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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Solve a problem of work and energy

A man pulling sled of his daughter by a massless rope, climbing a snowy hill whose slope is equal to 15 °. Considering that the mass of the sled is $4Kg$, the girl's $26Kg$ and $\mu _c = 0,25$, ...
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Why does the amount of energy transferred depend on distance rather than time?

The change in energy of an object can be determined by the work equation, where work is the change in energy: $$ W = F \cdot d $$ I conceptualize the transfer of energy as simply a series of small "...
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262 views

stimulated and spontaneous emission

I have a question regarding the dependence on energy density of the probability rate of emission/absorption while studying lasers in particular. It is given that,the probability rate of absorption as ...
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Elastic potential energy of compressed spring? [duplicate]

If a compressed spring is dissolved in acid what happens to the elastic potential energy of the spring?
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469 views

How is the energy distributed in a proton-antiprotion annihilation?

I know the products of the annihilation, but I don't know how much energy each particle has or gets. For example I know that 1876 MeV is released for each annihilation. Now, this energy is distributed ...
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430 views

Why doesn't intensity of light affect the emission of electrons?

So electrons of specific atoms have a minimum amount of energy needed to escape the atom, called the work function, W. Now let's say that you emit a certain frequency of light, and $hf<W$. However, ...
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274 views

Maximum separation when 2 unlike charges shot apart

I want to compute the maximum separation when 2 unlike charges are shot apart from each other ...
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242 views

Basic energy calculation for N identical spin system

We have a system that has N identical spins $n_i$, and each spin can be in state 1 or 0. The overall energy for the system is $\epsilon\sum_{i=1}^{N}n_i$. My understanding: There is only one ...
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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 \! \frac{d}{...
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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 ...
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4answers
237 views

Another question about energy(Improved)

I read this thread about energy What Is Energy? Where did it come from? EDIT: after some sleep I tried to read the answers again and found better questions to ask. so here you go As most guys have ...
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392 views

Specific electron energy gap values $E_{i+1}-E_i$ vs. photons with arbitrary energy $\hbar \omega$

The energy levels of electrons in an atom are quantized $E_i$. A photon of a specific momentum $\vec p$ and energy $$\omega=(E_{i+1}-E_i)/\hbar$$ hits an atom and gets absorbed. Okay now say the ...
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What arrangement of sound waves would be needed to heat air in a typical sized room?

From what I understand, sound is simply the jostling of the molecules that make up the air in a specific pattern, widely known as waves. I also know that these are longitudinal waves. If we were to ...
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664 views

Calculating laser wavelength/power to cause emission of light in a gas?

I've become interested in volumetric displays. In particular I'm looking at a pair of lasers scanning a volume of gas such that the intersection of the lasers would supply enough energy to cause the ...
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372 views

Why does an ice rink use a primary and secondary coolant?

A normal refrigeration cycle uses one coolant, so why does vapor compression cycle of an ice rink you a primary coolant and a secondary coolant?
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505 views

Maxwell-Boltzmann distribution and total energy per unit volume

We know that $$n(E) ~=~ \frac {2 \pi (N/V)}{(\pi k_B T)^{3/2}} E^{1/2} e^{-E/(k_B T)} dE,$$ where $V$ is total volume. If then, how do we derive total energy per unit volume from this equation?
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How did the energy/entropy/volume/pressure/temperature relationship exist at the Big Bang and how did it evolve thereafter?

According to the current Big Bang with inflation cosmological model? I was under the mistaken impression that there was very low volume, very high temperature/pressure, very low entropy and the Big ...
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371 views

2d or 1d conduction in this scenario?

There is a rectangular fin attached to a heat exchanger with a base temperature of 350K. The fin has uniform properties and experiencesa uniform heat generation. It also experiences heat transfer with ...
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In what situations do I use the characteristic length of a fin to find the surface area?

So I'm learning about fins in heat transfer and it seems that there are two separate formulas for the surface area of a rectangular fin of length L, width w and thickness t. The fin is attached to a ...
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963 views

Work and Area under a Curve relating to Hooke's Law

If it takes work W to stretch a Hooke’s-law spring (F = kx) a distance d from its unstressed length, determine the extra work required to stretch it an additional distance d (Hint: draw a graph and ...
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101 views

Terminology question about energy

I'm looking for the appropriate term to use for what gets "used up" as potential energy is converted to heat and work. For example, some of the the energy in solar radiation is converted by ...
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165 views

Will a photon emitted from something moving quickly have a shorter wavelength?

If a photon is emitted from a light source moving at any speed, the photon will nonetheless always move at c (assuming it is emitted in a vacuum.) If the speed of a photon's emitter cannot influence ...
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2answers
535 views

Spin angular momentum of a system of particles : Is there any energy associated with it?

Consider a system of point particles , where the mass of particle $i$ is $μ_i$ and its position vector is $\vec{r}_i$. Let $\vec{r}_\text{cm}$ is the position vector of the center of mass of the ...
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Energy transferring question

If I have a spring being compressed by two bodies, A and B, with different masses, how much energy would be transferred to each one when they are released and the spring expanded?
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Depalma Free Energy fields

Few years ago I read some papers about Free Energy, written by Bruce de Palma, a physicist who is said to be the inventor of the N-Machine, which is an device that works with free energy latent in the ...
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1answer
51 views

How is the energy efficiency in bicycles accounted for? [duplicate]

Bikes are more efficient than walking, but that's only because roads are flat so wheels make sense, right? Does it make sense to say that this fact is simply gained by putting a lot of energy into ...
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Equivalent beam rigidity for a 1D lattice structure

To model the behavior of continua, often discrete lattice models with nodes joined by 2-point spring elements (which resist tensile forces) and 3-point beam elements (which resist bending moments) are ...
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Is it possible in this Universe to communicate a bit of information with energy that scales sub-linearly with distance?

