https://physics.stackexchange.com/users/245289 2019-10-20T05:11:42Z 2019-10-20T06:11:34Z <p><img src="https://i.stack.imgur.com/BnAMJ.jpg" alt="this was the question asked in the entrance exam of Tata institute of fundamental research "></p> <p>The energy per oscillator of an isolated system of large number of non interacting fermions in one dimensional harmonic oscillator is <span class="math-container">$5hv/4$</span> then Entropy per oscillator is? </p> https://physics.stackexchange.com/q/509131 0 How is a computer able to replicate the sound of a human voice EXACTLY? What's in the computer that makes this possible? [on hold] Eric Sumarnkant https://physics.stackexchange.com/users/235152 2019-10-20T04:43:58Z 2019-10-20T07:12:47Z <p>I am a music producer. If I take a sound or sample and use a professional tool such as REVERB inside the DIGITAL AUDIO WORKSTATION to make a completely whole new sound that nobody's ever heard before, did I bring this sound into existence or did it already exist there in the universe?</p> https://physics.stackexchange.com/q/509130 -1 Space time and it’s existence [on hold] Ahitagni https://physics.stackexchange.com/users/212400 2019-10-20T04:40:33Z 2019-10-20T04:40:33Z <p>I was just thinking if we could analyse the ellipses in which all of the planets move around the sun and then see if they resemble a cone with the sun at the apex(coz that what it looks in the classic space time gravity demonstrations), the would that be a sigh for the existence of space time ￼ If we see the case shown in the picture and try imagining what it’s look from the side, it would look like a cone or at lease resemble one </p> https://physics.stackexchange.com/q/509129 0 What percent of solar energy would pass thru a typical one-way from the typical viewing side into the room that is normally viewed? rudy https://physics.stackexchange.com/users/245286 2019-10-20T04:36:46Z 2019-10-20T04:36:46Z <p>Suppose a one-way mirror (reversed with the half-silvered surface to the inside) is installed as an exterior window. What percent of solar energy would typically pass thru this one-way mirror/window from the bright sunlight outside into the less-energized/darker interior?</p> https://physics.stackexchange.com/q/509128 0 Inverse Lorentz transformation and Doppler effect SMRUTI RANJAN https://physics.stackexchange.com/users/152570 2019-10-20T03:14:47Z 2019-10-20T03:14:47Z <p>In relativistic Doppler effect if the frequency of light source in S-frame is n(n is the proper frequency ) if an observer in S'- frame moving with velocity v measures the frequency of the light source is n'(using Lorentz transformation). If the same event occurs in s'-frame (source is rest) we take "n' " as proper frequency and apply "v=-v" in above expression (inverse L.T) to find out " n "(where n is the observed frequency) for the 2nd case. My question is that in first case we take "n" is proper frequency (event is rest w.r.to s-frame) and " n' " observed frequency. If we choose inverse L.T to the first relation then does the frequency changes to observed frequency and vice versa or in both L.T and Inverse L.T proper frequency always remain as proper frequency. Please clarify my doubt.</p> https://physics.stackexchange.com/q/509126 0 Bicycle physics [duplicate] Pradeep Sharma https://physics.stackexchange.com/users/174126 2019-10-20T02:21:42Z 2019-10-20T06:00:49Z <div class="question-status question-originals-of-duplicate"> <p>This question already has an answer here:</p> <ul> <li> <a href="/questions/506/why-doesnt-a-bike-bicycle-fall-if-going-with-a-high-speed" dir="ltr">Why doesn&#39;t a bike/bicycle fall if going with a high speed?