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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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 ...
1k views

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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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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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 ...