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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Where does the majority of the mass of the usual matter come from? [duplicate]

I apologize in advance to experts for the naivety of the question. It should be a duplicate but I didn't find any satifying question or answer about that. The proton is composed by two up quarks ...
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Energy conservation of Virtual Particles - Quantum Fluctuation?

I (as a middle-school student) was wondering how virtual particles even conserve energy of the entire system? I don't mean just the particle's energy, but conservation with respect to the surroundings?...
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Is energy conserved in general relativity? Does $\nabla_aT^{ab}_{\rm matter}=0$ represent the conservation of energy and momentum?

For example, the radiation dominated cosmology, the energy density of radiation is proportional to $a^{-4}$ and the volume is proportional to $a^3$, where $a$ is the scale factor. So the total energy ...
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Mass converted to energy in a common fire?

In a common wood fire such as a campfire, is matter converted to energy or is it simply an exothermic chemical reaction and all the mass can be accounted for in the ash and soot?
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Numerical schemes, time integration algorithms and energy conservation

What does it mean when someone says a numerical scheme or a time integration algorithm is "energy conserving". How can a numerical scheme "gain" or "lose" or "conserve" energy apart from the numerical ...
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Does the mass lost by merging black holes depend on how they merged?

We've all heard the news about the detection by gravitational waves of two black holes, one 29 solar masses and the other 36 solar masses, spiraling into each other to create a single black hole of 62 ...
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Does expanding space cost energy?

Does the cosmic inflation reduce the energy density (inversely) proportional to the volume, or does the inflation "cost" energy? Is space itself "something" created at the expense of energy?
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Is energy extensivity necessary in thermodynamics?

Given a partition of a system into two smaller systems, the energy $U$ is devided into $U_1$ and $U_2$, with $$U=\mathcal{P}(U_1,U_2):=U_1+U_2,$$ so that $U_2$ is given by $U-U_1$. Here the ...
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Do magnets redshift light?

Do magnets redshift light? Suppose we have an extremely powerful magnet (say the size of the Sun) and we have a smaller paramagnetic material above it (say. Titanium Brick which is indestructible)...
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Was the Big Bang actually cold?

As I understand, from watching the Discovery Channel, the total amount of energy in the universe is zero. As such, people like Hawking explain that the universe can be created out of nothing because......
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Why can't a superconductor make a DC motor self sustaining?

Superconducting wire can host a low current magnetic field. I do not know if it supports a corresponding electrical field. Can a superconducting wire that sustains a current accelerate a DC motor? ...
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Why do I get negative height?

A baseball is thrown from the roof $50.1^\circ$ above the horizontal. The initial velocity is $11.5\ \mathrm{m/s}$. I'm trying to find how high the ball goes. I choose positive direction upwards. $$...
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Why don't we use positrons from beta plus radiation as a source of anti-electrons for energy?

Why can't we use the positrons (let off by beta plus radiation) as a source of anti-matter, such that we can collide it with an electron for 100% efficient mass to energy conversion?
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How is the conservation of energy formulated for electromagnetic waves?

The electromagnetic radiation consists of electric $\boldsymbol E(t)$, and magnetic $\boldsymbol B(t)$ fields. The magnetic and electric fields disappear and acquire max values at the same time points....
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Classical Mechanics contradicts Conservation of energy?

Imagine a Stanford torus rotating with 1 rpm so that centripetal/reactive centrifugal acceleration provides about 1.0g of artificial gravitational acceleration inside the ring. The picture below shows ...
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Inconsistency of PE to KE conversion in moving reference frames

Here's a nice trick question to keep you amused over the weekend. A trolley of unit mass with light frictionless wheels is released to roll down a ramp onto a smooth level surface. The PE lost equals ...
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Where does the energy go, when light is blocked by polarisation

I've been looking around about LCD monitors, and how they polarise light. When a pixel needs to be black, the light is "twisted" so that it can't go through the polarising sheet in front. What happens ...
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Does the uncertainty principle violate the law of conservation of energy?

What is the scientific view of the beginning of universe? Quantum fluctuation seems to contradict with the law of conservation of energy. Uncertainty Principle does seem to violate the Law of ...
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computer vs heater coil comparison

If I have a computer and a heater coil that consume exactly the same amount of energy, which would be more efficient at heating my room? This is assuming that they both have the same fans and heat ...
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Explicit time dependence of the Lagrangian and Energy Conservation

Why is energy(or in more general terms,the Hamiltonian) not conserved when the Lagrangian has an explicit time dependence? I know that we can derive the identity: $\frac{\partial \mathcal{H}}{\...
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Quantum perpetual motion

Perpetual motion describes hypothetical machines that operate or produce useful work indefinitely and, more generally, hypothetical machines that produce more work or energy than they consume, whether ...
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What is changing in latest light bulb technologies?

I'm confused with the latest home lightning bulbs. Understanding filament bulbs was easy. For example take 220V, 100W filament bulb: Power = $V^2/R$ Filament gets heated and emits energy in the form ...
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Energy Gain with capacitor?

I have a question about energy gain in capacitors. Assume the following system: As the electron gets accelerated inside the capacitor, it will have more kinetic energy coming out than going in. But ...
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Conservation of Energy in a Capacitor

Consider a parallel-plate capacitor in free space. A negatively charged point particle with initial velocity $v$ passes through the space between the pair of parallel plates (with an initial path ...
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What happens when you shake a can of soda?

