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 the ability to do work?

Here was my argument against this, the second law of thermodynamics, in effect says that, there is no heat engine that can take all of some energy that was transferred to it by heat and do work on ...
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1answer
180 views

Hamiltonian in 2-dimensions? [closed]

I am trying to construct a Hamiltonian for a system in 2 dimensions using Matlab. I am not sure how this Hamiltonian will look like in matrix form. If somebody can help me visualize this matrix that ...
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1answer
69 views

Electricity directly from heating a material [duplicate]

I am looking for some more information about how to obtain electricity from heat directly. This e.g. involves the Seebeck effect, as I have found it is called, where a material produces a voltage ...
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2answers
86 views

Is energy simple or composite?

From $E=mc^2$ it seems mass and energy are interchangeable. I can understand that mass is distinguishable, one type from another, such as protons and electrons. But in the reverse process, how is it ...
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1answer
222 views

What energy changes take place when you operate a jet-ski?

What energy changes take place when you operate a jet-ski?
2
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1answer
327 views

Fusion vs. Fission

I understand why fission generates large amounts of energy when the nucleus is split, but then why does fusion generate such large amounts of energy. If fission releases energy when some mass is lost ...
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1answer
186 views

Where did the concept of energy come from?

Energy seems to me to be a very abstract thing, and while it clearly works out very nicely, I don't understand how anyone would have thought to come up with it. Where does the concept of energy find ...
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2answers
199 views

What can be known about the formulas for energy only from the fact that it is conserved?

The question is to figure out how the energy can be derived knowing just one thing: There is a quantity called Energy that is conserved over time. The goal is to get an equation that somehow ...
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3answers
2k views

How does energy convert to matter? [duplicate]

To my understanding, matter and energy are one and the same. Shifting from $E$ to $M$ in Einstein's famous equation requires only a large negative acceleration. If $M$ really is $E/c^2$, does that ...
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0answers
42 views

Welding physics (black body and heat conductance)?

If I have two pieces of steel that I would like to weld with a laser (say $d$ in diameter). If I wanted to melt the two pieces to a depth of $d$ along the edge and then combine them together, how much ...
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1answer
2k views

How to derive the velocity in the double ball drop problem?

The double ball drop problem is as follows: A ball of mass $m$ is placed on top of a ball of mass $M$ (where $m < M$), and the balls are dropped simultaneously from some height $h$. When the ...
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1answer
4k views

How to calculate incident solar energy on earth in a given day with a spectral filter

The daily solar radiation irradiation is often quoted as being between $3.2 \text{ kW hours}/\text{m}^2$ and $7 \text{kW hours}/\text{m}^2$ per day . If you filter out a range of wavelengths, how can ...
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1answer
2k views

From where do the permanent magnets get energy from? [duplicate]

I have a doubt about permanent magnets. If a magnet is permanent it can attract some materials permanently. Attracting something involves energy. If a permanent magnet can do this forever, from where ...
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0answers
64 views

First-order pertubation theory

I'm having some trouble figuring this out, so I was hoping someone could help. I need to show that the first-order pertubation of the ground state energy is not changed by the pertubation $H'$, given ...
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1answer
33 views

Averaging energy density

Let we define energy density by, $$E= a^2 \cos2 t +b^2 \sin2 t+ c^2 \cos 3t$$ where, $E=$ total energy, $a,b,c =$constants, and $t$ is the time. I'm confused from someone's statement that ` There ...
4
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4answers
240 views

Why does it require such little energy to create the fastest thing in the universe?

I have noticed when I turn on the light switch in my house light comes from the bulb. How is this light created?(process occurring in the bulb) and why is this small amount of electricity enough to ...
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3answers
1k views

What does it mean if a body has kinetic energy?

What does it mean if a body has kinetic energy? Does it mean that the momentum vectors of each particle of that body has the same direction? What about angular momentum?
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2answers
582 views

Human power on treadmill

On an elliptical treadmill a regular person can easily burn 1000 calories in one hour (treadmill reports calories burnt). This translates into: $$(1\times 10^3\mathrm{cal/hr}\times ...
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0answers
58 views

Bass and Treble-Car Steroes

In a car which phenomenon, diffraction or the resonant frequency of the car, lends itself more to the ability of bass to go farther? Related Answer: Why do bass tones travel through walls?
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2answers
204 views

Street Light Interference Phenomenon [closed]

Is there a scientific approach that can explain the street light interference phenomenon? Everytime I walk past a Streetlight it turns off.
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2answers
948 views

Minimization of energy and maximization of entropy

Are maximization of entropy and minimization of energy equivalent? Or are they contrary? Why should the thermodynamic potentials such as $G$, $A$, etc, be minimum at equilibrium? I am confused. ...
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1answer
243 views

Definition of energy

What is the definition of energy $E$ given a dispersion relation $\omega=\omega(k)$ where $k=|\vec k|$ and $\omega$ is not necessarily linearly proportional to $k$? What about momentum $\vec p$? This ...
4
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3answers
1k views

When Hamiltonian and the total energy are the same

In which condition, the Hamiltonian is the same as the total energy of the system, or say $H=T+V$?
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1answer
783 views

Formulas for kinetic energy

I was reading ABC of relativity from Bertrand Russell and some formulas about kinetic energy caused me some problems. Here is the extract : The kinetic energy is, in the usual form ...
35
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5answers
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Why doesn't light kill me?

