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 the integral of $dK$ equal the work done?

$$\frac{dK}{dt} = \frac{d}{dt} (1/2 mv^2) = mv \frac{dv}{dt} = F\frac{dx}{dt}$$ $$\implies dK = F. dr$$ Also, work is defined as $W = \int F. dr$. So does this imply integral $dK = W$?
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90 views

Why isn't $E=mv^2$ a general formula for energy?

This is the usual derivation of the formula for kinetic energy: $$\begin{align} \mathrm{d}E &= Fv\,\mathrm{d}t \\ \mathrm{d}p &= m\frac{\mathrm{d}v}{\mathrm{d}t}\mathrm{d}t \\ \mathrm{d}E ...
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31 views

Combination of quantum numbers for a particle in a 3D box

For a second excited state, the three combination of quantum number corresponds to $$n_{1}=2,n_{2}=2,n_{3}=1$$ or $$n_{1}=2,n_{2}=1,n_{3}=2$$ or $$n_{1}=1,n_{2}=2,n_{3}=2.$$ This is from the text ...
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1answer
44 views

Which electron is first ionized $n=2,\ell=1, m=?$ [closed]

Eletrons in atoms are described by n,l, m, s quantum numbers? For noble gas 1s2 2s2 2p6 which is the first electron that will be ionized n=2, l=1, m=?, s = ?
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29 views

The relation between work and time

Work equals the force acting on an object times the distance that object moves. But I am a bit confused about the definition of the distance. is it the distance the object moves under the effect of ...
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1answer
97 views

How much energy would be required to make one tea cup full of Earl Gray tea at 100F?

On the TV show "Star Trek: The Next Generation", Captain Picard is often pictured using a replicator to materialize a cup of "Earl Gray tea, hot". Besides wondering what they do with all the empty ...
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36 views

Is a Battery to store photons possible?

@MISC {55768, TITLE = {Why can't we store light in the form of light?}, AUTHOR = {AldCer (http://physics.stackexchange.com/users/20409/aldcer)}, HOWPUBLISHED = {Physics Stack ...
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2answers
41 views

How much energy could be obtained by freezing water?

I know with our current technology energy is required to freeze water. But if the theories are correct we should be able to freeze water by extracting energy correct? How much energy could be ...
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0answers
21 views

What is the form of energy that is forming while i am holding a suitcase an not moving at all [duplicate]

If I am lifting a heavy suitcase and I am not moving at all. Abviously I am not doing work because I am not moving or rising the suitcase up and down. But abviously I am losing chemical energy since ...
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0answers
19 views

What is the form of energy that is forming while lifting a suitcase [duplicate]

If I am lifting a heavy suitcase and I am not moving at all. Abviously I am not doing work because I am not moving or rising the suitcase up and down. But abviously I am losing chemical energy since ...
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4answers
121 views

Turbine like device in space spinning with KE and converting to Watts. Acceleration is constant?

I have a question that I can't seem to get my head around. I'm really rusty on my physics, but I'm sitting here trying to think this out, but I'm missing a really big part of the equation. So lets ...
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2answers
77 views

Does a man run a same distance with different velocities spend the similar energy?

To simplify the question we assume the same man run on a straight and flat road with constant speed. Personally I don't believe the man would spend a similar energy. But how to prove it in physics? I ...
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2answers
69 views

If you lift something does the internal energy increase or remain unchanged?

So change in internal energy $\Delta U = q + w$; so in lifting an object you do work on it, thus increasing its internal energy. Is this correct? Sorry if this is a stupid question.
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3answers
4k views

Where is the flaw in this machine that decreases the entropy of a closed system?

I was thinking about a completely unrelated problem (Quantum Field Theory Peskin & Schroeder kind of unrelated!) when the diagram below sprang into my mind for no apparent reason. After some ...
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2answers
86 views

Question about distillation

I have got a question about distillation. In distillation as we know the mixture is boiled and the mechanism then goes like substances with high boiling points condense at the bottom and substances ...
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4answers
3k views

Does light loses its energy when it passes through denser medium

I know it does not because it emerges out of denser medium at 300,000 KM per second, but according to E=MC square and given that speed of light decreases inside denser medium with refractive index ...
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2answers
549 views

How much faster is the fusion we make on earth compared to the fusion that happens in the sun?

