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Questions tagged [energy]

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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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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316 views

Free Energy Landscape - Construction and meaning?

I struggle to understand the concept of free-energy landscape. It seems to me the concept makes perfect sense for energies, but not for (canonical) free energies. In my actual, hopefully to be ...
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518 views

What is the link between free energy and lagrangian?

Free energy is a generalization of energy when the system exchanges heat with the environment. Energy, in its turn, can be extracted from lagrangian under the symmetry of time $(\frac{\partial L}{\...
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366 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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115 views

Why do electrons abide by Hund's rule?

Is the reason why Hund's rule exists, that when electrons are in different orbitals (such as 2px, 2py, or 2pz), they are most stable (lowest energy)? If the purpose is stability/lowest energy, ...
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512 views

What is the third peak in the spectrum of Sodium-22?

I collected this spectrum from a sodium-22 sample in an instructional laboratory. The sample was set in front of a germanium detector which sends a signal into a pulse height analyzer and plots a ...
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304 views

Energy of a particle as measured by an observer at infinity

I'm wondering if it is possible to make a definition for the energy of a particle as measured by an observer at infinity. I've looked through Wald for this but wasn't able to find anything - I may be ...
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68 views

Meaning of zero temperature in terms of $1/T=\partial S/\partial E$

Suppose we have a $NVE$ system of a pure crystal. From the definition of the temperature in macroscopic thermodynamics, $$\frac{1}{T}=\left. \frac{\partial S}{\partial E}\right |_{V,N}$$ . Then ...
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68 views

Integrated vs. Differential Form of the Enthalpy

I have been under the impression that the definition of enthalpy as $$H = U + PV$$ is purely mathematical, the result of taking the Legendre transform of the internal energy $U$, and merely provides ...
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98 views

Can the Taylor expansion of Energy/Intensity be explained more in-depth?

In reference to the question, "Why is energy in a wave proportional to amplitude?" Ben Crowell answered with the answer attached. The original post is linked here. I did not respond to the original ...
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85 views

Power in a wind turbine

An wind turbine starts turning by a wind speed of $v_b$ and it stops turning (rated wind speed, to prevent overloading) by a wind speed of $v_g$. By wind speeds between $v_b<v<v_g$, the power ...
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85 views

Extracting work from a partition of equally energetic “colored” balls

I am referencing this answer by Ruben Verresen to this question: Does entropy depend on the observer? A bit down in that answer Ruben states that just being able to perceive (in theory - in your ...
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144 views

Variational Quantum Free Energy?

The quantum mechanical free energy $F$ is defined as $$ e^{-\beta F} = \text{Tr}\left( e^{-\beta(H+\mu N)}\right) $$ where $H$ is the Hamiltonian of the many body system, $\beta$ is the inverse ...
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128 views

Constant work function although changing Fermi energy

I'm working with the following book “Interfacial Electrochemistry” (2nd Edition) written by Wolfgang Schmickler and Elizabeth Santos and I'm trying to understand the process described in section "...
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159 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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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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560 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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57 views

Hamiltonian directly expressed in $(q,\dot{q})$ : how to find what is $p$?

I am reading a book about non relativistic quantization of E.M field. But first we do classical field theory. We directly wrote the Hamiltonian of our study, and a part of our Hamiltonian is the ...
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37 views

Questions about the energy density and momentum density in linear media

Precisely which energies are accounted for in the energy density of a linear medium given by $$u = \frac{1}{2}\left(\epsilon E^2 + \frac{1}{\mu}B^2\right)?$$ For example, is there a kinetic energy ...
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37 views

How can I draw the energy bands for the first and second zones of Brillouin? Is it conductor or insulator?

I want to draw the energy ($E$) diagrams for a simple cubic cell of parameter $a$, where each atom provides two electrons for the almost free electron levels for planes [100], [110] and [111]. I ...
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85 views

Percentage increase in wavelength leads to percentage decrease in energy. (Compton effect)

In this question: What percentage increase in wavelength leads to a 75% loss of photon energy in a photon-free electron collision? My understanding of this question is that $0.25\times E_i = E_f$, ...
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49 views

Hydrogen energy state transition times and probabilites

How correlated is the energy absorbed to the later energy emitted, and hence to the transition type? What is the scale of time it takes to make a downward transition in energy level? In a very cold ...
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171 views

Does an electron have a frequency (and hence an energy)?

The formulation is provocative, the question is similar to the question here. There I can follow the question, but not the answers, which for me imply that an electron in a momentum eigenstate does ...
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Is it possible to have a vortex with more than one attachment point?

In my study of two vortexes, fire whirls and tornadoes, I have come to hear a bit about vortex attachment. As far as I can understand, a vortex is attached to its "power source" for lack of a better ...
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96 views

Minimizing mechanical energy subject to fixed angular momentum

I may be in error, but this problem (ch. 4, no. 14 on p. 203), from Symon's Mechanics, 3rd edition, seems wrong to me: A mass $m$ of gas and debris surrounds a star of mass $M$. The radius of the ...
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Frequency Spectrum and Energy of an electromagnetic impulse

The energy associated with an electromagnetic wave, in its simplest form, is given by the relation: $$ E = h\nu $$ Where $h$ is Planck's constant. However, this energy relates to a definite frequency. ...
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Is there a limit for how much ways are there to transform energy?

