# Tagged Questions

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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### Are all kinds of energy quantized? [duplicate]

I was reading this answer of David Z about meaning of "Quantum" that this question came in mind: If light is quantized (i.e. is a integer multiples of some unit $\hbar c/k$), are other kinds of ...
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### trouble reconciling doppler shift for energy and momentum of light

[See late addition at bottom for what currently remains unresolved in this question.] I have seen the assertion in a couple of places that if you trap light in a reflective resonator, or any other ...
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### Does shortening the path length of an excited etalon do work? What about LIGO?

Start with a high-Q etalon excited on-resonance with a laser for a time long enough that it has built up an essentially stable standing wave. There is a constant outwards-directed force on each mirror,...
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### When we say particle in a box has quantized energy, is that kinetic or total energy?

In quantum mechanics, it is usually said that energy of the bound (constrained) systems such as particle in a box (infinite potential well) is quantized. It confuses me exactly what type of energy is ...
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### Why does positive work done by internal conservative forces $\implies$ decrease of potential energy?

Potential energy can be thought as the amount of work that the force can potentially do on the point because of its position. $$W=-\Delta U=U_{initial}-U_{final}$$ A positive work done by a force ...
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### Does a measurement always require the exchange of energy?

Can you measure something about something without adding or removing energy from it? This comment has got me wondering if there is a way to measure a property or a quantity of something without some ...
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### Is it possible to synthesize an EMW which is not quantized?

( I initially started to ask, "since according to Quantum-theory of light; the energy of a photon, depends only on the frequency of light-wave (E = h * nu), and no-mention of amplitude. So, does the ...
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### Energy Spectrum and Gauge Transformations

There is a question in SE about the fact that the Hamiltonian isn't invariant under the EM gauge transformations. I wanted to ask about its consequences here. I know that in general, the Hamiltonian ...
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### Is there a maximum temperature? [duplicate]

I have read about absolute zero. I understand that it is impossible to cool a system below absolute zero (or to absolute zero for that matter). However, is there a maximum temperature? My intuition ...
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### Transfer of energy from gravity back to other “more familiar” forms of energy?

In this question I've mentioned an account of the recently reported 2nd observation of gravitational waves, LIGO and Virgo Collaborations, Phys. Rev. Lett. 116, 241103, 15 June 2016, where 1 of the ...
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### Minimum gap between consecutive energy levels?

Assume a standard one-particle, non-relativistic Hamiltonian of the form $$H=\frac{p^2}{2m}+V(r)$$ and denote its eigenvalues as $E_{n,\tau}$, where $n$ is the principal ...
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### Doesn't quantum uncertainty disprove string theory? [duplicate]

String theory states that the oscillations of little strings are responsible for all the particles in and the evolution of the universe. The specific type of particle created by a string depends on ...
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### 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 $w_1$ radians per second around its center of gravity and blue solid is ...
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### Is the energy cost of desalination linear vs salinity? [on hold]

There is a practical limit to how salty water can be and still have a useful potential for desalination. For example, a recent study on the potential for water recover from known sources on Mars ...
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### Stuck with 2D kinetics problem [on hold]

I've been trying to solve this exercise for the last four hours and I'm totally stuck. The problems goes like this: Given the mechanism in the image, located in the vertical plane, the OB cord ...
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### how does an induced draft fan work in a coal

I have been studying a coal-fired power plant, and the power plant cycle has an induced draft fan that is used to create negative pressure to help get the flue gas travel to the stack. I am confused ...
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### 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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### Increasing a potential causes increase in energy levels

Suppose a potential $V(x)$, and suppose a bound particle so the allowed energy levels are discrete. Suppose a second potential $\widetilde{V}(x)$ such that $\widetilde{V}(x) \geq V(x)$ for all $x$ (...
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### How to compute this change of Gibbs energy for a Van der Waals gas?

The change of Gibbs energy at constant temperature and species numbers, $\Delta G$, is given by an integral $\int_{p_1}^{p_2}V\,{\mathrm d}p$. For the ideal gas law $$p\,V=n\,RT,$$ this comes down to ...
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### Do we have a better understanding of what energy is since Feynman's time?

When lecturing about conservation of energy in the 1960s, Richard Feynman remarked: It is important to realize that in physics today, we have no knowledge of what energy is. We do not have a ...
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### What is the root mean square energy?

How the root mean square energy of nucleon in a nucleous of atomic number A in ground state is depend on A ?
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### In relativity theory could we talk energy rather than frames of reference? [on hold]

In relativity theory, the space geometry, of space and time, is no longer absolute, if one observer is accelerating compared to the other. Could I say in other words that space geometry is changed if,...
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### Proper definition of Internal energy

In Internal energy of a gas ($U$), we have to include ALL types of energies possible like $KE$, bond energies, nuclear interaction energies, etc.., However we have derived that $U = (f/2)nRT$ from ...
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### Conservation of Energy with Rotation Caused by Friction

A book that I am using to study poses the following problem: Suppose you wish to use a spring with a force constant $k$ to launch a spherical ball or radius $r$ and mass $m$ up a ramp of inclination ...
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### If a wormhole connects between ground and sky, does an object require extra energy to pass through it? [duplicate]

As we know, if we bring an object from ground to sky (higher position) by normal methods, extra energy is required to overcome the gravitational potential energy. Then if a wormhole connects between ...
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### Why is the energy shift due to a 'sagging' potential negative and independent of box size?

Consider a box of width $L$ and the composed of the following potential $$V(x)=\frac{V_0x(x-L)}{L^2}, x\in[0,L]$$ and $V(x)=\infty$ elsewhere. Using perturbation theory - with a square box as the ...