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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Is Energy attracted to Energy?

Newton taught us that bodies with mass attract each other according to the universal law of gravitation (mass-mass attraction) and Einstein taught us that mass and energy are equivalent though his ...
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It's established that universal energy is not constant. But is the net change positive or negative?

Dark energy is density is constant and that's something like 75% of the universe, so I am pretty sure that the net change must be positive. But photons redshift and so loose energy. I assume other ...
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Solving the quantum well gives you eigen energies gives $E_n$, are these energies in conduction band or valence band?

I wonder if the energies $E_n$ that is derived from solving the SWE for the quantum well can be considered as energies in the conduction band or the valence band. In other words is $E_1$ is lowest ...
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Energy of a wave and Planck formula

Especially from this post I understand that the energy of a wave is directly proportional to the amplitude of that wave squared. Therefore, we can determine the total energy of a wave by summing the ...
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Where does the energy within atoms come from?

Most kinds of energy I know about involve a finite measurement and are transferred. In other words, energy in the universe is finite and is not lost or gained but only transferred through space one ...
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41 views

How much energy is required to compress one engine cylinder full of air?

I was recently wondering how much energy would be required to start a car engine by manually cranking it. I was talking to someone who said the largest load you would have to deal with in the ideal ...
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Energy stored in a coaxial cable before reaching breakout field

Yesterday I had a (multiple choice) exam and the following was one of the questions. We have a coaxial cable (basically two coaxial conducting cylinders) with the inner radius of $a$ (variable) and ...
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“Where” does dissipated enstrophy go?

We are all familiar with the kinetic energy dissipation and how it is converted into heat which can either be radiated away or go into the internal energy of the system. In the enstrophy transport ...
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What can take kinetic energy, transform it into potential energy when pressed on, and put back out as kinetic energy when released (besides a spring)?

A spring can only hold so much of the kinetic energy. For example, a 1 cm spring can hold less than 5 J. Is there anything that can hold a large amount of energy but be fairly small?
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Where does the energy of the electric cable come from?

I'm stuck on one really simple example, I can't figure out what's happening to energy here... (This is not homework) Let's consider an uncharged electric cable, we'll model it by an infinite ...
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How much energy does it take to hold a bag? [duplicate]

I'm holding a bag with some mass $m$. The force required for not letting it fall is equal to its weight $m \cdot g_L$ (where $g_L$ is the local gravity, assuming Earth this is $\approx 9.8 \; \text{m} ...
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How can we define the energy stored in a (conservative) force field?

I have come to know from my textbook that energy is stored in the E-field of a capacitor, in the B-field of an inductor and so on. Take the example of an inductor. The derivation bewilders me ...
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How to calculate the electric energy caused by eletrostatic field stored in a region, given $V$?

I have problem in apply the rules to find the energy stored in free space here is the problem Find the energy stored in free space for the region $$0.002<r<0.003m,\quad ...
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1answer
21 views

Why reduced atoms are more energetic

For example, in glycolysis, NAD+ is reduced to NADH and then it's more energetic. Why adding electrons to an atom gives it more energy?
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Possible Lack of Invariance in Two Inertial Reference Frame Scenarios

Two objects are connected together via a ideal spring which is compressed initially. Then the spring is released, the two balls are no longer connected and go their separate ways. Assume no other ...
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Superimposed hydrogen electron states

I have been following an Edx.org course on Quantum Computing. The Prof. has started with a Hydrogen atom qubit, assuming that the electron can only be in the ground state and the first excited state. ...
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1answer
43 views

Conservation of energy during rolling without slipping?

Say there's a cylinder laying on a flat surface, and the surface is rested on frictionless ice. Attached to the surface is an engine that is attached via a string to the center of the cylinder, and ...
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1answer
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What is negative energy?

From what I've read negative energy is based on the Dirac sea concept of virtual particles. Negative energy is referenced by Casimir effects of virtual particle concentration differences between the ...
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Energy for acceleration towards speed of light is relative?

This seems a very simple question - and I guess it will turn out to be so. It's a given that accelerating particles with mass towards the speed of light takes more and more energy (ultimately an an ...
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Kinetic energy of the object, but Potential energy of the system: Why is it so?

Examples from Principles of Physics (by Walker,Resnick,Halliday) will say it better: Let us throw a tomato upward. . .as the tomato rises, the work $\mathbf{W_g}$ done on the tomato by the ...
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The storage of kinetic energy in a flywhell?

I am reading a book on physics demonstrations and problems, and one of the problems deals with a flywheel which rotates at maximum angular speed. The density of the flywheel is uniform and the ...
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1answer
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Heat capacity $C$ at low temperature

The internal energy, $U = Nk_bT$ where $N$ is particle number, $k_b$ is Boltzmann constant and $T$ is temperature. Therefore, the heat capacity $C$ is given by $C=\frac{dU}{dT}=k_b$. However, in ...
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A problem related to Work done by falling bodies : Expert's attention much needed! [closed]

I'm having a lot of trouble with this question, that I've found in my textbook. I've solved it in my own way and it's very simple! But the solution in the book is totally different. It doesn't make ...
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Solar plants and energy conservation

I'd like to apologise if this question is stupid or if it was asked ten thousand times - I haven't found an answer to it. Quite possibly due to mediocre English or abominable knowledge of physics. ...
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1answer
40 views

Necessity of the Notion of Work and Kinetic Energy

I've worked through many example problems in my college physics text in the section on kinetic energy and work. I noticed that if the desired quantity is velocity or force, they can be solved entirely ...
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How much energy and momentum is carried by a mechanical wave?

