Questions tagged [energy-conservation]

The law of conservation of energy, which states that the amount of energy in a system is constant. For questions about Earth's environment, see the climate-science tag instead.

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Is this short paper correct?: "Heating the coffee by looking at it. Or why quantum measurements are physical processes"

I've just found the following paper in arXiv written by a Spanish politician who is also a physicist. In it he claims that you can setup a sequential Stern-Gerlach-type experiment in such a way that ...
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When can a closed system with a time-dependent Hamiltonian be mapped to an open system with a time-independent Hamiltonian?

Apologies in advance for what is not a short read. I hope someone will find it interesting enough to answer. Alternatively, if my misunderstandings are infuriating enough to warrant a response ...
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Energy conservation in cosmological phase transitions

Let us consider a cosmic phase transition, in which fermions $\psi_f$ condense and the vacuum expectation value $|\langle \bar{\psi}_f \psi_f\rangle |$ of the resulting fermion-bilinear field gives ...
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What would happen if I took empty space and stretched it?

There was a talk at my school by Rocky Kolb and he claimed that they derived (or it might have been ''experimentally found'', I don't remember) that the mass of empty space must be/is on the order of $...
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Non-locality of gravitational energy

Gravitational energy is non-local which is essentially because of the equivalence principle. The equivalence principle says that you can always transform your frame so that you feel like in a ...
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Swing: why does the body position modify the amplitude?

When a person swings, why does the amplitude of oscillations increase if the person changes the body position ? That is, when descending and approaching the vertical position, if the person extend his ...
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Is it possible to peform thermodynamic work by harvesting entanglement from the vacuum?

I have already asked two questions on this topic: 1. What happens to the vacuum when entanglement is harvested? [closed] and 2. Does entanglement harvesting require cooling the environment to ...
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Does $\frac{d}{dt}E=0$ always yield unambiguous equations of motion?

If you have a conservative system, one way you can derive the equations of motion is by using the fact that the total energy $E$ of the system is conserved, i.e.: $$ \frac{d}{dt}(T+V)=0 $$ For example ...
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Can an Opposing Current Create another Opposing Current?

In Inductors when current increases it's magnetic field induces a voltage which causes an opposing current that slows down the rise of the current that initially creates it, but can this opposing ...
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Why does the law of conservation of energy not hold true when the work-function $U$ depends explicitly on $t$?

[...] the infinitesimal work $\overline{\mathrm dw}$ comes out as a linear differential form of the variables $q_i$: $$\overline{\mathrm dw}= F_1~\mathrm dq_1 +F_2~\mathrm dq_2+ \ldots + F_n~\mathrm ...
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Non-local gravitational energy tensor

The well-known derivation of the Landau-Lifshitz gravitational energy pseudotensor, relies on several requirements: 1) that it be constructed entirely from the metric tensor 2) that it be index ...
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Avoiding Pseudo-tensors when addressing global conservation of energy in GR

Discussions about global conservation of energy in GR often invoke the use of the stress-energy-momentum pseudo-tensor to offer up a sort of generalization of the concept of energy defined in a way ...
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What quantum states can be measured using entanglement?

I have been reading about quantum entanglement, and I was wondering what quantum states can be "sent" using entanglement. I know you can measure the spin of one of the particles, and know the spin of ...
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Ball, Fixed Curved Track, and Non-Fixed Curved Track

I wonder whether the way I approach and solve this question is correct or not. Question: Case 1 As illustrated, a track of mass $M$ is fixed on a horizontal table. $AB$ is horizontal while $CD$ is ...
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On energy conservation and time-ordered interaction diagrams

I've found the following lectures given by professor Mark Thomson (here's his particle physics web page); in the third one, Interaction by particle exchange and QED, it's stated that time-ordered QM ...
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Negative-frequency waves

Let's consider a plane wave described by $E = E_0\exp(i(kx-\omega t))$. We can either consider this wave to move forward (i.e. along increasing distance $x$) with a positive angular frequency $\omega &...
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Mechanical energy conservation in an unconstrained system

I was going through energy conservation for unconstrained multi-particle systems in Gregory's Classical Mechanics ("Energy Principle", p266). In the section where he calculates the work done ...
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The so called "Energy Approach" vs "Force Approach", when and how should they be used?

I'm new to physics and I've been trying to solve a few high school olympiad questions. I've figured that I approach the questions by analyzing the forces acting on objects and trying to induce ...
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What mechanisms could keep Universe at 0 energy?

In my understanding is right, there is no obvious guarantee of the conservation of energy on the cosmological scale. The expansion of space-time causes the red-shift of radiation and therefore the ...
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Can some one explain to me why, when I include translational kinetic energy ($\frac{1}{2}mv^2$), I run into problems dealing with rotational motion?

Overall, the problems I know when to use it are when there is something like a rotating pulley disk which has a string wrung around it and a block at the end falling down. Another place I know when to ...
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Can the Casimir effect propel this system?

Between two extremely close metal plates, there is a force of attraction because the metal plates have to be nodes of any electromagnetic quantum fluctuations and so there are less virtual particles ...
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Does: capillary_action * surface * distance = (force*surface) / area*distance == capillary_energy?

I'm trying to get the hang of the conservation of energy. Looking for that aha moment when it makes more sense that energy is conserved than not. When reading it seems to be a consensus that energy is ...
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Time translational symmetry breaking

I have heard of time translational symmetry, which refers to the law of conservation of energy. Time crystals break time translational symmetry. I read on Wikipedia that it only breaks discrete-time ...
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1 answer
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Does this setup defy conservation of energy?