</a> <span class="question-originals-answer-count"> 10 answers </span> </li> </ul> </div> <p>From a long time, I have a question that bicycle is not stable at low speed or for that matter it's difficult to balance bicycle on its wheels in static position (with zero speed and no brakes). Can this be explained by physics</p> https://physics.stackexchange.com/q/509124 -1 How to find the initial speed on the y axis while only having $Dy$, $Dx$, angle, and acceleration? [on hold] Louka Papineau https://physics.stackexchange.com/users/245278 2019-10-20T01:45:25Z 2019-10-20T08:20:33Z <p>Is it possible to find the initial speed of a projectile on the y axis while only having <span class="math-container">$D_y$</span> (distance on the y axis), <span class="math-container">$D_x$</span> (distance on the x axis), the angle and <span class="math-container">$a_y$</span> (acceleration on the y axis).</p> <p>My data:</p> <ul> <li><span class="math-container">$D_y$</span> = 251.5cm</li> <li><span class="math-container">$D_x$</span> = 93.5cm</li> <li>Angle = 15 degree (starting angle)</li> <li><span class="math-container">$a_y$</span> = -9.8m/s2 (gravity)</li> </ul> <p>If it is possible would it be possible to show me the equation or how to do it?</p> https://physics.stackexchange.com/q/509121 0 Given acceleration, why must kinematics be used when solving for distance in terms of time rather than the literal definition of acceleration? Code123 https://physics.stackexchange.com/users/165584 2019-10-20T00:00:44Z 2019-10-20T07:48:08Z <p>I'm working with rotational motion, and one of the mistakes that I made in solving for distance in terms of time was substituting angular acceleration for (d/Rt^2) where d is distance and R is the radius of the wheel. I proceeded to solving for d.</p> <p>Apparently the correct method is to apply rotational kinematics, which I don't believe I understand conceptually.</p> <p>Going back to translational kinematics, If I want to solve the distance traveled in time t with an acceleration of 3, I would use d = 1/2(3)t^2, assuming initial velocity and starting distance was 0. Why does it not simply work to use the literal definition of acceleration (d/t^2) and say that 3 = (d/t^2) to solve for d? You get two different answers, and I know that the former is correct, however, I do not understand why. I feel as though an understanding here will carry over to the problem I am trying to solve that involves rotational kinematics.</p> https://physics.stackexchange.com/q/509095 2 Why do we still call the electron charge the elementary (EM) charge? Árpád Szendrei https://physics.stackexchange.com/users/132371 2019-10-19T19:41:56Z 2019-10-20T05:25:11Z <p>The electron is an elementary particle, part of the SM, pointlike, with no substructure, or spatial extent. Its intrinsic properties include its EM charge, which we still call the elementary charge.</p> <p>Originally the name electron comes from electric ion, and it was discovered around the 1900s.</p> <p>Though we still call it (the electron's Em charge) the elementary charge. Since we call it the elementary charge, it was believed that every single object in the universe had a EM charge that was larger (multiples) then the elementary charge, and the elementary charge of the electron was indivisible.</p> <p>The elementary charge is a fundamental physical constant.</p> <blockquote> <p>Charge quantization is the principle that the charge of any object is an integer multiple of the elementary charge. Thus, an object's charge can be exactly 0 e, or exactly 1 e, −1 e, 2 e, etc., but not, say, (1/2)e, or −3.8 e, etc. This is the reason for the terminology "elementary charge": it is meant to imply that it is an indivisible unit of charge.</p> </blockquote> <p><a href="https://en.wikipedia.org/wiki/Elementary_charge" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/Elementary_charge</a></p> <p>Now in 1964, we discovered the down quark, with EM charge of one third of the electron. Since then, the elementary EM charge should be that of the down quark, and the electron should have three times the elementary EM charge (of the down quark).</p> <p>Now I believe that the true elementary EM charge is the down quark and its EM charge should be the fundamental physical constant.</p> <p>I do understand that quarks are in confinement and have never been observed outside confinement experimentally, but still I believe that the down quark charge is the real indivisible elementary EM charge.