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 the [energy-conservation] tag if it is specifically about its conservation.

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What would be the theoretical energy required to make a "microscopic" temporary black hole that could move an atom?

I stumbled across some videos/news about The Large Hadron Collider and all the panic around the possibility of it creating a black hole. Of course, it is impossible to create a black hole and from all ...
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Two objects of equal and opposite velocities collide elastically. If the two objects have different masses, which one has a bigger final speed?

Question: Two objects, one less massive than the other, collide elastically and bounce back after the collision. If the two originally had velocities that were equal in size but opposite in direction, ...
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Multiple questions on equation 1.6 after deriving Planck's law on page 5 (Schwartz QFT)

After deriving Planck's law for the expectation energy of a single mode, Schwartz takes the limit $L \rightarrow \infty$ and turns the sums into integrals. The average total energy of the blackbody ...
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Can you combine photons to ionise electrons?

I was talking with a friend about the photoelectric effect. I know that only light of a certain energy will eject an electron from a metal plate. But consider this. A photon (red) had the exact energy ...
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How does the heat energy from burning a gallon of gasoline compare to the solar energy the resulting $\rm CO_2$ absorbs in the atmosphere?

I think essentially all of the energy from a gallon of gasoline burned in, say, a car is eventually converted into heat in the atmosphere. But I don't think the heat from burning fossil fuels ...
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·Can Physically Apply + Positive Newton's III Third Law of Motion as Cycle System in a Motor Engine [closed]

*Physically Can Apply Positive/in favor Newton's III Third Law of Motion as a Cycle System in a Motor Engine Device? 3rd Thermodynamic Law Applicate in a Power-Plant Device. The Reaction at From of ...
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Finding the stress tensor in the Stokes equation from free energy

I have found free energy for a system of particles. The free energy is a functional of a scalar field which is the area of the particles. So I have the following free energy $F(A,p)= \int dx dy \: f[A(...
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Understanding the Cahn-Hilliard equation in terms of units

The Cahn-Hilliard equation may be formulated as $$ \frac{\partial c}{\partial t} = M \nabla^2 \left(\frac{\partial \hat f}{\partial c}\right), $$ where $c : \Omega \to [0,1]$ describes the ...
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Is the definition of work related to the nature of the fundamental interactions?

I am having troubles trying to understand why is work defined as it is. So, I know how work is defined: $W = \vec{F}\cdot{}\vec{d}$ (F is the force, d the displacement) and I am okay with it. This, ...
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Dispersion of $E$ and $p$

Is the dispersion of $E = c$. dispersion of $p$? Following that: $\begin{aligned}E^{2}=c^{2}p^{2}+m_{0}^{2}c^{4}\\ \Delta E^{2}=\Delta c^{2}p^{2}\\ 2\Delta E=2\Delta c^{2}p\\ \Delta E=c^{2}\Delta p.\...
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Is it true that the Poynting theorem $P=E\times H$ is quite valid for DC circuits?

Can we assume that Poynting's vector theorem $P=E\times H$ is one of the universal laws of physics that applies to electromagnetic fields in AC and DC circuits.Is there A rigorous analysis of ...
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Electric Power and Energy

I was asked a question three times this week and I have been giving the same explanation over and over again, but I am starting to question myself. if you have data on a PV panel production in the ...
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Why are the electric and magnetic fields of a charged particle a single entity and should be mutually orthogonal?

I think Maxwell's four well-known equations dictate the correct answer. I guess the question is a general rule (no specific problem) in electromagnetism as well as Maxwell's 4 equations themselves. ...
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Energy and pressure relationship for cosmic rays [closed]

How to prove $E = 3P$ for cosmic rays? In other words $\gamma = \tfrac43$ for cosmic rays. Whereas for $\gamma=\tfrac53$ we have $E = \tfrac32 P$.
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Most efficient method to evaporate a metal in a vacuum?

If one had a metal block (lets say tantalum) in a vacuum chamber, what would be the most efficient way of evaporating it in regards to the proportion of the input energy directly going into removing ...
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Moving body is collided with a spring system.Why there is a difference in maximum compressed distance for different observers?