I have come across many websites that states that the proton-proton fusion which is the dominant type of fusion that powers the suns, is extremely slow and that is why the sun is still burning to that ...
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1answer
28 views

Does scanning a lightsource affect the scan result?

I'm very much a layman on this kind of topic, so my logic may be wrong, but here goes: Sample situation A 'pass app' on your smartphone, allowing someone to scan your ticket on your device. My ...
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4answers
3k views

Does a magnet contain (and potentially produce) energy?

Very quick question, does a magnet contain energy? The general consensus seems to be, it does not. And this is generally confirmed by the fact that it would break the first law of thermodynamics. ...
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4answers
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What is the relationship between Force and Kinetic Energy?

I am an Middle-School (Grade 8) student, who is self-learning classical mechanics. I saw this equation from Classical Mechanics: $\vec F=m \vec a$, the following equation is pretty straight forward ...
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40 views

Does it take energy to destroy the surface of a liquid jet? [closed]

Imagine we have a collision of a liquid jet with an obstacle Does it take time for the surface of the jet to be destroyed while collision?? Or it'll be destroyed immediately?
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3answers
374 views

Energy of a standing wave

I've been taught that energy of a standing waves remains between the nodes of the wave or in other words energy remains within the wave. I wanted to know how that's possible. Also if energy of a ...
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2answers
171 views

Kinetics energy with different reference frame (a fighter plane shoots a bullet)?

Suppose a plane flies at 1km/s and shoots a 2kg projectile at 1km/s, so to the Earth the projectile flies at 2km/s with 2*2*2/2 = 4MJ. The same gun, when fixed on Earth, shoots the same 2kg ...
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1answer
88 views

Why do particles in a medium have different speeds?

If thermal conduction occurs because of particles colliding with each other and imparting kinetic energy to each other, won’t the particles of an isolated medium collide with each other until the ...
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219 views

Why does light bend?

I have just read about dispersion of light by a prism and the thing which i donnot understand is that WHY does the light bend at all? Through prisms and slabs? I came to know that red light has the ...
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134 views

Why the electromagnetic waves propagate and do not stand still?

I know that the electromagnetic waves are alternating electric and magnetic fields. But I don't get why they can't stand still and alternate at the same point. I would like a logical and verbal ...
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2answers
635 views

What is “Energy” of a vacuum in the context of quantum theory?

I'm not a physicist or cosmologist, so I hope I am asking the right question with the right words here. My question regards the word "energy" as it pertains to quantum vacuum states, and to concepts ...
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0answers
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Candles: paraffin and flameless, which is the most environmentally friendly?

How could one compute (at least the scale of order) the well-to-flame energetic cost of a paraffin wax candle burning one hour (including CO2 and CO)? Can it be compared with a flameless candle (LED ...
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3answers
112 views

What is physical significance of energy?

John Rennie said, actually all particles are basically energy in the sense that they were created by adding energy to a quantum field. This applies whether you're creating a photon, an ...
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2answers
53 views

What is a qualitative description of energy? [duplicate]

In elementary physics courses one is taught that energy is a measure of an object's ability to do work (this in itself seems a little flaky as how does one then define exactly what "work" is, other ...
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0answers
23 views

Diffraction of rays [duplicate]

To all the physics experts here this might sound like a stupid question, but I give it a try: This is a sketch of how you usually learn about the effect of diffraction in an experiment: Now I was ...
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6answers
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How is momentum conserved when is is only dependent on mass and velocity, and so many other factors come into play?