</p> <p>Question:</p> <ol> <li>Why do we still call the electron the elementary EM charge and why is the -e still the fundamental physical constant (and not the down quark)?</li> </ol> https://physics.stackexchange.com/q/509047 1 Potential of hydrogen atom solution of the Laplacian: Missing boundary condition to fix integration constant $c_1$ Alessio Popovic https://physics.stackexchange.com/users/235334 2019-10-19T13:08:23Z 2019-10-20T08:39:37Z <p>I have following problem, </p> <blockquote> <p>I want to calculate the classical potential <span class="math-container">$\phi(r)$</span> of the hydrogen atom in its ground state. <br> The charge density is known: <span class="math-container">$$\rho(r)=\frac{-e_{0}}{\pi a^3}e^{-\frac{2r}{a}}$$</span> with <span class="math-container">$a$</span> being the Bohr radius, <span class="math-container">$e_{0}$</span> the elementary charge and <span class="math-container">$r$</span> being the distance from the hydrogen atom.</p> </blockquote> <hr> <p>My approach: I use the Laplace equation in spherical coordinates leading to: <span class="math-container">$$\Delta\phi=-4\pi\rho(r)$$</span> <span class="math-container">$$\frac{\partial^2\phi}{\partial r^2} +\frac{2}{r}\frac{\partial\phi}{\partial r }=\frac{4e_{0}}{a^3} e^{-\frac{2r}{a}}$$</span></p> <p>by using reduction of order the general solution is:</p> <p><span class="math-container">$$\phi(r)=\frac{e_{0}}{r}e^{-\frac{2r}{a}}+\frac{e_0}{a}e^{-\frac{2r}{a}}+\frac{c_1}{r}+c_2.$$</span> If I set <span class="math-container">$\phi(\infty)=0$</span> we obtain that <span class="math-container">$c_2=0$</span>.<br> Whats bothering me is that I don't know what I should do with <span class="math-container">$c_1$</span>.<br> My guess is that there may be a boundary condition I'm not aware of. <br> Any help would be appreciated.</p> https://physics.stackexchange.com/q/508891 0 Is linearity of the quantum state space a necessary postulate in the reconstruction of quantum theory? PFerro https://physics.stackexchange.com/users/245164 2019-10-18T15:02:15Z 2019-10-20T08:48:00Z <p>This question is about quantum reconstruction. I am new to this topic, and I decided to read some papers on it. </p> <p>I selected some works which follow an "information-focused" approach. The authors of these works try to formulate QM through an alternate set of axioms, which avoids the standard quantum state-space structure (in terms of Hermitian operators/linear spaces) and is centered on the idea that the physical state can carry only a limited amount of information. </p> <p>Some of these authors, after stating the postulates of the theory, use the standard QM formalism to get their results. This can be done because in their “central” postulate about information includes some extra (in some authors hidden, for example  and , see below) hypothesis about the linearity of the state space.</p> <p>Of course, since their works are on the reconstruction of QM, their aim is to obtain the same properties of standard QM, and linearity is among these properties.</p> <p>My questions are:<br> 1) When I say that linearity is somehow hidden, is my interpretation right?</p> <p>2) Are there other theoretical works on quantum reconstruction which follow this idea of “limited information” systems but avoid any hypothesis about linearity? (but, anyway linearity will be a consequence of the axioms of that theories)</p> <p>Here are some references:<br>  Caslav Brukner and Anton Zeilinger: <a href="http://quantmag.ppole.ru/Articles/Quo_Vadis_Quantum_Mechanics.pdf#page=60" rel="nofollow noreferrer">http://quantmag.ppole.ru/Articles/Quo_Vadis_Quantum_Mechanics.pdf#page=60</a></p> <p>Carlo Rovelli:<br>  "Relational quantum mechanics" <a href="https://arxiv.org/abs/quant-ph/9609002" rel="nofollow noreferrer">https://arxiv.org/abs/quant-ph/9609002</a></p> <p>Borivoje Dakic and Caslav Brukner:<br>  Quantum theory and beyond: Is entanglement special? <a href="https://arxiv.org/abs/0911.0695" rel="nofollow noreferrer">https://arxiv.org/abs/0911.0695</a></p> <p>Even in Hardy's most cited work ( <a