Suppose a mechanical system that contains a single particle that is at a human scale height above the ground. There are numerous ways to transform the energy contained in the system. One is to ...
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2k views

Derivation of the energy equation in the differential form

I'm trying to understand the derivation of the energy equation from fluid mechanics, that is presented in the book "Fluid Mechanics" (4th ed.) by Frank M. White (as you can see here, page 231). I can ...
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49 views

Zero point energy and volume integral of the expected value of intensity

In his article "The concept of the photon" Scully and Sargent say that the zero point energy of the quantized electromagnetic field $\hbar\omega/2$ is given by the volume integral of $\langle 0|E^2|0\...
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37 views

What is the order of magnitude of the energy released in Majorana fermions collision/annihilation?

Majorana fermion experiment It has been observed a quantum state in a one atom thick wire which in a certain energy range behaves like a Majorana fermion. It is a quasiparticle that arises out of the ...
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224 views

Hard Sphere Scattering in the high energies limit

We see that $\delta_1$ decreases by $90^\circ$ each time $l$ increases by one unit. Thus, for an adjacent pair of partial waves, $\sin^2\delta_1+\sin^2\delta_{l+1}=\sin^2\delta_1+\sin^2(\delta_1-\pi/2)...
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48 views

Doppler shifted light sail

If I have a probe being pulled by a reflective "light sail" and I need to derive an expression for the speed of the probe as the sail is hit by laser beams, how do I do this using relativistic ...
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Intuitive understanding and physical significance of Exergy, Gibbs free energy and differences

I have already browsed several questions on this topic and have not found answers to be totally satisfactory. I am interested in knowing physical significance and an intuitive understanding of Gibbs ...
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94 views

Different expressions for energy stored in an electromagnetic field

This question is motivated off my readings in Griffiths. I've tried to be thorough in my background, but my final question is at the bottom. In the electrostatics section Griffiths gives the first ...
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101 views

Thermodynamics: Relation between different types of energy: My Analogy

I have always been confused between the different kind of energies in thermodynamics so today I thought of finally getting it right. Here's my analogy(which is wrong and I need help in improving it, ...
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53 views

Where is the particle free and where is it trapped?

This is one of my homework problems: A particle of mass $m$ is acted on by a one-dimensional force: $\displaystyle\vec{F}=\left(b\sin\frac{2\pi x}{\lambda}\right)\hat{\mathbf{x}}$ where $b$ and $\...
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189 views

Surface integral in Regge-Teitelboim paper

Recently i'm trying to read Regge-Teitelboim paper (Role of surface integrals in the Hamiltonian formulation of general relativity) in order to understand the definition of energy and momentum in GR. ...
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130 views

Will it be easier to separate a low or high density plasma into negative electrons and positive ions

I know separating plasmas into positive ions and negative particles requires energy. Now, I was wondering whether it would be easy to separate a low or high density plasma. The first thing I thought ...
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How does Newton's cradle with balls of different mass behaves?

Conservation of energy and momentum governs Newton cradle. If $n$ balls were lifted, $n$ balls will bounce of on the other side. This happens if all balls have the same mass. Now, assume that you have ...
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92 views

Intensity of an ultrasound beam?

I am working through question 9.1 (link to Google Books) of 'Fundamental Physics for Probing and Imaging' by Wade Allison. The question involves finding the varies quantities (such as pressure ...
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435 views

electric field energy density for an electron

I am currently reading a chapter in a physics book covering the electric energy stored in a parallel plate capacitor. The author derives the formula for the energy density: $u = 1/2 \epsilon E^2 $ and ...
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123 views

How to apply conservation of energy to active noise-cancelling headphones?

Active noise-cancelling headphones add the opposite waveforms of the ambient sound in order to cancel out this ambient sound by destructive interference. I do not understand why adding more power in ...
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499 views

Zeroth component of 4-momentum and relativistic energy-momentum relation

As I understand it one is forced to use 4-vectors since we require objects that transform as vectors under application of Lorentz transformations and 3-vectors do not (technically they do under ...
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120 views

If a system has an internal energy equal to 0, does this mean it doesn't exist?

Enthalpy is a measure of energy in a thermodynamic system: It includes the internal energy, which is the energy required to create > a system, and the amount of energy required to make room for it ...
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113 views

How efficient is the forward transfer of energy in gravity (surface) waves?

Ocean waves (properly categorized as gravity waves since gravity provides the restoring force within the medium they travel) propagate the major part of the energy derived from winds along the air-...
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62 views

How much damage do high energy experiments impose on the LHC detection equipment?

I do appreciate that I am second guessing lots of experts who have already considered this aspect of high energy experimentation, but I have not seen a similar question, my apologies if I missed a ...
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Have Witten-type TQFT's nonconservation of energy and momentum in interactions?

Witten-type topological quantum field theories are based on cohomology theories. Every observable must lie in a cohomology class. May be $G$ a geometric field. Then every observable expectation value ...
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76 views

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 H-...
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261 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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417 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| \hat{...