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 the [energy-conservation] tag if it is specifically about its conservation.

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34 views

Noether's theorem and energy in four-momentum

In Newtonian physics, momentum and energy are often treated as distinct entities, which happen to be separately conserved. In relativity, energy is regarded as the "time" component of the ...
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What is the energy density of Cf-252 compared to Pu-239 and U-235? [closed]

Cf-252 has a critical mass of 2.7 kg compared to U-235 (52 kg) or Pu-239 (10 kg). one Cf-252 atom releases 3.7 neutrons. One U-235 atom releases 2.5 neutrons and one Pu-239 atom releases 2.9 neutrons.
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In which cases you use free energy to calculate pressure?

From thermodynamic identity you get: $$\left(\frac{\partial U}{\partial V}\right)_{S,N} = -P$$ But with Helmholtz free energy $F = U-TS$, we can also get pressure from this equation: $$\left(\frac{\...
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If kinetic and potential energy are relative, how do they exert a gravitational force? [duplicate]

I am imagining two bodies flying through empty space near the speed of light relative to their (distant) surroundings. Let’s say they are a bowling ball and a tennis ball. They are not moving with ...
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If we know the location of object at different times, what more can we find out from it?

I'm brushing up on my physics, and I've been struggling with this problem in my head. I hope you can help. Let's assume that we have some object/particle that only moves along a single axis ($x$). We ...
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Where does the energy go? [duplicate]

Suppose a typical case of a girl dancing and after sometime changing her $I$ (moment of inertia to rotate) faster, my doubt is in this case there is a non-zero difference in initial and final total KE ...
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Shouldn’t the net Poynting vector at a resistor or incandescent light bulb be zero?

I’ve read that the Poynting vector points into an incandescent light bulb at all instants of time, whether there is direct/constant or alternating current through the wire. This means the bulb is ...
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Why do we have to do work to remove iron nucleus from an inductor?

Why do we have to do work to remove iron nucleus from an inductor? Since the energy associated with the inductor is $E=\frac{Li^2}{2}$, and $L$ is proportional to relative magnetic permeability $\mu_r$...
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Equation for enegy loss from evaporation/sublimation in vacuum?

Imagine you have a solid that you are rapidly heating inside a vacuum chamber with some kind of thermal heater or a laser such that the material begins to evaporate or sublimate. Obviously radiative ...
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Can you calculate kinetic energy with the rocket equation?

Is it possible to calculate kinetic energy with the rocket equation? I am doing a quick school project on the efficiency of a rocket. What if we write velocity with the kinetic energy formula. $E_k=\...
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Energy used to boil a full kettle once vs boiling enough water for a cup multiple times

I have an electric kettle and a thermal carafe. The carafe itself is not heated, it's basically just a large thermal flask. I've noticed that when hot water is kept in the carafe, the water stays ...
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Do states with infinite average energy make sense?

Do states with infinite average energy make sense? For the sake of concreteness consider a harmonic oscillator with the Hamiltonian $H=a^\dagger a$ and eigenstates $H|n\rangle=n|n\rangle$, $\langle n|...
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Conversion of potential energy of charge into kinetic energy

Suppose I have a battery of voltage v which can flow $Q$ coulomb of charge through a metalic conductor When voltage is applied across this conductor the electrons inside it get accelerated and move ...
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What energy range is considered as a "high-energy" or "low energy" cosmic ray?

I have been studying cosmic ray muons lately, and I'm having trouble with making sense of all the eV values. What is the approxamate range (in eV) for "high-energy" and "low energy"...
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Efficiency of electric heaters

I was reading this question. A person was asking what is the efficiency of an electric heater, and to sum up, people were answering "All resistive heating, can be considered to have 100% ...
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Why is it easier to ship goods by water than by land? [closed]

It is much, much cheaper to ship goods over water than over land. (Specifically, I'm referring to the cost per unit mass of cargo per unit distance transported. I'm also assuming the large-quantity ...
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Mass: a form of energy [closed]

Einstein's $E=mc^2$ and law of conservation of energy implies that mass is a form of energy, but if it is a form of energy then why can we freely interact with mass while the other forms of energy are ...
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Comparing 2 different uncertainty of KE of particle equations

I come across the uncertainty of KE of a particle in a box as: $$\Delta KE = \frac 12m[2v\Delta v\ +\ (\Delta v)^2]$$ I try to apply this equation by assuming an electron was placed in a 1-D box has a ...
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With how much energy can a charged black hole launch a electron away with?