I was attending my philosophy class and in the middle of student presentations, I found myself mentally wondering off and thinking about light. After a few minutes of trying to piece together how the ...
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2answers
2k views

Collision between a photon and an electron

Looking through this AP Physics question, I was struck by how the 'collision' between a photon and electron looks so much like a macroscopic collision. Is this even physically possible? Look at the ...
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0answers
379 views

How does the volume change affect an adiabatic process?

A gas consisting of rigid diatomic molecules was initially under standard conditions. Then the gas was compressed adiabatically, 5 times the original volume. Find the mean KE of rotating molecules ...
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1answer
398 views

A Calorimetry Problem

I have a question in calorimetry from an old competitive exam. The question is: The temperature of $100$ grams of water is to be raised from $24 ^\circ$C to $90 ^\circ$C by adding steam to it. ...
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1answer
109 views

How do we know that time and distance are not discrete?

I know that it is believed that energy is discrete, in that it travels in quanta. I was wondering if there is any evidence which either proves or disproves something similar with both time and ...
3
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1answer
120 views

Energy Functional

I am a graduate student in pure mathematics, during my study on Ricci Flow I faced some functional known as energy functional. For example Einstein-Hilbert functional is called an energy functional, ...
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2answers
91 views

What is the derivation for the exponential energy relation and where does it apply?

Very often when people state a relaxation time $\tau_\text{kin-kin}, \tau_\text{rot-kin}$,, etc. they think of a context where the energy relaxation goes as $\propto\text e^{-t/\tau}$. Related is an ...
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1answer
38k views

How much energy is in a lightning strike?

According to Wikipedia an average lightning has 1TW, the whole world used 16TW in 2006. (I suppose this means the same as 16TWh in one year?) Sometimes the lightning reaches 100kA. This peak last for ...
5
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1answer
319 views

First law of thermodynamics?

The first law says that the change in internal energy is equal to the work done on the system (W) minus the work done by the system (Q). However, can $Q$ be any kind of work, such as mechanical work? ...
6
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1answer
321 views

Why do wind power plants have just 3 blades? [duplicate]

Why do wind power plants have just 3 blades? It seems that adding more blades would increase the area that interacts with the wind and gather more energy. (Image from Wikipedia.)
5
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1answer
841 views

Do electrons need specific energies to excite electrons

Photons need specific energy levels, equal to the difference between two energy levels to excite an electron in an atom. Is this the same case with electrons that collide with atoms?
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640 views

The Preference for Low Energy States

The idea that systems will achieve the lowest energy state they can because they are more "stable" is clear enough. My question is, what causes this tendency? I've researched the question and been ...
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2answers
357 views

Negative potential energy of gravity

Does the negative potential energy in the gravitational field have to be considered in calculating the total mass of the system in question (because of $E=mc^2$)? If so it seems to me that the ...
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2answers
699 views

What happens to the energy not absorbed by a radio?

If a radio tunes to a specific frequency, where does the excess energy go? If one continues to hit the resonant frequency, shouldn't the wire begin to melt at some point from too much energy?
16
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4answers
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Redshifting of Light and the expansion of the universe

So I have learned in class that light can get red-shifted as it travels through space. As I understand it, space itself expands and stretches out the wavelength of the light. This results in the light ...
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2answers
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1answer
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What lifting mechanism is likely to have the best energy recovery ratio? [closed]

Suppose I was designing an apparatus which needed to lift 250kg 5cm high, hold it there for a few seconds, and then lower the object back to the original height. Such a process would need to be ...
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1answer
297 views

Why are electrons in one atom attracted to negative electrons in another atom? [closed]

Why or for what purpose is this attraction purposeful or what is the reasoning behind why these energy particles are drawn to each other? Any proven results, or proposed theories of your own, or ...
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3answers
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Integration by parts to derive relativistic kinetic energy

I have come across a weird integration during derivation of relativistic kinetic energy. Our professor states that i can get RHS out of LHS using integration by parts: $$ \int\limits_0^x \! ...
0
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1answer
87 views

Pendulum system: how is derived the output as Energy?

Good day to everyone, I want to understand in which way the "Energy equation" is been implemented to this pendulum system. $x_1(t)$: The angular position of the mass $x_2(t)$: The angular velocity ...
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2answers
101 views

Energy behind door knocking?

How do we estimate the energy released from knocking on a wooden table or a door?
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2answers
136 views

energy of Russian meteor, does it matter whether it explodes or not?

The kinetic energy calculation for any meteor would be the traditional one half times the mass times the velocity squared. In terms of the events that occur on the ground, does it matter whether the ...
16
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2answers
1k views

Why can't Humans run any faster?

If you wanted to at least semi-realistically model the key components of Human running, what are the factors that determine the top running speed of an individual? The primary things to consider would ...
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3answers
701 views

Why doesn't a stationary electron lose energy by radiating electric field (as per coulomb's law)?

If an electron in a universe constantly generates an electric field why does it not get annihilated ? I am confused because I read that an accelerating charge radiates and loses energy. So, why won't ...
2
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1answer
182 views

Total energy is extremal for the static solutions of equation of motions

In physics total energy is extremal for the static solutions of equation of motions. Can anyone explain this sentence to me?
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0answers
282 views

How to convert Richter magnitude scale to approximate TNT?

I know the Richter magnitude scale is often used for measuring the strength of earthquakes. At the same time, explosive/destructive releases of energy are often quoted in equivalent amounts of TNT. Is ...