href="https://arxiv.org/abs/quant-ph/0101012" rel="nofollow noreferrer">https://arxiv.org/abs/quant-ph/0101012</a>), which doesn't follow an information-focused approach, there is a very strong "Simplicity" axiom which involves mathematical properties of the state space, closely related to linearity and Hilbert structure. </p> https://physics.stackexchange.com/q/508826 1 Thermal fission of gold? j4nd3r53n https://physics.stackexchange.com/users/175366 2019-10-18T08:31:27Z 2019-10-20T09:03:02Z <p>Here's a slightly silly idea that arose from this question "<a href="https://worldbuilding.stackexchange.com/questions/158511/what-elements-would-be-created-in-a-star-composed-entirely-of-gold/158514?noredirect=1">What elements would be created in a star composed entirely of gold?</a>":</p> <p>If we accumulate a lot of gold (really a lot) fast enough, it will heat up as a result of the gravitational energy being released, loosely speaking. I think, if the temperature gets high enough, gold atoms will start colliding with enough energy to break apart, releasing more energy in a sort of run-away, thermal fission process. But am I right?</p> https://physics.stackexchange.com/q/507841 -1 Reason for quantization of angular momentum [closed] user236793 https://physics.stackexchange.com/users/236793 2019-10-12T15:31:02Z 2019-10-20T06:02:13Z <p>Why does a stationary orbit mean that the electron orbits without losing energy?</p> https://physics.stackexchange.com/q/506276 -4 What temperature increase results from the current 410 PPM spike in $CO_2$? polcott https://physics.stackexchange.com/users/243666 2019-10-04T02:45:47Z 2019-10-20T04:24:14Z <p>I have had three semesters of college statistics as part of my BSBA degree. From what I recall from regression analysis the graph seems to show a very high coefficient of determination between CO2 and temperature would tend to be be a reliable predictor of future changes in one variable based on changes to the other.</p> <p><strong>The average coefficient of determination (R-squared) turns out to be 0.752</strong></p> <p>In statistics, the coefficient of determination, denoted R2 or r2 and pronounced "R squared", is the proportion of the variance in the dependent variable that is predictable from the independent variable(s).</p> <p><a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5840396/" rel="nofollow noreferrer">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5840396/</a> For the entire record, the 90% confidence ranges of the correlations (R-squared) of CO2 with δD, ΔTsite and ΔTsource are 0.68–0.73, 0.76–0.80 and 0.75–0.79, respectively.</p> <p><a href="https://skepticalscience.com/skakun-co2-temp-lag.html?fbclid=IwAR2A_wvpTcWfdMjZsY_64JELzKt592FpXX8Q1UtIopgvoq3t8TqaThBGyxc" rel="nofollow noreferrer">https://skepticalscience.com/skakun-co2-temp-lag.html?fbclid=IwAR2A_wvpTcWfdMjZsY_64JELzKt592FpXX8Q1UtIopgvoq3t8TqaThBGyxc</a> So while CO2 did lag behind a small initial temperature change (which mostly occurred in the Southern Hemisphere), it led and was the primary driver behind most of the glacial-interglacial warming.</p> <p>According to the Shakun et al. data, approximately 7% of the overall glacial-interglacial global temperature increase occurred before the CO2 rise, whereas 93% of the global warming followed the CO2 increase.</p> <p>So based on the above, what temperature increase results from the current 410 PPM spike in <span class="math-container">$CO_2$</span>?</p> <p><strong>One of the comments included this:</strong> <br> Climate Threat to the Planet:Implications for Energy Policy<br> and Intergenerational Justice Jim Hansen December 17, 2008<br> <a href="http://www.columbia.edu/~jeh1/2008/AGUBjerknes20081217.pdf" rel="nofollow noreferrer">http://www.columbia.edu/~jeh1/2008/AGUBjerknes20081217.pdf</a></p> <p><strong>Empirical Climate Sensitivity 3 ± 0.5C for 2 x CO2</strong><br> If we assume that the formula above still holds then:<br> </p> <p>Since we began to rise from 280 in 1800 then 560 would be +3C<br> 410-280/280 = 46% of the way to +3C which reaches equilibrium as 1.4C<br> 420 would correspond to 1.5C<br> 467 would correspond to 2.0C<br> 513 would correspond to 2.5C<br> 560 would correspond to 3.0C<br></p> <h1>Does the 2008 climate sensitivity still hold?