Suppose a body of mass m moving with velocity collides with a spring system.The event is being observed by two observers, one at rest and one moving with a velocity v opposite direction to that of the ...
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Probability of observing harmonic oscillator at a particular position

Consider a classical harmonic oscillator whose Hamiltonian is $$H=\frac{p^2}{2m} +\frac{1}{2}mw^2x^2$$ where $w$ is the oscillating frequency. I wish to find the probability of observing the harmonic ...
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Energy conservation in the Sachs-Wolfe effect

When energetic photons climb out of a potential well (eg Sachs-Wolfe effect) they lose energy. According to conservation of energy, where does this energy go? Into the gravitational potential making ...
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How can we say potential or chemical energy is part of an object?

I'm posing this question primarily in the context of special relativity. Also, I'd like to leave gravitational potential energy out of the picture since that would take us into a different direction ...
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Non-Work energy conversion

Are there any conversions of energy that are not considered work, (energy change with a force and displacement)? For example heat transfer does not convert the form of energy, can something such as ...
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Relativistic particle-antiparticle annihilation

If a particle and antiparticle annihilate they produce photons of certain frequency. My question is about the frequency of the photons if the particles move at relativistic speeds with let say gamma=5....
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Concept of work [closed]

How was the concept of work and energy invented in physics? What is the actual idea behind work and energy? Why are distance and force specifically used to define work, why not time instead of ...
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How do I calculate enthalpy and entropy from Gibbs energy? [closed]

If for a system the Gibbs energy depends on the temperature like this: $$G(T) = aT + b + c/T$$, how would enthalpy and entropy depend on temperature?
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What is the matter/energy flux on the cosmological horizons?

Consider the usual standard FLRW cosmology. Depending on the model, there may be two distinct horizons around any local comoving observer: the particles (or causality) horizon and the event horizon. ...
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How comes, that my book states that internal energy is dependent on volume?

Currently I am reading in the 11 edition of Physical Chemistry, Atkins in the chapter 1st law of thermodynamics. A few pages ago I learned the following: Makes sense for now, as far as I know it ...
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Why is Gibbs free energy not used in Statistical Associating Fluid Theory (SAFT) Equation of State (EoS)?

I am trying to understand Statistical Associating Fluid Theory (SAFT) Equation of State (EoS)?1. In this method, the properties of a substance is obtained from its Helmholtz free energy. The Helmholtz ...
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Currently self-studying QFT and The Standard Model by Schwartz and I'm stuck at equation 1.5 in Part 1 regarding black-body radiation

So basically the equation is basically a derivation of Planck's radiation law and I can't somehow find any resources as to how he derived it by adding a derivative inside. Planck says that each mode ...
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Negative potential energy

In fusion of hydrogen into helium, hydrogen has negative potential energy which adds up and results in a mass defect.. Doubts:- Why is there negative potential energy between the nucleons in a ...
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Mechanics | Use the work-energy principle to find the speed of the book as it hits the floor [closed]

Problem A small book of mass $m$ is held on a rough straight desk lid which is inclined at an angle $\alpha$ to the horizontal, where $\tan\alpha=\frac34$. The book is released from rest at a distance ...
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Does the quantisation of energy apply to everything? [duplicate]

Radiation is quantised according to Planck, so that's out of the question. However, I have seen many simplifications that claim Planck introduced quantised energy. Period. Has Planck really done that? ...
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How does doubling the de Broglie wavelength affect the kinetic energy of a subatomic particle? [closed]

I'm an AQA A-level student revising for my exam next week and I came across a question that I managed to solve, but a friend of mine used a different method and got the wrong answer. However, my ...
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A bottle weights 4.5kg, what's its mass in TeV (Terra Electronvolts)? [closed]

I'm not entirely sure about this problem. And the second one says: An apple falls down from a height of 1 meter. What's it's kinetic energy? My thoughts: 1/2gt^2= 1 meter -> solve for t, then v=s/t ...
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Time differential between two signals sent at two different near-relativistic speeds

There is something I don't quite get about relativistic velocities, which I am hoping to clarify with this question. Suppose you have an emitter and receiver both located somewhere far away from one ...
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Is there a way to manually change the energy of a particle?

