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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Can the internal energy of a system change by itself?

Thermodynamically it is not possible. Internal energy of a system can not change until and unless an external work is done on the system or by the system. But I came across a very strange process of ...
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1answer
175 views

Can electron jumps to higher energy state without absorbing photon?

The shorter the wavelength of the electromagnetic wave the more energy it carries, when it hits an atom and gets absorbed the electron gains kinetic energy and jumps to higher energy state. Are there ...
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1answer
355 views

How does energy exist if it can't be created or destroyed? [duplicate]

I saw this in a science book and it's been bugging me lately because how can energy exist if it can't have any creations? How do eternal things exist if they have no beginning or ending, like energy?
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How is it possible that the energy needed to stop a train is the same as the (chemical) energy in a pack of chocolate cookies?

Today my friend told me something that blew my mind completely. He said: The energy necessary to stop a train is equal to the energy in a pack of cookies. How is that possible? Is he right? I'm ...
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1answer
115 views

Differentiating displacement with respect to speed in order to obtain time

I have this problem where I am trying to calculate $d(t)$ and $v(t)$ of a mass m on a spring, dropped from a displacement $A$, without using anything else than Hooke's law and energy calculations. ...
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40 views

Expression deduction for energy density per wave length

Energy density per frequency is defined by Planck formula as: $$u(\nu,T)=\frac{8\pi h}{c^3} \frac{\nu^3}{e^{\frac{h\nu}{kT}}-1}$$ The relation between wave length, $\lambda$, and frequency, $\nu$, ...
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1answer
171 views

Does a rotating object have more inertia, mass and gravitational pull?

When an object is rotating on an axis, it has stored rotational energy in it. Since energy and mass are related, does this stored rotation increase the mass of the object? And if so, will it be harder ...
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2answers
138 views

Can dark energy be converted into energy source? [duplicate]

Dark energy is an unexplained phenomena. But can it be used as a energy source?
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1answer
60 views

Is kinetic energy related to chemical potential?

I wonder if the kinetic energy written as $\frac{d\mathbf x}{dt}\cdot d\mathbf p$ is related to chemical potential? I ask because if I use $\mathbf p = m \frac{d\mathbf x}{dt}$ as a constitutive ...
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2answers
69 views

Does decomposition of motion rely on Pythagorean theorem?

As an example, when analyzing a simple projectile motion with initial horizontal velocity in Newtonian mechanics, I'm enabled to decompose the projectile motion into the vertical and horizontal ...
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1answer
289 views

Increase in kinetic energy of a system when there is no external force

If a man starts running on a boat with an acceleration $a$ with respect to the boat, there is no external force that acts on the Boat+Man system (assuming friction due to water to be zero and ...
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1answer
399 views

Work functions of ceramics/insulators

Is anyone aware of or know of a good source or means of estimating the work function of a ceramic material? Typically, work functions are given for pure elemental metals, rather than for compounds, ...
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Which travels further, a football or a soccer ball, when thrown?

I recently answered this question: Laminar Versus Turbulent Flow and it caused me to think of the following situation. Given that an (American) football weighs between Football 400 to 430 grams, and ...
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1answer
495 views

Kinetic energy (KE) in atomic orbital

Within an atomic orbital, electrons must obviously have relative differences between points in space due to potential gradient. But there is kinetic energy as well. If we choose a particular point as ...
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3answers
663 views

Have the Rowan University “hydrino” findings been replicated elsewhere?

In 2009, Rowan University released a paper claiming to replicate Blacklight Power's results on energy generation using hydrino states of the hydrogen atom. The paper (link now dead) appears to ...
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2answers
138 views

Source of energy for magnetic work?

I set two magnets on my desk such that they are experiencing attraction to each other, but due to friction with the desk, are just outside of the distance where they would snap to each other. Then I ...
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2answers
2k views

Why can't I evaporate water without wind, just heat? (not boiling,evaporating!) Or can I?

So here is the thing, I searched all over the internet for this but all the sources say that I need wind because the process of evaporation goes as follow: Water particles at the top layer with ...
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2answers
166 views

Work done by reaction forces between objects

Assume that there are no friction forces. If we had a particle sliding down a wedge that is free to move on a smooth surface, why do we ignore the work done by the reaction forces on both the particle ...
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4answers
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What is negative Energy/Exotic Energy?

So I have been researching around a little as I am highly interested in Astrophysics and I came across an energy I have never heard of before; negative energy also commonly known as exotic energy. Now ...
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3answers
488 views

Is space-time a special form of energy?

I know space-time can be influenced by matter and energy, so it must be somehow mingled in with the mix of it all, but does space-time have a fundamental particle? Can we make a little bit of space-...
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1answer
160 views

When i tear a paper, can i accidentally create nuclear fission?