<br> Can it be applied linearly as it has been applied above?<br></h1> <p><a href="https://i.stack.imgur.com/98C9w.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/98C9w.png" alt="enter image description here"></a></p> https://physics.stackexchange.com/q/419679 1 Capillary tube and mercury going up Abbas https://physics.stackexchange.com/users/202208 2018-07-27T06:36:22Z 2019-10-20T09:01:00Z <p>My question is quite straight forward: What forces are pushing mercury up a capillary tube and then preventing it from going higher as tube gets thinner?</p> <p>Setup: We've got a container filled with mercury and a capillary tube placed in it</p> https://physics.stackexchange.com/q/391258 0 How does an increase in potential difference increase the resistance of a non-Ohmic conductor? IK-_-IK https://physics.stackexchange.com/users/182004 2018-03-09T23:16:01Z 2019-10-20T08:01:50Z <p>I am a little confused with the reasoning of why an increase in potential difference (P.D.) increases the resistance of a non-Ohmic conductor, namely a filament lamp. </p> <p>From what I've seen this is the reason for why <strong>Temperature</strong> affects resistance</p> <p>The increase in temperature increases the vibrational energy of the positive ions causing them to vibrate more vigorously causing the flowing electrons to collide with the ions more frequently, which causes more loss in kinetic energy of the electrons, which reduces the current.</p> <p>Is it enough to say that an increase in P.D. simply increases the temperature of the conductors causing the effect as described above? or am I missing a few point? </p> https://physics.stackexchange.com/q/388525 1 Schrödinger's equation - Time reversal spyro386 https://physics.stackexchange.com/users/129427 2018-02-25T01:13:41Z 2019-10-20T08:50:50Z <p>While reading a book about interactions I've come to this paragraph:</p> <blockquote> <p>In the Schrodinger representation, the state function satisfies the equation <span class="math-container">$$\mathrm{i}\frac{\partial}{\partial t}\phi(t,x)=H\phi(t,x).$$</span></p> <p>Invariance requires the transformed wave function <span class="math-container">$\mathcal{T}\phi(t,x)$</span> to satisfy the same equation with <span class="math-container">$t$</span> replaced by <span class="math-container">$t^\prime=-t$</span>. The question is, how is <span class="math-container">$\mathcal{T}\phi(t,x)$</span> related to <span class="math-container">$\phi(t,x)$</span>?</p> <p>The simplest possible postulate, <span class="math-container">$\mathcal{T}\phi(t,x)=\phi(-t,x)$</span>, leads to <span class="math-container">$$\mathrm{i}\frac{\partial}{\partial (-t)}\phi(-t,x)=-H\phi(-t,x)$$</span></p> </blockquote> <p>How exactly did we get the minus on the right hand side? The way I think it should go is if we reverse the time and replace all <span class="math-container">$t$</span> by -<span class="math-container">$t$</span>, we get <span class="math-container">$-t$</span> in all arguments of the wavefunction and one minus in the derivative with respect to time. Where does the 4th minus on the right hand side of the equation come from? The only explanation I could somehow come up with is that we only changed <span class="math-container">$t$</span> to <span class="math-container">$-t$</span> inside the argument of wave function and left the derivative untouched and only then mulitplied the whole equation by (<span class="math-container">$-1$</span>) to get the minus in derivative.</p> <p>What am I missing? :(</p> https://physics.stackexchange.com/q/359192 0 Control volume example Sujata https://physics.stackexchange.com/users/166114 2017-09-25T11:28:27Z 2019-10-20T04:04:20Z <p>Do lungs of human body can be an example of control volume? I think it's not as for a control volume the mass can flow and energy too through system but i mean volume should be fixed .WHereas the volume of our lungs changes so it should not be control volume system.