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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6
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+50

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 ...
2
votes
3answers
289 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 ...
3
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5answers
573 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 ...
1
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1answer
202 views

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 ...
0
votes
2answers
48 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." ...
1
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1answer
98 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.
3
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2answers
35 views

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 ...
59
votes
1answer
3k 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 ...
-2
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2answers
56 views

Electricity from vacuum [on hold]

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 ...
1
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1answer
84 views

Maximum angle for highway lane change

I am preparing to fight a traffic ticket from a speed camera. There is a lot more to that story, but the info I need right now involves the angle at which a car can change lanes, in terms of vehicle ...
4
votes
2answers
70 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. ...
0
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1answer
126 views

Explanation of force amplification inside a solenoid

For a system being actuated by a motor, the force can be amplified by gearing. The energy is being used for force instead of distance, so it produces more torque but moves slower. For a system being ...
0
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0answers
17 views

Determining photon flux at a particular distance from a source, given frequency and transmitter wattage

I want to check my process and formulae for determining the observed number of photons per square meter per second, when the signal frequency is $f$, transmitter power is $P$ and the distance from the ...
2
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0answers
38 views

interpreting the dirichlet-principle

In obstacle problems (see e.g. wikipedia here) we are minimizing the dirichlet energy $$ I[u]=\int_D\|\nabla u\|^2dx\tag{1} $$ The Dirichlet-principle states that minimizing $I[u]$ is equivalent to to ...
0
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0answers
24 views

Storing ice for HVAC

Apologies if this question is outside of the scope of the site. Assume my home is a $20 \textrm{ ft} \times 40 \textrm{ ft} \times 10 \textrm{ ft}$ block. Further assume that it has continuous ...
-1
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2answers
45 views

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 ...
-4
votes
3answers
150 views

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 ...
3
votes
2answers
26k views

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 ...
0
votes
1answer
26 views

Where Does the Energy Come From in this Example?

I can't figure out where the energy comes from for the work in a video I found about how the pyramids could have been made. (I do not have enough rep to post the link of it as well as the picture but ...
0
votes
2answers
29 views

Explanation for the spectrum of the sun?

My Question is, why exactely does the intensity vanish below 240 nm ? If i look at the plank's law, obviously the intensity for $\lambda \rightarrow 0$ and $\lambda \rightarrow \infty$ will vanish, ...
0
votes
0answers
33 views

Change without Work [on hold]

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? (Note that ...
1
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1answer
18 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 ...
-2
votes
1answer
51 views

Capacitor Charging another Capacitor to a higher Voltage Level? [on hold]

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 ...
1
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3answers
65 views

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. ...
16
votes
4answers
4k views

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 ...
2
votes
7answers
848 views

Why is momentum conserved (or rather what makes an object carry on moving infinitely)?

I know this is an incredibly simple question, but I am trying to find a very simple explanation to this other than the simple logic that energy is conserved when two items impact and bounce off each ...
1
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1answer
38 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 ...
0
votes
3answers
68 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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0answers
244 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 ...
0
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2answers
35 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 ...
0
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0answers
12 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 ...
36
votes
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 ...
0
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1answer
40 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 ...
29
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2answers
3k views

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, ...
0
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1answer
591 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 ...
0
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0answers
19 views

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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0answers
46 views

Work done by a constant force [on hold]

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 ...
0
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3answers
92 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 ...
2
votes
3answers
169 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 ...
1
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2answers
145 views

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 ...
1
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3answers
603 views

Example of law of conservation of energy

Law of conservation of energy states that "Energy of the universe remains constant. Energy can neither be created nor be destroyed, but it can be transformed from one form to another". So I ...
0
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1answer
37 views

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 ...
-3
votes
2answers
81 views

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 ...
0
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1answer
46 views

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 ...
2
votes
1answer
99 views

The ADM Energy of Gravitational Waves?

I have been looking for books about this question for several days. However, almost all books use Landau–Lifshitz pseudotensor to calculate the energy of Gravitational Waves.And they said the result ...
0
votes
1answer
55 views

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 ...
0
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1answer
27 views

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 $y:\mathbb{R}^2\to\mathbb{R}$, ...
0
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1answer
47 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 ...
2
votes
3answers
291 views

Can the vibrational energy of a engine be used to increase efficiency?

In whole mechanics, we never talk about wasted vibrational energy! As we see in our daily life, a lot of energy is wasted as vibrational energy in every engine, for example as we fire our car's ...
0
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2answers
2k views

What is the relationship between keV and kVp?

On X-ray topics we get used to talk about "energy", but what is keV? and what is the relationship between keV and kVp?