Energy is the conserved quantity associated to time-translation invariance and represents the work a system is capable of doing. Use this tag for questions about energy, and consider adding [tag:energy-conservation] if it is specifically about its conservation.

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Positivity of Total Gravitational Energy in GR

I read the following statement in the introduction to an article: Over the last 30 years, one of the greatest achievements in classical general relativity has certainly been the proof of the ...
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If the energy of an ant is smaller than an elephant, does that mean the ant is more stable than the elephant?

I know that when a system is in its lowest level of energy, it is most stable. However, what if system 1 has lower energy than system 2, does it keep meaning so? Or do we need to examine their binding ...
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Could energy be stored into (not extracted from) the quantum zero point field (like a battery)?

In order to explain the question clearly, I will make a short introduction. In 1962, Josephson predicted that for a sufficiently thin insulating layer, it should be possible for Cooper pairs to ...
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How much energy is radiated when matter falls into a black hole

When a black hole consumes matter, it can form an accretion disc. Is there a relationship between the mass of the falling matter $m$, the mass of the black hole $M$, and the energy radiated during the ...
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206 views

Sum of energy for 2 solids in rotation

I would like to compute the sum of energy of the following case: Two solids are turning (disks). Yellow solid is turning at $w1 rd/s$ around its center of gravity and blue solid is turning at ...
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283 views

How to convert Richter magnitude scale to approximate TNT?

I know the Richter magnitude scale is often used for measuring the strength of earthquakes. At the same time, explosive/destructive releases of energy are often quoted in equivalent amounts of TNT. Is ...
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341 views

Dirac action and conventions

I have a (possibly) fundamental question, which is driving me crazy. Notation When considering the Dirac action (say reading Peskin's book), one have $\int ...
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64 views

Is refraction of light a thermodynamic process?

Is refraction of light a thermodynamic process? Can it be explained by conservation of energy? If so, does temperature has an effect on refraction of light?
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142 views

How to calculate the heat transfer rate between the liquid and the gas?

Let water (liquid) be in a chamber (with no heat transfer to the walls). The gas is the residual space above the liquid, meaning that the contact surface is the surface of the water. Given the gas ...
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117 views

Building free energy directly from lagrangian

Energy can be built from lagrangian when considering the symmetry of time $(\frac{\partial L}{\partial t}=0)$. Free energy is a generalization of energy when the system exchanges heat with the ...
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567 views

Practice AP Physics B Exam Question regarding Momentum

I am trying to review momentum for the AP exam coming up. I will be taking the AP Physics C exam for Mechanics, but I was just practicing on any free response questions I could find and I came across ...
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102 views

does a rocking kettle cause the water to boil more slowly?

When I boil water in the kettle on my electric stove, sometimes it rocks back and forth making an annoying sound at a frequency of about 6Hz. When that happens, I move the kettle slightly to make it ...
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675 views

How do I determine the Internal Energy of ammonia at a pressure in temperature when the steam table doesn't say it

I just bought a steam table for my thermodynamics class since they don't allow use to use the one from the book for the tests. This one is different from the one in my textbook as it doesn't give the ...
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Would impact angle matter on relativistic impactor?

I'm trying to calculate (for fun) a comparison between a kinetic impactor and an H-bomb. I would assume this to be a fairly straight forward problem involving kinetic energy and a table of various ...
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55 views

How to relate internal energy to atomic motion?

I am trying to conceptualize how atomic motion leads to the thermodynamically-defined internal energy (denoted as $U$ below) through some broad mathematical relationships. I get that the internal ...
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124 views

Is Energy attracted to Energy?

Newton taught us that bodies with mass attract each other according to the universal law of gravitation (mass-mass attraction) and Einstein taught us that mass and energy are equivalent though his ...
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38 views

What role did the Higgs boson play in the Big Bang?

Scientists say that inflation stopped when a very peculiar energy was mobilized, did Higgs boson play any role in this? What is it's relation to the Big Bang? Some physicists, such as Michio Kaku, ...
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53 views

Estimate of the second shallowest bound state?