Let's assume there is a wave spreading across in a substance. Described by some partial differential equation. How can I calculate the momentum and energy carried by this wave? Or maybe the energy and ...
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Why does a gold leaf appear blue if made very thin?

Is this to do with excitation of electrons and emission of photons? Or is it more to do with the structure of the gold I.e. Only small wavelengths being able to pass through gaps between atoms? EDIT: ...
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If kinetic energy is equal to force x distance, does that mean a mass traveling in the vaccum of space keeps getting kenetic energy

This is something I don't understand about it because if the speed remains constant, nothing changes about the objects energy.
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Are sound waves adiabatic or isothermal?

I am doing a presentation on sound waves and I need to know if they are adiabatic or isothermal. I know that they can generate heat, but is the amount of heat created so small that it can still be ...
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1answer
62 views

permanent magnet energy field [duplicate]

Please explain to me this: What is the physical mechanism of the field and atraction force; in which way a force (electro, magnetic or gravity) is transmited between two objects? I will try to ...
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Is the work-energy theorem valid for only particles or rigid bodies as well?

Is the work-energy theorem valid for only particles or rigid bodies as well? Most places where I have read this seem to claim the latter. But an example I thought up has been troubling me. Consider ...
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1answer
247 views

Is force the derivative of energy?

In my lecture today my professor briefly mentioned that force is the derivative of energy but I did not really get what he meant by that. I tried to express it mathematically: ...
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44 views

Where does energy goes when I work out?

Let's say I am using a machine to lift weight by pulling a bar. When I do that, the state of the machine is exactly the same at the beginning and the end of the workout; no heat has been generated ...
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How to compute minimum shallowness of quantum well to have at least one bound state?

Given a potential $V$, how does one compute how shallow the potential can be such that it allows at least one bound state?
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Relativistic kinetic energy versus classic kinetic energy [closed]

I have a homework problem where I am getting the wrong answer and I just want advice on the route I took. So I am told that electrons in a television set are accelerated through a potential ...
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1answer
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time factor of heat increase (specific heat)

It takes 4 joules to bring 1 gram of water up 1 degree, but for how much time - 1 second? If not, how much and how to calculate it?
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Pump sizing for slurry removal

on our farm we need to pump slurry from an underground tank up into an open air temporary slurry holder. I understand that to calculate the power required to pump the slurry up out of the tank is ...
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1answer
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Kinetic energy for rotating round body having its COM not at the centre

Say I have a round object, whose center of mass is NOT in its center. This can be caused due to a hole or non uniform distribution of density. The object rolls on the ground with velocity of ...
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Are there any theoretical limits on the energy of a photon?

Is there any lower or upper limits on the energy of a photon? i.e. does the mathematical framework we currently use for Quantum Mechanics blow up when a photon surpasses a certain upper limit of ...
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1answer
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In special relativity what is the energy of a macroscopic body?

Is the energy of a macroscopic body in special relativity still given by: $E=\gamma m c^2$? If so why do we not need to consider the motion of the individual particles that make it up? Is this because ...
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Has $E=mc^2$ been experimentally verified for macroscopic objects with potential energy?

In relation to this question: What is potential energy truly?, I'm wondering if $E=mc^2$ has been experimentally verified to hold true for macroscopic objects with increased potential energy? I'm ...
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What are high energy photons?

I have often read the term High Energy photons, does this mean that there do exist photons with low energy? Aren't they supposed to have constant energy according to $E= h\nu$?
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Is it possible to calculate the energy of a spark?

I am not a physicist, but this question came to me and I need to answer it, and I think that you guys can help me. Actually my question is just like in the title. Is it possible to calculate the ...
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1answer
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Increasing the efficiency of photovoltaic power cells

Given that solar cells use one or more semiconductor materials to convert light to electricity, and that the efficiency of that conversion is dependent on the material and the wavelength of the light, ...
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1answer
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What happens to kinetic energy of an object when i stop its motion? [duplicate]

Assume an object is moving and it is stopped at a point . Now kinetic energy will transform into some other energy . what can be those energies and on which cases these will happen respectively ? ...
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Angular momentum effect on quantum energy

I'm doing some computational research into quarkonium states and I've written a code that determines energy levels by finding a solution to the Schrodinger equation for a given angular momentum. I.e. ...
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2answers
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Does a reflecting object move faster than one that does not reflect?

Suppose you have two objects with the same mass and shape, such as a ball, and the only difference between the two is that one reflects light and the other does not. Would one object's velocity be ...
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1answer
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Does radiating heat to outer space through the atmospheric transparency window have any impact on heat loss?

I was reading this article: Passive radiative cooling below ambient air temperature under direct sunlight The authors are improving upon the age-old idea of reflecting incoming sunlight while ...
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Elementary questions about where energy is stored in solids

I have the following question, I have no relation to the study of Physics in any way, but the question has been teasing me for some time. Please accept my poor physical terminology. Here we go... ...
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Do we really not know why atoms 'decide' to produce a photon?

I was watching the Cosmos documentary where Neil deGrasse Tyson explained how certain energy photons get absorbed by an atom, which causes the electrons of that atom to climb into a higher energy ...