Questions tagged [energy]

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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Conservation of energy violation Convection currents [closed]

There are three sources of heat in the Earth's core, impacts of the planet when it was created (impacts made heat), radioactive decay of elements, and the extreme amounts of pressure and friction ...
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Calculating Laser Beam Intensity Absorbtion

This question pertains to the ability to calculate the laser beam intensity , at a particular point in time when using lasers for the purposes of Cutting and Engraving Materials. The parameters at ...
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Why the energy and momentum conservation equations do not include terms reflecting the particle nature of the photons? [closed]

It is well known that light has a dual nature. However, the energy and momentum conservation equations embody the wave aspect of light but not the material aspect of the photon. Should there be ...
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Validity of the derivation of time-energy uncertainty principle?

Background The top voted answer to this question seems to make some assumptions I'm uncertain about (to say the least): What is $\Delta t$ in the time-energy uncertainty principle? Assumptions The ...
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Derivative of energy in General Relativity

I have found this equality that is the derivative of the energy for a local observer in GR, with energy defined as $E=-p^\mu u_\mu$: $$dE/d\tau = -p^\mu p^\nu\nabla_{(\mu}u_{\nu)}$$ trying to derive ...
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Work on an object in equilibrium

I heard somewhere that when an object is in equilibrium on the flat bottom of a hill that it will require work proportional to $dx^2$ when moved the small distance of $dx$. I thought that it would be ...
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How did Hamilton come up with his idea of summing up the kinetic energy and potential energy?

When we browse through any classical physics textbook, the Lagrangian comes first where the potential energy is subtracted from Kinetic energy. Was Hamilton playing around with energy functions and he ...
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Where does the energy go that is associated with a shift in a FRET spectrum (Forster Resonance Energy Transfer)?

In the commonly used Jablonski energy diagrams, some sort of loss during resonance energy transfer is usually indicated via horizontal lines below the excited state energy of the oscillator (see for ...
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Is the equivalence of mass and energy a direct consequence of SR or not?

Special relativity gives us the invariance of four-vectors. Consistency with Newtonian physics implies the conservation of four-momentum. The spatial part of four-momentum is $P^0=m\gamma(v)$ ...
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von Neumann on the Hamiltonian

I'm reading von Neumann's book on QM and I'm slightly confused by a simple point. He writes, "The energy is a given function of the coordinates and their time derivatives: $E = L(q_1,..,q_k;\dot{q}...
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Is potential energy always inversely proportional to separation?

This is a question from Class 11 Physics NCERT. The answer is that only graph (v) is correct and the reason given here is that potential energy varies inversely with separation. But does that always ...
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Prove that group velocity is the velocity of energy transport in wave

Generally, the group velocity $v_g = \dfrac{\partial \omega}{\partial k}$ of a wave is the velocity of energy transport. In "Introduction to Solid State Physics", Kittel following is stated: The ...
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What is the physics interpretation of work done of zero when moving a charged particle on an equipotential surface?

Assume I have a test charge of + 1 coulomb. If I move the test charge from a point with potential of +9 volt to another point with potential of -1 volt then the work done by me is -10 joule. This ...
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Why aren't gravity batteries everywhere? [closed]

The problem of energy storage, especially with regard to renewables, remains unsolved. Or at least there do not seem to be any popular solutions. Why not gravity batteries? I love the idea, and ...
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How is 0th law of thermodynamics possible?

If a body of gas A (say at $25\ ^\circ \rm C$) is brought in contact with another body of gas B (say at $10\ ^\circ \rm C$), then B absorbs heat from A until they both reach $17.5\ ^\circ \rm C$ and ...
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Why are velocity and kinetic energy not proportional? [duplicate]

In the equation $E_k=\frac12mv^2$ the fact that the kinetic energy of a moving object is not proportional to it's velocity seems counter-intuitive to me. Put another way, it seems intuitive to me ...
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Gibbs free energy: reduction from elastic solids to thermodynamics

The book on elastic solids that I'm reading defines the Gibbs free energy per unit mass as $g = e - Ts - \frac{\mathbf{P} : \mathbf{F}}{\rho_0}$, where $e$ is the specific internal energy, $T$ ...
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Energy release/gain in superposition (in Stern-Gerlach)

Let's have a quantum system in superpositon of two states $|e>$ and |g> with coefficients a and b. Now the system interracts with a field and is projected on either |e> or|g>. The question is what ...
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Where do particles get energy to vibrate nonstop?

The question is in the title. This sentence has no conncection to the question. I wrote it because the site required the body of question have some text.
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How is the energy output released in a proton-proton fusion reaction?

$$\rm p + p \to {}^2H + e^+ + v_e$$ The net energy output for this reaction, not including the electron-positron annihilation that likely would occur, is 0.423 MeV - according to my math and sources ...
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Free-falling elevator and jump at the last second: 2 different arguments, are they flawed?

There's a particular myth regarding an elevator and a person in it that goes like this: If a person is in a free-falling dive in an elevator (say the steel wires have broken and the emergency ...
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Kinetic energy in Lagragian mechanics [duplicate]

In my classical mechanics class they asked why the kinetic energy for an holonomic mechanical system has the homogeneous quadratic form. Of course for a autonomous standard system( system that is ...
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Energy change as ball rolls down a curved ramp [closed]

At the top of the ramp, the ball has gravitational potential energy. As the ball rolls down the ramp, the potential energy converts to kinetic energy. Due to friction, some energy is also converted to ...
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What is the theoretical maximum efficiency of converting electricity to EM radiation? Is there such a thing?