Suppose we have a 1D potential $V(x)$ of finite range, i.e., $$ V(x) ~=~0 $$ for $|x| > b $. The potential is assume to support at least two bound states, but might have more, say $n\geq 2$. ...
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47 views

Making An Energy Momentum Plot For A Rashba Model (Using Discretization)

I want to make a plot of the Energy versus the Momentum of the Rashba model, using discrete matrices. First Ill show how I did this for the free particle. Subsequently I will show what goes wrong for ...
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62 views

Force and Energy in robots

There are two similar and hypothetical robots that move with wheels powered by motors, Robot A and Robot B. Robot A has a gear ratio of 3:1 (The gear connected to the motor is three times larger than ...
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74 views

De-excitation and vibrational frequency

By utilising energetic beams of particles it is possible to set atomic nuclei into vibration. For a certain nucleus the subsequent de-excitation results in the emission of photons of energy $1$ $MeV$. ...
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75 views

Mean energy harmonic oscillator

I know that for a particle under the potential $$V(x,y,z)=\frac{k}{2}(x^2+y^2+z^2)$$ the equipartition theorem says that it contributes to the mean energy to $\frac{3k_BT}{2} $ (one half for each ...
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58 views

What's the connection between being homogeneous in energy and having a proper time scale?

Towards the end of page 8 of this scientific paper, I have found the following sentence: ...given the fact that the system is homogeneous in energy, or equivalently, that it has no proper time ...
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216 views

Finding internal energy using equation of state

I have an equation of state as a function of $p$ (pressure), $T$ (temperature) and $V$ (volume) for a real gas and need to find an expression for the internal energy $U$. I know this is given by the ...
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156 views

What are the physical limitations on energy density for a useful, safe battery, and are we anywhere near them?

Energy storage seems to be a fairly major bottle neck in technological development at the moment, especially for important things like clean-energy powered transport, but also for consumer goods. Are ...
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147 views

Heat equation with Neumann boundary conditions

I would like to explore some ideas with different geometries exchanging energy mainly by radiation. This means solving Laplace equation for the steady state. However the boundary conditions are always ...
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138 views

How efficient is the Crookes radiometer?

I remember many years ago, I think at 8th grade, seeing the teacher show us a Crookes radiometer. I remember it being very fascinating. Today I read the wiki article on it, after looking up what it ...
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89 views

The particle mesh ewald method in two dimensions

I am attempting to implement the particle mesh ewald method (http://dx.doi.org/10.1063/1.470117) in two dimensions. I am wondering what needs to be changed in the method from three dimensions to two ...
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689 views

Bungee jump physics

Question: A bungee jumper jumps from a bridge. The length of the loose rope is 30 m. When the jumper reach the lowest point possible, the rope stretches 10 m. What is the final stretch of the rope, ...
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387 views

Maxwell's Demon - laser cooling

There’s an interesting article on Scientific American that tells how to cool individual atoms to within a very tiny fraction of Absolute Zero. It uses two laser beams acting on a very cold rarified ...
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33 views

Why do particles in a medium have different speeds?

If thermal conduction occurs because of particles colliding with each other and imparting kinetic energy to each other, won’t the particles of an isolated medium collide with each other until the ...
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30 views

Internal heating effects of a circuit element?

The heat-up of a solid circuit component in an electric circuit (for which no chemical reactions, phase changes reactions or other stuctural changes take place) may include: Joule heating $\dot ...
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43 views

Is shaking a phone enough energy to charge a smartphone?

Is shaking a phone enough energy to charge a smartphone? For example I'd like to use some kind of power generator to charge smartphone's battery but I'm wondering if shaking (by hand, or motion when ...
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27 views

What does it mean to perturb the electromagnetic energy with respect to a change in a Lorentz frame?