I know it's a stupid question... but when I tear a paper i may coincidentally split it and create nuclear fission. When i tried experimenting, by tearing the paper for HELLKNOWS how many times, I ...
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1answer
443 views

Water bottle rocket: Where does the energy go without water?

In Portland's OMSI there is a hands-on water bottle rocket station. (https://www.youtube.com/watch?v=cdtmVY76_PQ). The rockets are normal PET bottlers. The visitors fill their bottle with an amount of ...
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0answers
56 views

Does Light have Weight?

If I light a torch and hold it against a wall, will the wall feel any force? If yes, what factors does the magnitude of such a force depend upon? (Area of the wall covered by the beam, intensity of ...
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How to calculate the thermoelastic energy loss when applying displacing force

I'm trying to calculate the motion of an arrow (or similar shape) in orbit. My understanding has taken me to the following assumptions: There will be drag on the arrow (the drag in low earth orbit ...
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215 views

Work done by flow of a fluid

I'm looking at the power a flow of steam in 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 $v$ be velocity ...
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5answers
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A force is exerted on a body, kinetic energy increases but no work is done by the force. Why?

After reading the article, I was totally perplexed . I was reading the External forces and internal energy transfers in Principles of Physics by Resnick,Halliday,Walker. It goes like that An ...
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2answers
360 views

Carnot cycle: What happens to the rest of the energy?

In a Carnot cycle, the work potential is $$W =Q_\text{in} \left(1 -\frac{T_0}{T_\text{R}}\right)$$ where $T_0$ is the temperature of the sink, $T_\text{R}$ is the temperature of the source heat ...
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Peskin eqn 7.2 contradiction

They state $$\langle\Omega|\phi(x)|\lambda_{\bf p}\rangle=\langle\Omega|e^{iP\cdot x}\phi(0)e^{-iP\cdot x}|\lambda_{\bf p}\rangle \tag{7.4}$$ where $|\lambda_{\bf p}\rangle$ is a state of momentum ${\...
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5answers
865 views

Integrating factor $1/T$ in 2nd Law of Thermodynamics

How would you prove that $1/T$ is the most suitable integrating factor to transform $\delta Q$ to an exact differential in the second law of thermodynamics: $$dS = \frac{\delta Q}{T}$$ Where $dS$ is ...
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2answers
231 views

What does negative energy imply?

I know energy comes in a number of forms and every form of energy is defined uniquely. But if I had to give a broad definition of energy, I would define it as "ability of a system to perform work." ...
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1answer
209 views

why doesn't liquid metal vaporize in a vacuum?

I am wondering why molten metal in a vacuum of electron beam and machines never turns to gas like liquid water does when exposed to a vacuum.
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how can $\frac{kqq_o}{r}$ be $U$ for both a particle and a system of two particles?

If there is a charge $q$, at a distance $r$ from it there is a voltage and the potential energy of a test charge $q_o$ would be $\frac{kqq_o}{r}$. Now if I construct a system of two charges a ...
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Electricity from vacuum [closed]

Can I generate electricity in complete vacuum without using any conventional sources? Using Casimir theory, according to him, he kept two plates nanometers apart in a vacuum. According to the law of ...
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193 views

Do individual rays of light lose energy via the inverse square law?

We've all heard of the inverse square law, but apparently that refers to the flux or intensity or number of photons hitting an imaginary surface area. This is not exactly what I want to ask about. I'...
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2answers
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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
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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 [closed]

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 ...
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Is Keshe's technology for real?

http://www.youtube.com/watch?v=UrN99RELqwo (Video Title: "Keshe Foundation Promo Intro Video (english with multiple subtitles) === PLEASE SHARE ===") In this video they claim that they can create ...
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1answer
69 views

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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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. $$V=\frac{-Gm_eM_\oplus}{\sqrt{(x_e-x_\oplus)^2+(y_e-y_\oplus)...
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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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3answers
169 views

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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288 views

Simple: What do these ballistic impact numbers represent?

I bought physics for game programmers, found here, today to study ballistic impact, but I'm confused as to where a few numbers are coming from. I'm basically looking to understand what the values ...
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2answers
175 views

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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199 views

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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5answers
6k views

Why doesn't light kill me?

Why does each individual photon have such a low amount of energy? I am hit by photons all day and I find it amazing that I am not vaporized. Am I simply too physically big for the photons to harm me ...
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1answer
63 views

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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1answer
860 views

Can negative energy explain black holes?

I think black holes are peculiar and I want to know if there is a relation to negative energy. Does negative energy have positive mass and therefore can explain the nature of black holes? IS there any ...
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3answers
701 views

Definition of “first excitation energy”?

I was solving a problem but I didn't know what the term "first excitation energy" means. The first excitation energy of the hydrogen atom is $10.2\text{eV}$. Calculate the speed of the slowest ...
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3answers
258 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 ...