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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Change without Work

Is it possible for the state of the universe to change without any work at all being done? Or, more specifically, is it possible for some physical change to occur without work being done?
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Work done by tension when a particle is tied to a vertical cylinder using a thread and given a velocity perpendicular to the thread

For a particle attached to a rope moving along a circle that has the length of rope as radius, the tension provides centripetal force and work done by tension is zero since velocity of particle is ...
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Capacitor Charging another Capacitor to a higher Voltage Level?

Ok, I admit I don't have a degree in physics and I'm not well versed in a lot of calculus formulas and sometimes I don't understand formulas when I see them so I usually skip over them, yes I know ...
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Is there an example of a situation where you need a continuous spectrum?

If you had a hydrogen atom you could say that you want to be able to ionize them. But if you then add the potential due to the earth, e.g. ...
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What was the motivation behind the work formula?

Surely there must be a reason we decided to use this as a metric for mechanical energy.How was it developed and what made it more acceptable than other work formula candidates (Like force over time, ...
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Why does electrostatic energy not obey superposition principle? What does the extra energy term account for?

Energy stored in an electrostatic field is given by $\frac{\epsilon_0}{2}\int\text{E}^2\text{d}\tau$ where the integration is over all space.If $\vec{\text{E}_0}=\vec{\text{E}_1}+\vec{\text{E}_2}$ and ...
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Question related to charged concentric conducting shells

There is a charged conducting spherical shell given a certain positive charge.Another larger spherical shell is given the same negative charge.The first shell is now placed inside the second shell so ...
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37 views

Entropy reversal in magnets

Entropy is an irreversible phenomenon without any energy supplied to reverse it. I was reading about paramagnetic substances and how dipoles align inside them on application of magnetic field. My ...
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Why does the sun make me feel warm?

For a while I thought that the reason I felt warmth from the sun was because my skin was being hit by photons, but then I realized that photons also hit me when I take an X-ray, but I don't feel any ...
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Open systems (Conservation of Energy): is a temperature change possible if a tube is thermally isolated?

Full question: consider an perfectly thermally isolated cylinder. Water is flowing through this cylinder from the entry to the exit. Is it possible for the water flowing through the exit to have a ...
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Given the Newton constant G, the speed of light c and the Planck constant h, construct an energy of the system [on hold]

How do I use dimensional analysis to construct an energy for the system given the Newton constant G, the speed of light c and the Planck constant h? I don't know of any energy formulas containing the ...
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Controlling the flow of powder from a container, based on grams selected [on hold]

was wondering how you would control the flow of powder from a container (with a small cylindrical dispenser at the bottom). The container is filled with powder. As the container drains, less ...
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Work done by flow of a fluid

I'm looking at the power a flow of steam when used by a turbine. I originally assumed that the kinetic energy of a moving section of steam was equal to (let $V$ be volume, $\rho$ be density, and $v$ ...
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Pouring oil on choppy water to calm it , does it work and if so how?

Near where I live, local fishermen often bring cans of castor oil with them, to calm the water around their boats, if they feel bad weather is due. They claim this method of sea calming works, ...
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Battery charging a Capacitor

Ok I know there is a similar question already on here, but mine takes it just a step further. What I would really love to know is exactly how much energy is being transferred from a battery into a ...
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3answers
166 views

Is there an intuitive explanation of the work formula?

Upon learning calculus, I decided it was time to derive all of classical mechanics to give myself a good understanding of physics. What I found was that, while trying to do so, I would need some ...
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Matter to energy conversion

Okay so I have a question, during a nuclear explosion or particle/antiparticle annihilation, matter is converted into energy. How do I determine if a from a explosion will come lets say a small ...
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+50

Peskin eqn 7.2 contradiction

They state $\langle\Omega|\phi(x)|\lambda_p\rangle=\langle\Omega|e^{iP\cdot x}\phi(0)e^{-iP\cdot x}|\lambda_p\rangle$ where $|\lambda_p\rangle$ is a state of momentum $\textbf{p}$. They then rewrite ...
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Kinetic and potntial energy of an airplane

If an airplane is flying horizontally at an altitude of H, and with a constant velcotiy of V(no air resistance) horizontally. Is there a relationship between the potential energy of the airplane due ...
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Work done by a constant force

Is a constant force always conservative? What I tried (Assuming $F$ and $dr$ to be in same direction ) $W=\int Fdr$ Now since $F$ is constant hence it becomes $F \int dr=F \times ...
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Why is there no $1S_{-1/2} $ state of the hydrogen atom?

Heyho, i found this term scheme for the hydrogen atom: https://upload.wikimedia.org/wikipedia/commons/9/94/Wasserstoff_Aufspaltung.svg I was wondering why there is only a $1S_\frac{1}{2}$ state and ...
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1answer
45 views

Why do systems with a fixed gear-ratio still use gears?

From my understanding, there are two uses of a gearing system: to change the speed of output rotation (trading it with torque), and to change the axis of rotation. Now, in a car, for example, it is ...
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Power of a wave in a string

My text of physics, Gettys', shows how the energy, both kinetic and potential, of a small element $\Delta x$ of a string, through which a wave (whose wave function is $y:\mathbb{R}^2\to\mathbb{R}$, ...
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Could it be that all energy is seeking ground? [closed]

Edited version. If an electrical charge like a lightning bolt seeks ground (think of a lightning's rod's purpose), and if all matter is seeking ground (gravitational pull), and if all energy and all ...
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Kinetic energy of a falling object in water

I am considering the impact energy of an object falling vertically downward with impact velocity $v$ in water. The mass of the object is $m$, and the hydrodynamic added mass from the displaced water ...
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How can Kinetic energy formula and the Work formula be derived without assuming the other to be true?