If we look at all the ways that people do communicate information, they all seem to have a cost "at least linear in distance." For example, communicating over a wire has attenutation, so the energy ...
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How to apply conservation of energy to active noise-cancelling headphones?

Active noise-cancelling headphones add the opposite waveforms of the ambient sound in order to cancel out this ambient sound by destructive interference. I do not understand why adding more power in ...
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1answer
35 views

Work done relation to potential energy

I know work done is negative of change in potential energy, I.e., $W=-(∆U)$. It means that Work done against a force (or work done on a system) increases its potential energy. And Work done by a ...
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Energy Quantization

Why a free particle gives rise to Continuous Spectrum energy eigenvalues where particle in the Bound States provide the quantization?
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Model/formula for bouncing ball

I'm programming an animation of a bouncing ball, and I want it to be as realistic as possible. I fully understand the physics while the ball is rising and falling: It accelerates downward at 9.8 ...
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46 views

What is the cause of discrete or quantized energy levels in an atom? [duplicate]

I understand how it is that electrons move from one energy state to another, however I've not been able to find anywhere that describes why an atom has any particular states. Why should an atom of ...
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39 views

Mechanical energy in an harmonic wave and in normal modes

I think I miss something about energy of a mechanical wave. In absence of dissipation the mechanical energy transported by an harmonic wave is constant. $$E=\frac{1}{2} A^2 \omega^2 m$$ But, while ...
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Proof for fluctuations in vacuum

I'm not a physicist. My understanding of Heisenberg's uncertainty principle and its proof (that is given by an imaginary microscope) is that for example: at a specified time determining the exact ...
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change of energy in changing frame of reference

Let's imagine a car that can jump onto, or off a moving train. The train moves at 10m/s. The car, on a road next to the train, accelerates to the same 10m/s, jumps off a ramp and lands on the train, ...
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How can removing energy from a system do work? (ice)

Water freezing into ice expands, damaging roads, breaking rocks, etc. To my understanding, this is doing work (in a physics sense) by removing energy from the system. How is this possible?
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Gravitational collapse (conservation of energy)

During gravitational collapse, gravitational potential energy of the gas is converted to its internal kinetic energy so the internal energy of the clous of gas is said to be increased But isn't ...
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Interaction of atoms of the same material

So I've been reading a section about energy levels and came across this statement: "As the atoms of a material are brought closer together to form the crystal lattice structure, there is an ...
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Zeroth component of 4-momentum and relativistic energy-momentum relation

As I understand it one is forced to use 4-vectors since we require objects that transform as vectors under application of Lorentz transformations and 3-vectors do not (technically they do under ...
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1answer
35 views

Variation of Veritasium bullet block experiment

I'm new here. this is in reference to the video posted by veritasium on the bullet block experiment. i realise there is already a thread on this but i want to ask a variation of the question. so ...
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Thermal de Broglie wavelength - definition

The thermal de Broglie wavelenght is often defined by the formula $\lambda=\frac{h}{\sqrt{2\pi mkT}}$ but equally frequently is it defined as de Broglie wavelength for a free ideal gas of massive ...
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Energy efficiency in modern houses [closed]

What is an energy efficient way of heating modern houses in a winter scenario and how could we experiment it? For example, my group tried heating a beaker of water with a hot plate, microwave,and an ...
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1answer
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What are the assumptions behind the Lagrangian derivation of energy?

What are the assumptions behind the Lagrangian derivation of energy? I understand that we're searching for a function $L$ that describes a set of physics so that solving the energy minimization ...
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Light bulbs Flashing on when disconnected

For the 2nd time I've had a lightbulb flash on in my hand after removing from the power socket. The first time the overhead light in my hallway was out for a couple of days before I got around to ...
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1answer
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Why does the kinetic energy of a particle moving in circular motion increase when the turn radius decreases and no torque is acting?

Why does the kinetic energy of a particle moving in circular motion increase when the turn radius decreases and there is no torque acting? E.g. if a planet is rotating about its axis and it shrinks to ...
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6answers
105 views

Why does positive work done by internal conservative forces $\implies$ decrease of potential energy?

Potential energy can be thought as the amount of work that the force can potentially do on the point because of its position. $$W=-\Delta U=U_{initial}-U_{final}$$ A positive work done by a force ...
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2answers
59 views

How does conservation of energy work?

I understand that the law of conservation of energy states that energy cannot be created or destroyed, it can only change from one form to another. This means that the total energy before a certain ...
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Does energy produce a gravitational force [duplicate]

$E=mc^2$. From this, I would assume that any form of energy (not just rest-mass energy, but kinetic energy as well) would produce a gravitational force. Am I being too naive in my application of ...
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Reason behind the heat capacity of water

Water has a high specific capacity. Why can water release a large amount of energy without a great change in temperature?