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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Is the intensity of a light wave related to frequency of the wave?
My problem is: How can I resolve these following ideas?
Energy of photons in an EM wave is proportional to the frequency of the wave
Intensity of an EM wave is proportional to the energy that is ...
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In an elastic collision, is conservation of energy and momentum enough to define the behavior of a system?
An example I was thinking of is two balls of equal mass M, where the first ball has an initial velocity of v_1 m/s, and the second ball has 0 velocity. The final velocities are v_2 and v_3 ...
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When an electron absorbs a photon and gets excited to higher energy level what does happen to its kinetic and potential energy?
We know for a fact that if an electron absorbs a photon with enough energy it gets excited to a higher energy level, which increases that electron's total energy and we know that energy can present ...
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Is the expression of elastic energy in this paper correct? [closed]
Elastic properties of Ni2MnGa from first-principles calculations
I am reading a paper investigating the linear elasticity of a crystal. However, I am a little bit confused over the expression of ...
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What is the explanation of dark energy for the Unified Field Theory? [closed]
Found out an explanation for the Unified Field Theory. How come time was never seen like this before?:
Unified Field Theory (Part 1)
Unified Field Theory (Part 2)
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Commutator of energy and unitary time evolution operator
Edited:-
$\hat{H} = -\frac {\hbar^2}{2m} \frac {\partial^2}{\partial^2 x}~+~V$
$\hat{U}(t) = e^{-iHt/\hbar}$
The Robertson Uncertainty Principle states:-
$\sigma^2_A\sigma^2_B \ge (\frac {1}{2i}[\hat{...
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Gibbs energy calculations for compounds
I am studying machine learning during my master's degree and have a simple task. I am using the FactSage Pure Substance Database (https://www.crct.polymtl.ca), which provides a list of phases for a ...
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Relation between Star's Mass and the radius of an orbit of a Planet? [closed]
I have tried using the Gravitational force formula, Energy and even Circular motion equations but I am getting D as the correct answer. Please help me with the question.
How does the mass of the star ...
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where did earths water comer from? [closed]
Water on earth
We can create water by igniting hydrogen gas and oxygen. The reverse process, separating water into hydrogen and oxygen, requires the presence of salt.
This reaction gives off a great ...
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Magnetic energy expression in frequency domain
I have a question concerning the integral expression of the magnetic energy in frequency domain, I sometimes find this expression:
$$ E_m = \tfrac12 \int_V \bar{\mathbf H}\cdot\bar{\mathbf B} ^* dV $$
...
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Can internal energy and entropy move in opposite directions?
Is it possible for there to exist system for which there is some quasistatic process which causes this system to have $\Delta U < 0$ while $\Delta S > 0$? No constraints on the type of system as ...
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Statistical proof of the principle of least action
In statistical thermodynamics we can prove that the evolution of a system minimises some potential with units of energy (e.g. energy). This can be done purely statistically, by using the first two ...
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On the non-quasistatic transfer of heat
Suppose we have two bodies at different temperatures, and we let them interact thermally in such a way that the process is not quasistatic (e.g. two different metal spheres touching). Do we arrive at ...
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Why should this thermodynamics problem be indeterminate?
Callen asks us to consider the following
A cylinder of length $L$ and cross-sectional area $A$ is divided into two equal-volume chambers by a piston, held at the midpoint of the cylinder by a ...
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Work and energy question along circle [closed]
The 10N force is in a constant direction, but when calculating work we need the angle between the force and the direction of movement. Here the direction of movement (the tangent to the circle) is ...
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Elastic potential energy formula
From the Wikipedia page on elastic energy, we can find a bunch of formulas to describe it. For example, in the continuum section it talks about energy per unit of volume (density?):
$U=\dfrac{1}{2}C_{...
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In inelastic scattering between photons and electrons, how does light only partially transfer energy?
In the case of elastic scattering or absorption between photons and electrons, the incident light is either fully reflected or fully absorbed and hence it either retains its wavelength or ceases to ...
