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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Energy concept and calculuation

Could someone please explain how the concept of energy was conceived and how it evolved into our modern understanding of it ? Also, how did people come about various ways of mathematically ...
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Could sound be considered a kind of renewable energy? [closed]

Is sound energy useful as a source for generating electricity? If so, could it be a renewable resource?
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84 views

Why won't this Casimir effect perpetual motion machine work?

So the Casimir effect is caused by an imbalance in radiation pressure(caused by virtual particles) on two sides of a metal plate. This is typically shown with two close plates getting pushed towards ...
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1answer
62 views

Energy of hydrogen atom - Schrodinger equation [closed]

The wavefunction of the electron in the hydrogen atom is $ k* exp(-r/a)$ (k is the normalization constant), but it does not take n into account, whereas the solution of Schrödinger's equation ...
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Self-powered heat exchanger w/energy profit? [closed]

It is possible, considering the laws of physics, to build a heat exchanger that runs on the power it gathers from the environment? A typical heat exchanger uses less power to harvest heat for a ...
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2answers
89 views

Heat diffusion in a house

I was wondering about this problem (which is NOT homework, it's one of my mental problems due to what surrounds me): supposing I'm in a room, and it's winter. Heating is on, the boiler works and the ...
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2answers
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energy of a ball [closed]

What is the energy of a ball lying on the ground at rest? As it is not in motion, it will have zero kinetic energy and as it is lying on the ground its potential energy will also be zero, taking the ...
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2answers
77 views

What's the minimum kinetic energy required to overcome static friction?

Suppose I have a still (granite) cube on a flat surface with friction with gravity evenly pulling down on everything. I know how to calculate how much force something such as electromagnetism would ...
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1answer
55 views

Given a current velocity and a fixed input energy, how much faster will a relativistic particle be?

The relativistic kinetic energy of a particle with mass $m$ and velocity $v_0$ is $$m c^2 (\gamma_0 - 1) \textrm{ where } \gamma_0 = \frac{1}{\sqrt{1 - \frac{v_0^2}{c^2}}}$$ I would like to know how ...
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Do we know how much energy is released in internal conversion (fluorescence)?

In the Jablonski Diagram, when photons hit a surface and certain frequencies are absorbed, then internal conversion occurs to the excited portions of the surface and energy is released as heat (If I ...
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1answer
54 views

Calculating speed of an object at the bottom of a curved slope? [closed]

Say you had a slope and it was like a ski slope that started shallow, got steeper and shallow again, and you knew that the top of the slope was a certain height vertically upwards from the bottom of ...
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1answer
49 views

Four momentum in particle physics

I have seen four momentum defined in two different ways. One is $P =(E,\vec{p}c)$ and the other is $P =(E/c,\vec{p})$. Which one is correct and if they both are then why and when is it more convenient ...
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1answer
68 views

How does potential energy relate to kinetic energy on a rollercoaster?

This is appearantly how the potential and kinetic energy are in the different positions of the rollercoaster, if I am not mistaking. However I don't understand why the total energy is energy and ...
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1answer
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Which system's mechanical energy is conserved?

As the figure shows, $m$ starts falling down from its current position at the original speed of zero. And $M$ is a curved side trapezoid. All friction can be ignored. |# <- This hash stands for ...
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How am I able to stand up and walk down the aisle of a flying passenger jet?

The energy of a moving object is $E = mv^2\;.$ That is it increases with velocity squared. I walk at say 3 miles per hour, or lets round that down to 1 meter per second for a slow walk. I weigh less ...
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2answers
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What happens during mass-energy conversion?

A mass is another form of energy. When a mass ceases to exist as 'matter', it exists as energy - in the forms of energy we generally know (light, heat). But is this so simple? When a mass exists in ...
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1answer
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How Do Low Kinetic Energy and High Kinetic Energy Atoms Attraction differ?