Suppose we have two wire loops of the same dimensions placed adjacent to each other. The first loop is connected to an AC source, the medium inside the first loop is air and the medium inside the ...
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Positivity of $dt$?

Background I had an idea to write time $dt$ in the first law of thermodynamics. So let us proceed with the $1$'st law in differential form: $$ dw = T dS - d U $$ where $w$ is the work, $T$ is ...
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What is a reason for the energy to not conserve in QM perturbation theory?

Consider the transition rate of the evolution of the state $|i\rangle$ to a state $|f\rangle$ - the Fermi golden rule: $$ d \omega_{if} = 2\pi |\mathcal{M}_{fi}|^{2} \delta(E_{f} - E_{i})d\nu, \quad \...
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Max velocity before toppling risk

I want to find out the maximum velocity a machine can travel before there is a risk of toppling. Say, worst case, is that the castors hit a dead stop and the machine is brought to rest '...
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If negative mass should exist, how would conservation of momentum work?

Imagine that a particle with negative mass has been discovered. It is known to obey both the equivalence principle and Newton's second law. This means that the particle is a subject to repulsive ...
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Can two semi-infinite plane waves undergo perfectly constructive interference?

This is building off of a question I asked here. When discussing the linked problem with some friends, the consensus seemed to be that the reason two identical semi-infinite plane waves cannot ...
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How to estimate the kinetic energy loss of a rigid cube after it colliding with a heavy spring at rest?

In an environment without gravity, there is an unfixed, motionless, uniform spring with non-negligible mass m, length L, and stiffness k suspending and a rigid cube with mass M moving with velocity v ...
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Work done by conservative and non-conservative forces

Work done by conservative forces changes one form of mechanical energy into another. Is it correct to assume that work done by non-conservative forces changes one form of energy to another, for e.g., ...
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Inflation and dark energy have constant energy density - energy is created out of nothing without any source or mechanism?

I can understand that a photon in an expanding universe can be redshifted and lose its energy by its wavelength being stretched from space expansion. But in the case of inflation and dark energy, it ...
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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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Potential Energy in a System and its Relation to the Total Energy

In this answer it was stated that potential energy is a property of a system and not an individual particle. If we have two particles (1 and 2) interacting via a conservative force, we can write an ...
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How do I derive the formula of coefficient of restitution?

I have learnt in collision mechanics about the term Coefficient of restitution, $\mathrm{e=\frac{v_{2f}-v_{1f}}{v_{1i}-v_{2i}}}$. But the sad part is that, in my book only the formula is there. My ...
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Fluid approximation for inflaton?

In cosmology there is a widely used fluid approximation which applies when the mean free path is very small compared to the scale of observation and then all the properties of stress energy tensor can ...
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Does the additivity property of Integrals of motion and Lagrangians valid in all situations?

I would like to know if the additivity property of an integral (constant) of motion valid in all situations ? It works for energy but does it work for all other integrals of motion in all kinds of ...
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Law of Conservation of Energy

I find it hard to comprehend the law of conservation of energy. Allow me to explain my confusion. I understand that the law of conservation of energy states that energy is neither created nor ...
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Friction and work from torque

I would like to understand where is the error in the third case, for that I gave 2 easier cases where I'm able to find the energy from heating is equal to the energy lost by torque. Case 1/ Purple ...
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How many properties like charge, mass etc any quanta or particle must have?

How many properties are required to measure full energy of a fundamental particle? I know $E=mc^2$, but what about charge, spin, etc? Which full equation would give me all parameters of any particle?
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Momentum of capacitor in a uniform magnetic field

We are observing ideal, charged, parallel plate capacitor placed in uniform magnetic field parallel to plates. Whole system is at rest and isolated (we have forces that hold plates separated, but net ...
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Would a large, small mass object in orbit experience induced rotation

Imagine a large (multiple earth radii), very small-mass ring orbiting The Sun. Half of the ring would be closer to The Sun than the outer half. Since orbital velocity decreases with distance, two free ...
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James Joule's Conservation of Energy

Is there a manifest or documentation of the original Conservation of Energy as stated by James Joule word for word? Edit 1.1: I found Joule's Memoir on google, skimming through to find any conclusive ...
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Escaping the Gravity well of the Moon

How much energy would it take to get 1 kg from the Moon's surface to Earth? Would You aim for Lagrange Point 1 or would you launch in the opposite direction that the moon is orbiting?
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Binding energy converted to kinetic energy/mass loss

My particular question is regarding nuclear fission, but applies to other nuclear processes as well. I understand that in nuclear fission, the two fragment nuclei have a higher binding energy per ...
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Does laser trap atom cooling change the electron shell levels?

Do the electron orbits change during the cooling of the atom in a laser trap? When cooling down, do the electrons come closer to the nucleus of the atom or do they move away? Or do the electrons leave ...
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Some questions related to the energy of a viscoelastic bar

Let's consider a problem of free longitudinal vibrations of an one dimensional infinitely long elastic relaxing bar (Maxwell material) with constant cross section. The bar's displacement $u$ and ...
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1 answer
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Will the heat generated from solar panel electricity eventually generate wind energy?

I'm curious about limits for extracting solar energy, but I never studied this stuff so I'm going to speculate a lot. My first question deals with the things like wind energy. I assume this energy is ...
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General definition of a standing wave

Question "What is standing wave?" has already been asked. Intuitively we all know what it is, but how would we define such a wave in the most general way: for arbitrary geometry, dispersion, ...
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How does energy conserve during superposition of wave?

Let's consider two overlaping particles of mechanical wave, since it's a mechanical wave, we can think of it as spring-like so energy of particle $ E= \frac{1}{2} k A^2 $ where A is the amplitude of ...
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