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About the energy dependence of differencial cross section in QFT?
We know that the famous formula of differential transition probability per unit time (DTP) in QFT is written as:
$$
DTP= (2\pi)^4 \delta^{(4)}(p_f-p_i)|A_{fi}|^2 \prod_{final} \frac{d^3 \mathbf{p}_f}{(...
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If two opposing equal forces act on an object, how does one calculate the energy exerted? [duplicate]
If energy exerted is a function of distance and force, the object would not move in this case. But would energy not still be exerted?
Simplified the example for sake of clarity; but specifically ...
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On using intensive variables to describe the macrostate
I am currently reading through Callen and, though Callen is very thorough, I find myself puzzled at the following question.
In Callen's treatment, the thermodynamic formalism is introduced ...
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Interpretation of many-body Green function in energy space
In real space and in equilibrium, the retarded Green function $G_{i,j}(t)$ gives the amplitude probability for propagation of a particle transfering from j to i during time t. The Laplace ...
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Kinetic energy in the semiconductor band diagram
In Modern Semiconductor Devices (available online from the author), it's stated that "any energy above $E_C$ is the electron kinetic energy", accompanied by the figure below. Additionally, ...
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Can heat flow from colder to hotter body breaking the second law of thermodynamics? [closed]
In an isothermal process, the whole heat is converted to work but not in a cycle so it doesn't break the 2nd law of thermodynamics.
$$dQ=dW$$
But let's imagine a scenario with it. If we connect an ...
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Can pipes with pressurized fluid act like an electric grid?
Electricity is used for many things. One of the biggest uses is transporting energy, almost instantly, from a power plant to the machines in my home and many others'.
I was wondering if a similar ...
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Free energy of isotropic superconductivity ($s$-wave)
As far as I know, $s$-wave superconductivity has a constant order parameter (or a gap function) $\Delta$.
Ginzburg-Landau free energy takes a form $$(\nabla \Delta)^2 + |\Delta|^2 + |\Delta|^4$$
(I ...
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Is $TdS = dQ$ true for processes involving mass transfer?
In his famous thermodynamics textbook, Callen writes
The identification of $-PdV$ as the mechanical work and of $TdS$ as the heat transfer is valid only for quasi-static processes.
My question -- is ...
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What are the exact conditions for 1D elastic collisions? Is there a list for characterizing when elastic collisions occur?
We know there are elastic, inelastic, and partially elastic collisions. In a fully elastic collision, both momentum and kinetic energy are fully conserved. However, that tells us nothing for which ...
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Energy in reconfiguring a system of charges
Let's all agree that the formula $ \frac{1}{2} \iiint \rho_{macro} V_{macro} d\tau $ is a decent approximation to the work done to build this system of charges (basically a considerably large charge ...
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Pushing off another object — why does the other object do work on you without expending any energy?
Suppose you are in space (or on a frictionless surface) next to another object of the same mass $m$ as you. Take the reference frame of the center of mass of you and the object (so there is zero ...
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Why and How is there heat loss due to friction? [duplicate]
In this question, the answer is given 432 which is double of the expected 216.
This appeared counterintuitive.
Why is the work done by the motor double of the difference in final and initial kinetic ...
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How do we justify that work is a "transfer of energy" in the general case?
By the work-energy theorem, we can justify that the work on a particle due to the net force equals the change in kinetic energy of the particle. In compact notation,
\begin{align}\tag{1}
W_{\text{net}}...
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Why can't entropy decrease for an adiabatic and closed system?
In equations (10) and (11) of this article by Curzon on the adiabatic piston, Curzon uses that a system which is thermally insulated cannot have its entropy decrease (irrespective of anything else ...
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Influence of slide mass to recoil of a pistol
All other things being equal, how does the mass of a pistol's slide effect the recoil imparted to the shooter?