It turns out LEDs can have an efficiency of more than 100%, i.e. they output more light energy than electrical energy goes in. The 'missing' energy is taken from their thermal energy so the LED cools ...
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Energy in a vibrating part of an engine

I have sensor data of the vibrations of the parts of an engine. I was wondering whether I could calculate how much energy a part contains in a given time window. Now the energy of a wave given by ...
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Can I derive the 1 dimensional Maxwell-Boltzmann Distribution using this method?

I am aware that the 1D MB Distribution is usually derived by counting the momentum states in various directions. However, I would like to know if the following method using the 3 dimensional speed ...
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Energy and momentum

Does a muon or an electron with the same energy have the higher momentum? According to $E^2=m^2+p^2$ if you increase the restmass, the momentum must decrease. But if we look at a nonrelativistic case:...
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Work - Energy confusion when movement is vertical/up a slope

I've always thought of work done on an object as the energy transferred to it. This explanation, however broke down when I came across the following situation : A block of mass $m$ is transported to ...
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Fan and venturi effect

A simple calculation shows the power $P$ you can get from wind at speed $v$ passing through an area $A$ is (in Watts) $$P = 1.23\,v^3 \frac{A}{2}.$$ Using the above equation and taking the Venturi ...
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1answer
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What is an isolated system?

I am confused about the concept of conservation of energy in an isolated system. 1) Does an isolated system mean that it does not have any non conservative forces? If yes, then, is mechanical energy ...
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Definition For Internal Energy Of An Ideal Gas

We know that the internal energy for an ideal gas depends only on its temperature. What confused me was the correct defination of the internal energy for the ideal gas. Is it: $dU = nC_vdT$ OR $U ...
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Different energy levels in quantum mechanics

I have a doubt regarding energy levels. I saw that translational energy levels are quasi-continuous, rotational energy levels are discrete and vibrational energy levels are more discrete. I want to ...
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Vector field $\textbf{F}$ with energy density proportional to $|\textbf{F}|$ — elementary arguments that this is unphysical?

In Newtonian gravity, electrostatics, and magnetostatics, we have some vector field $\textbf{F}$ with an energy density proportional to $|\textbf{F}|^2$. What is the simplest, most elementary argument ...
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If an atom can have only discrete energy values, why it can move on any speed?

Atom, as a quantum system can have only certain energies. Also, energy, specifically kinetic energy depends on velocity, which should mean, that an atom should move only on some strict velocities, and ...
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How do we tell which part of kinetic energy gives rise to temperature?

I know that macroscopic temperature is a measure of kinetic energy of particles at very low scales (let's call it microscopic kinetic energy). But how can we derive which part of this microscopic ...
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1answer
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Why is the relation between coefficient of restitution and air pressure of a basketball logarithmic?

Perhaps this is due to the spherical shape of the ball but yea, i've conducted the experiment for school and i'm trying to work through justifying this but need assistance. edit: here's the data
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How can amplitude affect wavelength $\lambda$? [closed]

This was a question published by Cambridge International for an AS Level question last year: For (II), the mark scheme says that $v=0.5\lambda × f$, thus giving double the wavelength as supposed the ...
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How does a quasar convert mass to energy?

I'm trying to understand where the energy for a quasar comes from and specifically if it comes from converting mass to energy. I know that as matter falls into a black hole gravitational energy is ...
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How do we calculate energy of food?

Here is the problem, according to me: In a classic hamburger, according to my internet research, approximately 906 kJ are contained. Now take a car of 1 000 kilograms, and push it to 30 m/s. It has ...
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What has entropy to do with internal energy?

I'm trying to get an intuitive understanding of what entropy is and, frankly, I'm starting to get desperate for the click in my mind that allows me to connect all that stuff. This is my current ...
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1answer
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How to convert $\rm kg/m^3$ to $\rm J/m^3$ for natural gas and biogas? [closed]

I want to convert from mass density to energy density: Natural gas has a density of $0.8\: \rm kg/m^3$. Biogas has $1.15\:\rm kg/m^3$. I want to convert them into $\rm J/m^3$ so that I can ...
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Can bound states have zero energy?

Suppose a system is under the influence of a potential which vanishes at $\pm \infty$. Now we know that if the energy of the system is negative ($E<0$) then the system is in a bound state and the ...
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Is their a theoretical minimal amout of energy to keep some information?

The goal here is to keep a bit of information $b$ at the temperature $T$ during a time $\delta t$. I don't know if this question makes a lot of sense, considering the diversity of ways to encode ...
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Doesn't electrical energy gets lost due to slackening of local electrical wires hinged between street electric poles?

I am talking about the wires which get somewhat loose or somewhat like parabolic shaped due to earth's gravity. What I think is due to slackness of wires there is a net increment in resistance which ...
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Why does the phenomenon of localized energy producing particles and antiparticles lead to the creation of an infinite number of particles?

Penrose says the following (paraphrased) in "Road to Reality" on pg 611 of the first American edition A particle and anti particle may combine to produce energy, as given by general relativity. ...
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Why is there a 1/2 in the expression for electrostatic energy U?

The expression for electrostatic energy is $$U= \dfrac{1}{2} \times \int \rho\,\phi\, dV$$ where $\rho$ is the charge density and $\phi$ is the potential at that point in ($dV$) Let me explain what I ...
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First law of thermodynamics clarification on internal energy

Starting from the experience of joules we know that the mass before falling has a potential energy $mgH$ where $H$ is height from floor. Once we reach the height $0$ we observe that the kinetic energy ...
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Why are certain quantities so fundamental to physics? [closed]

Apart from having a qualitative description of quantities such as momentum, work and energy, why are these quantities considered so fundamental? What is the reason to define them in the first place? ...
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Work a medium of energy conversion? [duplicate]

Is work a medium to convert one form of energy to other? i.e. to say without work you can't convert type of energy? Similar type of question pertains to heat.heat is also medium of conversion of ...