I've been trying to get a good grip on the difference between conservation laws. Momentum is particularly tricky, I don't understand how quantities like $m\mathbf v$ can be conserved when other things ...
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55 views

Theta-parameter vacuum energy

Suppose we have $\theta$-field in QCD (in a special case of constant $\theta$ it reduces to ordinary $\theta$ parameter): $$ \tag 1 Z_{\theta} = \int D[\psi_{QCD}]e^{iS}, $$ where $$ \tag 2 S = \int ...
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1answer
30 views

Calculate the applied force associated with uniform distortion of an area

I am exploring Hooke's Law and considering how it might apply to an arbitrary area (perhaps as a slice of some volume). Since $$ F = -kx $$ Would it be correct to calculate the force, $F$, ...
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3answers
211 views

Is it feasible to transfer energy from power stations to communities via photons instead of electrons?

Electrical wires are relatively inefficient in transferring energy--especially when the place of production is quite far from communities. Would it be possible to transfer that energy via photons? I ...
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1answer
52 views

Fusing Hydrogen with lightning

I've been reading about fusion recently (Specifically Deuterium fusion) and a friend of mine asked me if it was possible to fuse two Deuterium atoms with a lightning strike? Now this question has a ...
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1answer
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How are these marbles being accelerated?

This question refers to an effect visible starting at around 5m45s in this video1. (The question will make little sense if one has not first watched the clip.) The observation At around 5m45s we ...
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0answers
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Solving question from a energy perspective [closed]

Im having some trouble with a question i was given for homework. Ive already solved it from a motion perspective but now i need to solve it from a energy perspective. The question states that from a ...
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3answers
149 views

Work done changes between reference frames?

(This is not homework; a friend shared with me this puzzler and neither of us can figure it out.) Suppose you are in a plane traveling at velocity $v_1$ relative to the ground. The flight attendent ...
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4answers
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How can there be heat in a vacuum?

I keep reading in the Physics World focus issue on vacuum technology about scientists creating high temperatures in the vacuums etc. If heat is caused by thermal energy being radiated from particles ...
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1answer
46 views

Fundamentally speaking, what are all the essential factors for waves (of any nature) to exist?

Considering all systems in which waves can exist, from atomic scale (such as DeBroglie Waves for example) to cosmic scale (gravitational waves for example), and those that require media for transport ...
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2answers
48 views

Travel by Earth rotation [closed]

why we can't go outside the atmosphere and travel with earth rotation .. and can we travel in the opposite direction of atmosphere by the same speed of atmosphere it’s moving at about 1,000 miles an ...
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3answers
34 views

Hot Chocolate- cooling itself down

If a hot cup of hot chocolate is just standing there, can it cool itself down by transferring the kinetic/thermal energy that the liquid has into the mug/cup?
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1answer
187 views

Is work energy theorem $W=\Delta K$ or $W=\Delta E$?

I know that work is the change in kinetic energy. I saw on an equation sheet that work is equal to the change in total energy. Is this true? If so, how?
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1answer
41 views

Energy/entropy in transport of molecules across cell membranes

I'm asking a "Maxwell's Demon" question. I've read statements that sorting molecules and sending them to one side or the other of a barrier requires work, but none of these statements have explained ...
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3answers
4k views

In nuclear fusion reaction, what is the percentage of mass converted to energy?

I read somewhere that it is about one percent of the mass, but I find this too high. Also I have done some calculations, for example, the Tsar Bomba was 50 MT bomb and weighed about 27 tons. Although ...
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1answer
42 views

Can we transfer energy wirelessly?

Now-a-days solar energy is good alternating source of energy and it is free without any harmful effect to our environment. But earth based solar energy systems are dependent on a major factor, which ...
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1answer
32 views

Could one, practically or hypothetically, capture energy from heat using magnetic fields? [closed]

I have had this idea, and I'm not sure if it would work. Heat is a form of chaotic kinetic energy, right?. Would it be possible to create a magnetic field within a gas or liquid so that when the ...
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2answers
33 views

Tension at topmost point of a vertical circle

I read that for a body to move in a vertical circle, the minimum tension needed is 0 at the topmost point. Can someone explain to me what it means to have positive or negative tension on top. I'm not ...
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4answers
72 views

Why do coal power stations not directly use combustion to turn the turbines?

Why do coal power stations run by creating steam rather than from the expansion due to combustion? I understand that in order to generate electricity in a coal power plant, steam is usually produced ...