Will atoms of 2 difrent energy levels electron and/or Kinetic attract more than 2 Atoms at he same energy level? Do 2 high energy level atoms attract more than 2 low energy atoms?
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1answer
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Why it is good to have the frequency of cyclotron independent of the energy of the particle

In my book it is shown that the frequency of revolution of the charged particle in a magnetic field is independent of its energy. Now I understand that why if a particle is in a magnetic field then ...
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Energy of a relativist electron [closed]

I need to find a way to obtain $E = mc^2(\gamma -1)$ by starting with : $E = mc \cdot p = mc \cdot v \cdot \gamma$ with $ \gamma = \frac {1}{\sqrt{1-\frac{v^2}{c^2}}}$
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1answer
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mass attached to spring rotates in a horizontal circle [closed]

So here is the question I have: A mass of 2kg rotates at 1m/s in a horizontal circle on a table at the end of a spring with an elastic constant of 50N/m. If the original length of the spring is 2 ...
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4answers
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Why is 7 TeV considered as a big amount of energy?

Considering that $7$ TeV is more or less the same kinetic energy as a mosquito flying, why is it considered to be a great amount of energy at the LHC? I mean, a giant particle accelerator that can ...
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1answer
28 views

Calculating Dark enegy, dark matter and the rest [duplicate]

I have read that Dark energy composes roughly 68% of the Universe and that Dark matter composes about27% and everything else is less than 5%. Where and how did they come up with these figures?
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Electron-Positron Annihilation in a Gravitational Field

When an electron and positron annihilate what happens to any gravitational energy they have? In a way the energy is 'shared' between these two particles and all the matter in space. It must take a ...
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3answers
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Kinetic energy of a rotating object

in an exercise, a linear molecule is being subject to a force applied on the edge in its axis. Then $K_1=\frac{1}{2}mv^2$, all is well. Then in the second point of the exercise, the force is applied ...
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Basic PE question about moving a planet.

If you lifted enormous masses (eg. many gigantic rocks) at one point (A) of the planet's equator. Then after 180 degree planet rotation, released the potential energy by lowering the rocks (B). ...
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Is energy related to work or is it an entirely different thing?

I was reading about work done and energy, and I got to know some things. Let's take a simple example: Suppose I move an object to a certain height. Here, I apply a force against the force of gravity ...
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Electron energy from beta decay

I read in my IB-physics book that the average energy for an electron in the beta decay of Potassium-40 is 0.44 MeV. However this would imply the electron have a velocity of 3.9E8 m/s, i.e. faster than ...
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1answer
48 views

determine the mass of ice that melts during the impact

During a storm, a mass $m = 2 g$ hailstone falls to the ground. Its speed just before coming to ground is $v = 18 m / s$. its speed just after is zero. Assume that the hailstone is pure ice, the ...
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43 views

Conservative/Non-conservative nature of hinge force [closed]

Can total mechanical energy be conserved for a hinged rod free to move in the $X-Y$ plane? Because, there's the hinge force. If it is non-conservative, energy won't be conserved. Is it conservative?
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Statistical specific heat as energy fluctuation in spin glasses

Consider the specific heat (in statistical sense, as energy fluctuation in the canonical ensemble) of a complex model, something similar to a spin glass. Is the specific heat defined on fluctuations ...
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52 views

Could the Big Bang be the result of a decrease in entropy?

In my freshman engineering physics class I learned that a decrease in entropy, though hypothetically possible, is less statistically likely than either any or almost any other possible single ...
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2answers
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Elevator Cab Problem

I faced this problem in Fundamentals of Physics (Halliday and Resnick): A $0.250\ \mathrm{kg}$ block of cheese lies on the floor of a $900\ \mathrm{kg}$ elevator cab that is being pulled upward by ...
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1answer
133 views

How do momentum get transferred?

Simple Question , Consider two objects namely $A$ and $B$ where $B$ is stationary and $A$ is moving towards $B$ with velocity $v$. When the two objects touch each other what does actually happen ...
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1answer
54 views

Can we measure the energy of one of several identical particles?