Lets say with have a black hole with a strong negative charge, then a say a neutron falls towards it. Just before the neutron reaches the black hole's event horizon it undergoes beta decay, this ...
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How nucleons get excited? [duplicate]

Gamma rays can be released when nucleons at higher energy states fall down to lower energy states, but how do nucleons get that much high energy to release gamma rays?
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How can radio radiation ionize gases?

Reading the operational mechanism of Variable Specific Impulse Magnetoplasma Rocket (VASIMIR), there is a section which statesI: "The propellant, a neutral gas such as argon or xenon, is injected ...
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Degeneracy of energy levels for a "particle in a rectangular three-dimensional box"

In class we studied the degeneracy of energy level for a particle in various "boxes", including a rectangular three-dimensional one. Whenever we tried to quantify the degeneracy of the ...
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How is gravitational potential energy $mgh$?

I know the derivation that $W=Fd$, hence $F=mg$ and $d=h$ so energy gained by the body is $mgh$ considering the body on the ground to have $0$ gravitational potential energy. But the definition of ...
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After Todd Rider's thesis, why are companies still working on aneutronic and non-equilbrium fusion?

Todd Rider's thesis ("Fundamental limitations on plasma fusion systems not in thermodynamic equilibrium") clearly states in the abstract that: ...In virtually all cases, [the] minimum ...
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How do Photons Move Solar Sails?

Ok so, this is something that I've been researching for quite some time now and, no matter where on the internet I end up at, no matter how many sources I visit and read, this still doesn't make even ...
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Photon energy and transmit power of an optical transmitter

I have a big doubt. I have found an answer of other community a question about a relation between wavelength and transmit power. My doubt is according to the reply If you have a low wavelength and ...
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Will a rocket need the same fuel and energy going 1 g upwards on Earth as a rocket 1 g in space?

Will the gravity of Earth change the result (making the rocket on earth need more fuel and energy than the one in space)? Or it is the same?
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Energy loss in a circuit during critical damping [closed]

I would like some intuitive explanation regarding impedance in an $RLC$ circuit. So far, my understanding has been limited to underdamping situations where both resistance and reactance are present. ...
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What is the proof of $g_{xx} = a_{xx} - v\kappa a_{vx}^2$?

I am reading this paper (Phase and Conformational Behavior of LCST-Driven Stimuli Responsive Polymers, by David Samuel Simmons, PhD dissertation, UT Austin) and on Page 8, the author writes $$g_{xx} =...
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A rolling stone on a rollercoaster

A ball rolls from point $A$ ($v_0 = 0 \; m / s$) to the right along a friction-less path. We can then calculate the speed of the ball at the points $B$ and $C$ using the law of conservation of ...
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Why is there Energy? [closed]

I am a layman and this is my first ever question! Although, it is simplistic in nature, I hope that there is a beauty in its simplicity which could give a simple answer. Alan Watts, the philosopher, ...
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Could you use a chaotic pendulum to generate energy? [closed]

Could you use a chaotic pendulum to generate energy?
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Relation between kinetic energy and momentum in relativity

In a recent discussion a friend of mine claimed that kinetic energy ($K$) an momentum ($p$) in relativity can me expressed by $$K=\frac{p^2}{(1+\gamma)m} \tag{1}$$ This equation if holds, has some ...
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Franck-Hertz experiment