Background: firearm enthusiasts often assert that the larger the "reciprocating mass&...
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The "geometry" of thermodynamics
I have a vague notion that thermodynamics is best captured in some language like differential geometry or something of the sort, but I am unfamiliar with said language. That being said, it seems to ...
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What is the difference between $E$ in Einsteins theory of relativity, and $E_k$ in the equation to work out kinetic energy? [duplicate]
If $E=mc^2$, and $E_k=\frac12mv^2$, what is the difference between $E$ and $E_k$?
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How to determine when a body has only kinetic translational energy or only rotational kinetic energy or both?
does the rod have translational kinetic energy when it falls flat?
since I thought gravity acts on the body center of mass should have some downward linear velocity only but the solution given says ...
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Statistical Mechanics, Canonical Ensemble: probability the system will have a particular energy is not well explained
A typical statistical mechanics explanation for the probability ($p_i$) that a system (S) in contact with a reservoir (R) will take on a particular energy ($E_i$) is as follows (quoting from Greiner's ...
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Is it possible to deduce the chemical potential from a molar fundamental equation?
In Callen, it's shown that for a simple system, the entropy can be written as $U(V,S,N) = Nu(v,s)$ where $s = S/N$ and $v = V/N$. One can then prove (as I asked here) that both the temperature and the ...
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Change in potential and kinetic energy after a photon is absorbed by a electron
This is a question from the AP Physics 2 2022 FRQ. Why does the potential energy of the atom and the kinetic energy of the electron decrease after a photon is absorbed? Shouldn't both of them increase?...
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Force not applied on center of mass
I have a question about rigid body motion when a force acts on off-center of mass.
I read the answer to the post force applied not on the center of mass but I'm still confused.
I understood that ...
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On the additivity of entropy
In Chapter 1 of his famous textbook on thermodynamics, Callen gives (among various other posulates) the following postulate:
Postulate III The entropy of a composite system is additive over the ...
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Gas leaking out of a cylinder at constant pressure and volume [closed]
The gas is leaking out in such a way so as to maintain the cylinder at a constant pressure and volume, i.e the product of nT is constant.
If at a time t, the temperature of the gas is T and the number ...
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Calculating energy of a spring? [closed]
A toy of mass 20g attached to a spring is compressed and when the toy is released it jumps 0.7 m into the air. Calculate the energy of the spring. Assume the limit of proportionality is not exceeded.
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Shouldn't falling water heat up eventually?
The average internal kinetic energy of an object refers to its temperature. If the kinetic energy increases, the temperature of the object increases and vice versa. If that's right, then falling water ...
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Helmholtz free energy change inside box with constant volume and a partition
Consider a box with substance inside. It has a fixed, constant volume and is at constant temperature. The box is split into two halves by a piston.
How does the Helmholtz free energy change if the ...
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On definitions of interactions in thermodynamics—changing the energy of a closed system without work or heat?
On page 22 of his famous textbook on thermodynamics, Callen asks us to consider a particular gas (a simple $(N,V,U)$ system) enclosed in a cylinder. Callen then tells us that
a small paddle is ...
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Work done by electrical forces, when separating plates of disconnected capacitor
Asking this because I could not understand the correct answer for a question.
Question goes like this:
A capacitor is charged with direct current, disconnected; plates are separated further ...
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The sign of the energy released by an explosion
On a simple system with no external force, we can calculate the energy released by an explosion as the difference of the sum of the kinetic energy before and after.
With kinetic energy $K_i$ (initial) ...
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Potential energy density of a fluid in motion
If I have a fluid in motion at a given time and I know its pressure $p$ everywhere, I know its dynamic viscosity $\mu$ and I know its velocity field $\vec V$.
This system must hold some intrinsic ...
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Force in the Casimir effect
In the Casimir effect, after performing the regularization, it is found that the zero point energy between two conducting plates in a distance $L$ from eachother is (in the 1D case),
$$E=-\frac{\hbar ...
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