I am perturbing the energy of an electromagnetic field using \begin{align} \dfrac{\partial U}{\partial \zeta} = \dfrac{1}{\mu} \boldsymbol{H}\cdot \dfrac{\partial \boldsymbol{H}}{\partial \zeta} + ...
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35 views

Energy density question

I have looked for these formulas everywhere but cant seem to find them... What is the formula for the energy density of the electric current ,and the energy density of the mechanical wave?
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Vertical wall at 3 different temperatures - do the same natural convection coefficient correlations apply?

I have an vertical enclosure that consists of 3 shelves, each with its own temperature and are thermally isolated from each other. We will call the shelves A, B, and C with A being the base shelf. The ...
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30 views

How much energy does a PV cell need to work?

How much energy does a $1m^2$ photovolatic/solar cell need to work? Can it work using a bunch of Laser lights? Edit: Ok so this PV cell is $1m^2$ is dimesions. The N-type has silicon doped with ...
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54 views

What is the smallest engine theoretically possible?

Is there some law of physic forbiding small engines that use say petrol? I would like to know of some analogy between electric motors and heat engines. In the latter we can make a motor so small that ...
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Energy density in electrostatics

Imagine two hollow spheres with radius $R$ with charges $q$ and $-q$. $L>2R$ is distance between them. Potential energy of the each sphere is \begin{equation*} W_1=\frac 1{8\pi \epsilon_0}\frac ...
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47 views

Superimposed hydrogen electron states

I have been following an Edx.org course on Quantum Computing. The Prof. has started with a Hydrogen atom qubit, assuming that the electron can only be in the ground state and the first excited state. ...
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Negative temperature thermodynamics

I asked this question about thermodynamics. I considered $$T=\frac{\partial U}{\partial S}$$ at constant $V$ and $N$ (number of particles), but now I can't figure out the change of $U$ with respect ...
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67 views

What does the relation between mass and energy of a free particle mean?

What does the Hamiltonian for a free particle mean? Does it mean that the kinetic energy of the particle is in reverse relation with mass? $H$ or $E=\hbar^{2}k^{2}/2m$. Or better to ask: what's the ...
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Marvin the Martian vs. the Death Star: how much energy will they actually need to disintegrate the Earth?

According to a detailed analysis by Dave Typinski, Marvin the Martian’s Illudium Q-36 Explosive Space Modulator will require $1.711 \cdot 10^{32}~\text{J}$ to shatter the Earth into a gravitationally ...
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101 views

Help with derivation of the Casimir Effect?

I am at the very last part of a relatively long derivation of the Casimir effect, and I just don't understand the final step D: So far, I have derived the ground state energy to be $$\langle 0| ...
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210 views

Energy of RLC circuit

If you are given a general differential equation for an RLC circuit, for example, $$L\left(\frac{d^2 Q}{dt^2}\right) + R\left(\frac{dQ}{dt}\right) + \frac QC = V\cos(\omega t),$$ which is a driven ...
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55 views

Does a pulled rubber band contain as much energy as a twisted rubber band?

lets say I take two similar rubber bands. One of them I pull until it almost reaches its breaking point. The other I twist until it almost reached its breaking point. Do both of these rubber bands ...
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34 views

Is energy since the big-bang still here in the present?

Based on the concept that energy can never be destroyed and is only transferred. Does it mean that energy since the formation of literally everything still here today?
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Determine the particle velocity of a pressure wave

I am using constant density wave propagators to model seismic waves in the subsurface. What I want with these acoustic waves is to estimate the energy of them at a certain grid point at a given time. ...
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45 views

Polarizable molecule in E-field

If we have a linear molecule with a dipole moment $\mu$ in a static electric field $E$, the potential is given by $V = - \langle \mu,E \rangle$. What is the appropriate equation for the potential if ...
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Radius dependence of Energy levels of spherical Quantum dots

I have frequently read in articles that the energy level of spherical quantum dots depends on radius. We simply match the wave functions inside and outside the particle and find the intersection of ...