Suppose we have a many-particle system described via a many-particle wavefunction that involves single-particle states $\lvert\lambda_{a}\rangle$, $\lvert\lambda_{b}\rangle$, ...
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Simulating Debris Burn Up Upon Atmospheric Reentry

For work I need to model Debris (1 gram to no more than 1000 km) burn-up upon reentry. I can't seem to find much information regarding this. I feel like I am looking for the wrong terms or something. ...
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Can you grow more Biomass in a brickhouse only powered by solar cells than in a greenhouse? Will we ever be able to? [closed]

Right now I read that 1m² of solar cells generates roughly 100Watts of energy. There are really efficient Grow LEDs that use only two spikes in the lightspectrum to make plants grow. I have seen some ...
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Energy contribution due to matter transfer

I have a very important question that lingers in my mind for a long time: In the fundamental relations of Gibbsian Thermodynamics, I simply don't understand how the last term appears. I have ...
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2answers
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Why are the allowed energies a continuum in the region $V_{\_} < E < V_{+}$?

I'm studying quantum mechanics and I don't quite understand why there's an energy continuum in the region $V_{\_} < E < V_{+}$ in the following example: It was explained that because of the ...
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2answers
67 views

Is an x-ray maser possible?

I was wondering if lasers evolved from masers. They applied the same principles to a different wavelength. Would it be possible to create a maser at a higher wavelength, using x-rays, gamma rays, ...
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Why are heavy elements used in nuclear power plants? [duplicate]

I've seen that the energy produced by breaking an oxygen atom's nucleus is equivalent to burning a LOT of coal, 4.6 million metric tons if I remember correctly. (Crash Course Chemistry) If that is ...
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1answer
42 views

Reference frame and conservation of energy

Say spaceship $\alpha$ burns a portion of its fuel to leave planet A and is cruising through space at 10 m/s relative to the surface from which it launched. Spaceship $\alpha$ is being observed by ...
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1answer
50 views

Kerr Effect; Electro-refraction

First post here. is is possible to observe the Kerr effect in transparent materials, at low voltages (such as 3.8V)? If so, can you refract light enough to distort an image, in a way similar to that ...
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1answer
131 views

What happens to a sound wave going from air to vacuum?

Let's say you are in a space ship. The inside is filled with air, a medium in which sound waves can exist and travel. Then there's a wall, still a medium in which sound waves can exist and travel. And ...
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2answers
53 views

which is efficient on solar electricity by photo voltaic or thermal

As we know solar energy used to generate electricity in two major ways: photovoltaic and thermal energy. The most small applications or personal use are mostly using solar cells. But when it comes to ...
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1answer
47 views

Help with my bouncy ball lab (I know the factors just not how to approach them) [closed]

In my physics lab we need to determine the factors that account for the energy "loss" during a high bounce ball bounce. I know that energy is "lost" (not really) to heat, air resistance, and sound. ...
5
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1answer
207 views

What is gravitational energy in general relativity?

In GR the curvature of spacetime "is gravity". This curvature is expressed via the Riemann tensor (or the Ricci tensor + Ricci scalar). The curvature is connected via the Einstein Field Equations with ...
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2answers
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Inelastic collisions

During inelastic collision the kinetic energy does not remain constant. In many texts, I have seen it is because there is loss in energy from the system. If there is loss in energy, then how can mass ...
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1answer
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How does energy levels of an electron differ from atoms?

I would like to know how does energy levels of an electron differs from atom. I've read that electrons with almost similar energy orbit in the same orbital and have a discrete set of energy levels. ...
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To derive the relation between work function and potential energy

I'm reading "The variational principles of mechanics- Lanczos", The author mentions a relation between Work-Function $U(q_1,q_2,\cdots,q_n,\dot q_1,\dot q_2,\cdots,\dot q_n)$ and the potential ...
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How to detect transient X-ray?

I have a Mercury UV lamp (ENF-260C/FE) which is supposed to emitte UV light with wavelength of 254nm and 365nm (http://www.chescientific.com/ecat/Eng/spectroline.pdf). However it is found that during ...