</p> https://physics.stackexchange.com/q/322727 -2 Does a force do work if the direction of displacement is not in the direction of it (except the case of 90 degree)? Immortal Player https://physics.stackexchange.com/users/29762 2017-03-31T18:47:52Z 2019-10-20T08:40:59Z <p>From Work (Physics) - Wikipedia: </p> <blockquote> <p>In physics, a force is said to do work if, when acting, there is a displacement of the point of application in the direction of the force.</p> </blockquote> <p>According to the mathematical formula of work: $$W = F . S = FS Cos \theta$$, even if the displacement is not along the direction of force (except in the perpendicular direction), work will not be zero. </p> <p>Then, <em>a force can do work even if, when acting, there is a displacement of the point of application in any direction w.r.t force except 90 degree.</em> But Wikipedia seems to claim (in the above statement) work to be done only in the case of $\theta$ equal to zero.</p> <p>I have got confused. Where am I wrong? Or is there a problem in wikipedia statement?</p> <p>Edit: According to above Wikipedia statement, is force not said to do work, if the displacement is not in the direction of force?</p> https://physics.stackexchange.com/q/310067 0 Irradiance and current Shanks K https://physics.stackexchange.com/users/144552 2017-02-06T11:59:59Z 2019-10-20T06:00:37Z <p>Is there a relationship between Ir-radiance of an LED and the forward current supplied to it? I am designing a photo-therapy device for neonatal jaundice treatment. The required quantity is Ir-radiance. However, I'm unable to measure it in our college because of non-availability of a radiometer. So I was wondering if there is a relationship between these two. Then I can supply the required current to the LED to get the desired irradiance.</p> https://physics.stackexchange.com/q/300766 1 How does a transformer work? FanBoy https://physics.stackexchange.com/users/140144 2016-12-24T20:54:30Z 2019-10-20T05:03:15Z <p>As voltage is given to primary coil, current flows through coil and consequently magnetic flux changes. Due to change in magnetic flux a back emf is also induced in the primary which opposes the applied voltage. Thus applied voltage =- induced EMF, i.e. $$V_p=E+IR \, ,$$ and if $R=0$ then $V_p=E$. If an equal and opposite back emf gets induced then how does change of flux takes places through iron core? Shouldn't it suppress the applied voltage completely?</p> https://physics.stackexchange.com/q/200948 1 How can we find velocity, acceleration etc, of a revolving particle with respect to an observer inside the circle(not at center) Dost Arora https://physics.stackexchange.com/users/89177 2015-08-17T17:15:55Z 2019-10-20T07:00:50Z <p>A particle is revolving in <em>horizontal</em> a circle of radius $R$ with constant speed of $|\vec{v}|$ and constant angular velocity $\omega$. There is another observer standing inside the circle, at a distance $x$ from the center of the circle.</p> <p>Will this observer see the particle revolving around the circle with same velocity, acceleration and angular velocity or will it differ, if it differs then how can we find the function of <strong>velocity, acceleration, speed, angular velocity and $\phi$</strong><em>(refer diagram)</em>, in time; for this observer in his frame; i.e. how can we find $\vec{v}(t)_\text{observer}\text{, }\vec{a}(t)_\text{observer}\text{, }|\vec{v}|(t)_\text{observer}\text{(speed), }\theta(t)_\text{observer}\; \&amp; \;\phi(t)_\text{observer}$</p> <p>Assume that an ideal(massless &amp; inextensible) string is attached to the revolving particle and it has mass $m$. At $t=0$ the particle is at a point where the ray from center to observer cuts the circle: initial position(refer diagram) and revolves in anticlockwise direction. <a href="https://i.stack.imgur.com/6iuZk.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/6iuZk.png" alt="enter image description here"></a></p> <p>$$\alpha = x$$</p> <p>Edit: can we find a relation between $dx \; \&amp; \;d\phi$</p>