We have a relation, $E=h\nu$, where $\nu$ is the frequency and $E$ is the energy of any particle. If we have a function, $\psi$, that is an eigenstate of energy, the particle has a definite energy, ...
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“Standard kinetic energy term of nonrelativistic quantum mechanics” $[\mathbf p\cdot \mathbf A + \mathbf A \cdot \mathbf p]$

On page 187 of Peskin and Schroeder, a certain term in an expression for the amplitude of an electron scattering from a classical magnetic vector potential is identified with “the contribution of the ...
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Proof that minimum free energy corresponds to equilibrium

I've tried to derive the relation between free energy, $F$, and equilibrium of a closed system. From the derivation i've written below it seems that if $T$ is constant and the system can't exchange ...
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Gear Ratio And Torque Relationship

suppose a gear teeth ratio of (1:10) >> then it is obvious that the smaller gear will be faster than the larger gear >> And the higher gear has more torque which means a higher ...
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Does the vacuum really have infinite energy density?

I said: As far as I understand it quantum field theory says that the vacuum has an infinite energy density. r/AskPhysics RedditorAbstractAlgebruh said: But wouldn't that be due to the way we do the ...
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What law of thermodynamics is broken here? [closed]

This question came in the Dhaka university admission exam 18-19. A heat engine in each cycle does positive work and loses energy as heat with no heat energy input. Which law of thermodynamics does the ...
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Unable to make sense of - Heat, a form of energy [duplicate]

I have referred to some textbooks on Thermodynamics and they more or less explain heat in the following manner: Consider a system which has some energy content. In order to increase the energy content ...
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1 vote
2 answers
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Nuclear fusion mass defect and energy production

In researching the topic of nuclear fusion, I have a few doubts related to the mass defect, 'negative' potential energy and resultant energy released as a by-product of nuclear fusion. When two ...
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Can someone please explain energy of electrons in Bohr's model?

Energy of and electron is $$E = \text{kinetic energy} + \text{negative of potential energy}. \tag{eq-1}$$ But energy of electron in the $n$th orbital is also $$E = -\frac{13.6}{ n^2} \tag{eq-2}$$ ...
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Why is angular momentum equal to mass times radius times velocity?

When momentum is mass times velocity, why is angular momentum mass times radius times velocity?
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How is the internal energy changed if an atom is charged?

I read this statement from an exam: A neutral carbon atom gains 4 electrons, this atom has a higher internal energy after the addition than before. I am not sure whether it is correct, in particular, ...
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In an isothermal process there is no change in internal energy, but why in an isobaric one?

In an isothermal process, the heat Q added (which increases the temperature and internal energy) is exactly cancelled out by the work W done by the gas in the environment (which lowers the temperature ...
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Pressure wave speed $v=\sqrt{\frac{B}{\rho}}$ rearranged $\rho v^2=B$ is similar to kinetic energy: $E_k=\frac{1}{2}mv^2$. Is this a coincidence?

The speed of a Pressure wave speed is given by: $v = \sqrt{ \frac{B}{\rho}}$, which seems rearranged very similar to kinetic energy: $\rho v^2 = B$. How can this be understood, is this a coincidence? ...
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Why does applying the kinetic energy operator to a free particle result in a divergent integral?

The wavefunction of a free particle is just $$\psi = Ae^{i(kx-\omega t)}$$ and when you plug this into the Schrodinger equation you get the dispersion relation $$E = \frac{\hbar^2 k^2}{2m}$$ However, ...
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Energy of electric point charge at and not at the origin

This questions comes from reading through the fourth edition of Griffith's Introduction to Electrodynamics. Consider a point charge q located at the origin. Its potential is given exactly by $V_0 = \...
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Is there a temperature difference when I change the volume instantaneously? [duplicate]

If I were to instantaneously remove the wall as drawn above, then there is no loss of molecular velocities --> T1 = T2. But I have a problem imagining that there really is no change in temperature. ...
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What is the exact ratio of dark energy to mass?

I need to know the exact ratio between mass and dark energy (total dark energy in the universe / total mass in the universe). I could only find it to 2 decimal points (0.68). I need this to make this ...
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