In the Franck-Hertz experiment why are the peaks much sharper than the troughs? Also our data analysis suggested that the distance between the peaks is smaller (nearer to $4.9 \ $ eV ) than the ...
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Kinetic energy of an electron inside the sun [closed]

I have to tell if an average proton or electron at 15 million degrees kelvin be treated by relativistic mechanics. The criterion is that the kinetic energy differ from the classical kinetic energy by ...
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Potential energy at a certain height

If an object is levitated above the surface of the earth with constant velocity so the net work done is equal to zero then the object must possess no potential energy =0 at a certain height $h$ ...
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If Potential Energy Is only defined For Conservative forces, can Non-conservative forces Increase spring potential?

Consider Two Cases Of collision When Energy is conserved before And after collision,we can say That The forces that acted between them were Conservative. During deformation their kinetic converted to ...
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Saturated vapor and liquid densities (VLC curve) from Helmholtz free energy equation of state

I have been reading about the statistical associating fluid theory (SAFT) which computes the Helmholtz free energy ($A$) from the SAFT EoS (a hell lot of equations) for a molecule of interest, and is ...
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Current from guiding center drift under gravity

Wikipedia says: when there is a force on the particles perpendicular to the magnetic field, then they drift in a direction perpendicular to both the force and the field. This is called "guiding ...
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What happens to Kinetic Energy in Perfectly Inelastic Collision?

I’m going over a chapter on linear momentum in my physics course right now and am somewhat puzzled with what happens with some of the kinetic energy that is lost in a perfectly inelastic collision. ...
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Confusion regarding ball bouncing off a wall with respect to Newton's Third Law

This is more of a confirmation post whether my ideas are correct or not. Suppose a ball of mass $5 kg$ travelling with uniform velocity $5 ms^{-1}$ through vacuum hits a block of mass $200kg$ with ...
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Why work can't be extracted from thermal energy?

As I understand it, thermal energy is simply molecular agitation, and therefore nanoscopic mechanical energy. So why can't this form of energy be used to extract work? I consider here an isolated ...
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Energy in nuclear fusion - mass change vs potential energy explanation

Trying to understand how the energy related to nuclear reactions. In particular why particular fusion of small nuclei release energy. So far, there seems to be two explanations which I have ...
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Could photons generated from the many trillions of stars be some how contributing energy to dark energy? Where do they go and what becomes of them?

What is dark energy and can photons contribute to it. When the total number of stars start to diminish will dark energy perhaps begin to diminish as well. What does happen to all those photons with ...
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Kinetic Energy of wavefunction

Suppose we have been provided the form of some wavefunction on a graph, but not the exact mathematical expression of the wavefunction $\langle x|\psi\rangle $. Now I'm asked to find the average ...
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How can we make the energy levels of an one-dimensional infinite square well potential equispaced?

We know that the energy values for an one-dimensional infinite square well potential is given by $$E_n = \frac{n^2{\pi}^2{\hbar}^2}{2ma^2}$$ where $a$ is the width of the well. Now, as we can see that ...
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Why is the half of the total height taken as effective height in case of certain mathematics regarding gravitational potential energy?

A problem gives the depth and diameter of a well full of water, states that a pump can empty the well within a certain given time, and asks to find out the power of that pump. In the solution, while ...
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Confusion Regarding Reaction Forces and Newton's Third Law of Motion [closed]

So I was confused about reaction forces for a LONG TIME and I have finally been able to clear some of my confusions, but the concept is still not totally clear to me. Can someone confirm whether my ...
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Question regarding Energy as function of magnetic field

We have an exercise where we have to work with energy as a legendre transform in the form $$E(B) = \tilde{E}(m) - mB$$ where $\tilde{E}(m)$ are the lowest energy of states with given magnetization $m$....
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Mechanical work done when standing up

Suppose you squat. As you stand up, about how much mechanical work do your muscles do? Your weight is $500 N$. Your center of mass moves at a constant speed from a height of $H_{CM}=0.2 m$ to $H_{CM}^′...

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