After searching how to derive the formula for Kinetic energy, I found that most derivations required the use of the "work" formula. After searching how to derive the work formula for a bit, I found ...
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What's the difference between vacuum particle-antiparticle annhiliation and regular annihilation?

Why does vacuum particle-antiparticle creation and annihilation result in nothing rather than photons? What is the difference between that and regular annihilation that does result in photons.
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Kinetic energy as $\pi k_B T$

In the derivation of the Thermal De Broglie Wavelength on Wikipedia, I come across the following: "In the nonrelativistic case the effective kinetic energy of free particles is $E_K=\pi k_B T$ ...
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How to calculate the work of the electrostatic forces in a parallel-plate capacitor?

The expression of the energy stored in a parallel-plate capacitor is: $$U = \frac{e_0\cdot A \cdot V^2}{2d}$$ with $e_0$ the vacuum permittivity, $A$ the surface of the capacitor, $V$ the applied ...
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Is it possible to measure temperature using sound?

Is it possible to measure the temperature of something using sound, by blasting sound waves at solid objects, liquids and gasses, then measuring how much sound is reflected or absorbed, to give an ...
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If there is no definite speed in the universe, only relative speeds, how does energy increase when velocity approaches the speed of light?

Is the concept of energy increasing as it approaches the speed of light based on the fact that this is only true relative to the observer? Lets say, there's a scenario where a person in a rocket ...
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1answer
65 views

Why is gravity sensitive to absolute energies?

In QFT absolute energies play no role in the physical set-up, only relative energies (i.e. energy differences) are important. However, in general relativity this doesn't appear to be the case, I've ...
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4 momentum in particle physics, collision of positron and electron

If I have a positron striking an electron at rest to create 2 pions( + and -) and I want to calculate the minimum kinnetic energy that the electrons can possess to create these pions, then the created ...
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Differentiating between standing waves (stationary waves) and progressive waves [closed]

The question is ''Differentiate between standing waves (stationary waves) and progressive waves?" I do not understand what exactly they would like to hear. Should we talk about the frequency, ...
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Along with electrons, do molecules also directly take in photon energy from a source?

Let's take a black object for example. When waves of light (whether from the sun or a lamp) come into contact with the object, is it just the electrons (not the molecules) of the object that directly ...
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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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What's faulty in my reasoning about energy and momentum in this problem?

The question (from Sparknotes SAT Physics): An athlete of mass 70.0 kg applies a force of 500 N to a 30.0 kg luge, which is initially at rest, over a period of 5.00 s before jumping onto the ...
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What is the relationship between the velocity of an object and its Sound Intensity Level in an inelastic collision?

Assuming the mass, acceleration, and height of the dropped object are constant and the area of the measured sound wave is constant, would the relation be linear, exponential, logarithmic, square root ...
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1answer
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Does energy exist? [closed]

None of this is going to be rigorous or have a right answer, so please don't take it as though I'm demanding one. I'm just interested in everyone's thoughts. It feels to me (and possibly you) that ...
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66 views

Work - constant or zero speed

Say I'm riding a bicycle at speed $v$. There's air that causes drag force, let's suppose it's constant, equal $F$, and it doesn't change with speed (we know it does increase quadratically with speed, ...
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Forster rate dependence on temperature

In the Forster rate expression, what values are dependent on temperature?: Forster orientation factor, k^2 the refractive index of the solvent, n The spectral overlap, J the quantum yield of the ...
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1answer
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How can I interpret $P(t) = \frac{1}{Q(t)} \frac{dW(t)}{dt}$ physically?

Generally speaking, one can calculate the work, $W$ (energy) required to compress a volume, $V$ of gas by integrating the pressure-volume 'loop' in a phase-space as $$W=\oint PdV$$ where $P$ is the ...
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Internal energy of an ideal gas as a function of volume

Okay so I've been reading a bit on Thermodynamics and I found something that I couldn't wrap around my head. For an ideal gas, the change in internal energy is equal to $$\Delta U = Q + W$$ And ...
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Conservation of Energy vs Conservation of Momentum in Rotational Dynamics

It is clear to me why angular momentum is always conserved, and how in some cases energy is not necessarily conserved within the system (in those cases where bodies deform, or friction is involved). ...
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How did physicists arrive at conclusion that the product of mass and velocity is equal to momentum?

How did physicists arrive at conclusion that the product of mass and velocity is equal to momentum? What is the intuition behind $p=mv$? I had trouble finding any sources that state the actually ...
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work and energy dilemma in an inclined [closed]

An example problem done in my book: A driver was driving a car of mass 1000 kg through an inclined plane which makes an angle of 30° with plane with a velocity of 25 m/s . Then he saw a boy 50 m ...
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How much of this 'Slow Death of the Universe' was predicted?

In short: the energy generated in today's universe, is "about half of the produced two billion years ago" and found that "this fading occurs at all wavelengths" as yesterday news: ESO - Charting the ...
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Could light be limiting towards a more fundamental speed limit? [closed]

Could it be possible (as in strictly speaking, no evidence going against this) that light could be limiting towards a more fundamental speed, and that light truly does slow down when a mass goes a ...
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What is an EM wave? [closed]

How does an EM wave carry energy? What is an EM wave? (Is it a collection of photons?) What are the mechanics behind it? I am an engineer and I've been taught to think of light (light is my area ...
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Mass, Spin, Internal Energy and 1-Particle States in Galilean Quantum Mechanics

I have been reading an article discussing the unitary representation of Galilean group and non-relativistic quantum mechanics. The link to the article is given below. http://arxiv.org/abs/1107.2442 ...