Of course the CO2 comes out of the liquid, and it will even build up a lot of pressure if needed. The question is what happens at a molecular level to cause it to leave the liquid?
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Half wave plate and angular momentum

Given: A half wave plate freely floating in space. Circularly polarized light, falling perpendicularly to it. The plate changes polarisation of the beam to the opposite one. Therefore it receives ...
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Why does a rolling sphere stop?

I've just started studying Rotational Mechanics and one thing I'm confused about is that if friction is equal to zero when a body is rolling purely.. then why does a rolling body e.g. sphere.. stop ...
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Energy loss in Capacitors

We have this formula in our textbook for loss of energy when two capacitors are connected together. They mention that it is due to heat dissipation. However, we have not considered any such term in ...
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What happens when two high-speed objects going opposite directions crash into a black hole?

Imagine the following: A-> [] <-B where [] is a black hole, and A and B are two objects of same mass, traveling with the same speed right at the black ...
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Are information conservation and energy conservation related?

as evident from the title, are both, conservation of energy and conservation of information two sides of the same coin?? Is there something more to the hypothesis of hawking's radiation other than ...
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When to use Conservation of Energy vs Conservation of Momentum

I am confused as to when each should be used to solve problems.I am not looking for the answer to this question, but I will include it here so you have all the information about what I am asking about....
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What “happens” to the energy of a photon after it is absorbed?

The simple model of the colour of reflected light from objects (yes, colour perception is a function of the eye/brain) as I understand it is: Firstly I will write what I understand happens - which ...
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Particle sliding on a sphere

I believe most of you probably solved the following problem using energy conservation as shown here. It states A particle starts from rest at the top of a frictionless sphere of radius R and ...
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What is the result of a classical collision between THREE point particles at the same precise instant?

Classical Mechanics is said to be deterministic, a statement that nearly always is followed by that quote from Laplace, something like If at one time, one knew the positions and velocities of all ...
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Work done by the Magnetic Force

The magnetic part of the Lorentz force acts perpendicular to the charge's velocity, and consequently does zero work on it. Can we extrapolate this statement to say that such a nature of the force ...
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Can I use the expansion of the universe to create or destroy energy?

Imagine I have a setup like this: (The objects are not attached to each other. The red objects are much heaver than the black one, and the setup is balanced.) This setup has a non-zero ...
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Energy conservation without action principle?

The normal tagline for energy conservation is that it's a conserved quantity associated to time-translation invariance. I understand how this works for theories coming from a Lagrangian, and that this ...
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extracting energy from cosmological expansion

This question is a more concrete reincarnation of an old question about energy conservation in GR. Are there mechanisms to extract energy from the cosmic rate of expansion? putting some extremely ...
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Capillary action and conservation of energy [duplicate]

When I dip a paper towel in a cup of water the water gets drawn up due to capillary action. How is this reconciled with conservation of energy, as it would seem on the surface that the potential ...
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Does the dynamical Casimir effect generation of real photons create energy from nothing?

The experiments reported in recent years demonstrating the creation of real photon pairs from the vacuum via the dynamical Casimir effect have made me wonder if this process demonstrates the creation ...
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Is there a valid Lagrangian formulation for all classical systems?

Can one use the Lagrangian formalism for all classical systems, i.e. systems with a set of trajectories $\vec{x}_i(t)$ describing paths? On the wikipedia page of Lagrangian mechanics, there is an ...
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Is there a fundamental reason why “dynamic energy storage” is lossy?

Consider springs and moving masses - both can be used to store energy, the spring via tension $E=\frac{1}{2}kx^2$ and the mass via kinetic energy $E=\frac{1}{2}mv^2$. But day to day experience tells ...
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Experiment in the Dark Fringe Raises Questions

I set up a single slit and produced this interference pattern on a distant wall. I then placed another single slit in the deconstructive portion of the pattern. Here's a pic. The slit is between the ...
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What is rotational potential energy?

I don't understand the concept of rotational potential energy. During linear motion, when a force is applied, the work it does gets converted to kinetic energy and there is no change in the ...
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Understanding potential energy

I am self-studying the classical mechanics using the book by Taylor, and I have a question about the potential energy. The book (pg 111) says: If all forces on an object are conservative, we can ...
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Getting energy from decelerating a space-ship

This question comes from another answer. The proposed system is one where the moving space-ship decelerates and some object which is free to move within the ship feels the g forces of this ...
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Bremsstrahlung vs energy conservation

From Wikipedia: Bremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic ...
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Conservation of energy in a non-linear oscillator

I have a homework question about a "non-linear oscillator". I actually have an answer to this question, but the answer I get is stronger than what is needed according to the question. The question ...
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(Why) would unmaintained water heater use more electrical energy?

I'm specifically thinking about lime/sedimentation at the bottom of water heater, and calcification of heating elements (and not possible thermal insulation deterioration). It is very often claimed (...
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Why doesn't a hydraulic lever violate conservation of energy?

Suppose I apply some force on one side of Hydraulic lift where area is less, and the fluid in the lift raises some heavier object on the other side where area is more, Now work done is $Force\times ...