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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Do we have a better understanding of what energy is since Feynman's time?
When lecturing about conservation of energy in the 1960s, Richard Feynman remarked:
It is important to realize that in physics today, we have no knowledge of what energy is. We do not have a ...
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Magnetostatic energy density -- derivation without introducing inductance?
I was looking for a derivation of the expression for the energy density at any point in a static magnetic field. I do know that it is
$$u_B=\dfrac {1}{2 \mu_0}\left|\mathbf{B}\right|^2,$$
I was just ...
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Calculating Energy of a Wave
So my physics examinations are coming up and I was going through my notes on waves, but I realized that there were some discrepancies.
In my notes, the energy of a wave is directly proportional to ...
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Work term in First Law of Thermodynamics
The first law states that
$\text{d}Q = \text{d}W + \text{d}U$
where $\text{d}W$ is the work done by the system on the surroundings.
As far as I remember from my chemistry courses,first law in ...
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Why does a conservative force return the work done against it by a body to that body?
Newton's 3rd law of motion:
Newton's third law of motion, or the law of action and reaction, implies that there is no isolated force in nature. Whenever there is any force at all, there must be at ...
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Why the rubber ball bounced higher than the glass ball?
For a teaching degree we did the following experiment:
Drop a rubber and a glass ball of approx. same size from approx. same height onto laminate flooring.
As we expected, the rubber ball bounced ...
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Relationship between temperature and energy
What is the definition of temperature in relation to energy? I'm mostly interested in general dimensional terms.
Is temperature the kinetic energy per mass?
Or kinetic energy per volume?
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Are neutrinos affected by gravity?
Layman here, but EE and BS physics. I know that light is affected by gravity. But are neutrinos? During the collapse of a star into a neutron star, as the electrons join protons to form neutrons (e....
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What is Energy made of?
We have the famous equation $E = mc^2$, and we also believe that matter is made of particles.
Then, What is the energy made of? If the two are interchangeable, there must be some common building ...
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About Work and Energy
I have a problem when it comes to work and energy I honestly cannot get the idea behind them, when I read about work it is defined as the ability to transfer energy, But when I read what energy is I ...
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Why can't we observe a fraction of a particle?
I have a train of thought which leads me to believe that we should be able to observe a fraction of a particle. Pelase help me:
In Quantum Field Theory, we model particles as total energy eigenstates ...
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Numerical calculation of a quantum field's observables
Okay so QFT is definitely beautiful and elegant theory, its mathematics is rich and ingenious, but there is so much one can do with symbolic manipulations of mathematical objects only, how can I ...
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Isn't energy absolute according to Thermodynamics?
I was taught that Internal Energy $U$ is a relative quantity: only changes in $U$ are meaningful. It doesn't have an absolute value, since it always comes with an arbitrary constant (for example $U = ...
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Connection between $\Delta x \Delta p \geq \frac{\hbar}{2}$ and $\Delta E \Delta t \geq \frac{\hbar}{2}$
Is there a way to derive second equation from the first one? I mean is there a connection between those two uncertainty relations?
\begin{align}
\Delta x \Delta p &\geq \frac{\hbar}{2}\\
\Delta ...
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Do black holes violate the conservation of mass?
This question has confused me for quite some time now. I have searched it up online, and the basic answer is: 'Mass is a form of energy. When black holes die they release the amount of energy that ...
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The time-energy uncertainty relation on a compact space
This post follows this answer of Ben Crowell.
The problem with the time-energy uncertainty relation $$[H,T]=i\hbar$$ is that it implies that the spectrum of the energy operator $H$ cannot be ...
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Are the concepts of kinetic energy, potential energy etc not valid in general relativity?
In Newtonian physics, we come across different forms of energy, such as kinetic energy, potential energy etc. But in general relativity, we find only the total energy that is obtained from the energy-...
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Do virtual photons have a frequency?
Real photons do have frequencies, which is directly related to its energy. So, can virtual photons that take part in EM interactions have frequencies too?
When my hand is pressed up against a glass ...
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Can we get energy from the Earth's rotation?
Is there any way to harvest large amounts of energy from the Earth's rotation?
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Impulse from absorbing a photon? Is there an increase in rest mass?
I'm going through A P French's special relativity. In one chapter (6) the following is set up:
Suppose that a stationary particle of mass $M_0$ is struck by a photon of energy $Q$, which is ...
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What are the "inexact differentials" in the first law of thermodynamics?
The first law of thermodynamics states that
$$dU=\delta Q - \delta W$$
I have only just graduated high school and I am finding the above form of the equation rather difficult to understand due to ...
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What is "A" in AGeV?
AGeV means GeV per nucleon. But why A letter is used for such a short cut? Why not NGeV, for example?
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What is the highest energy position for a double pendulum? And for which energy positions is it chaotic?
Math/physics teachers love to break out the double pendulum as an example of chaotic motion that is very sensitive to initial conditions. I have some questions about specific properties:
For a ...
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How is gravitational potential energy $mgh$?
I know the derivation that $W=Fd$, hence $F=mg$ and $d=h$ so energy gained by the body is $mgh$ considering the body on the ground to have $0$ gravitational potential energy.
But the definition of ...
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How far can light go?
When a ray of light is projected, (say) from the surface of Earth to outside in space. The condition is that, there is no obstruction to it till infinity (it travels only in vaccum). My question is ...
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Hydrogen energy levels and energy-time uncertainty principle
Some hydrogen atom exists in some excited quantum state, and after some time $\Delta t$ it's de-excited, emitting a photon carrying the energy difference.
It is claimed that this photon will carry ...
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How would normal matter behave under conditions found in the core of the sun?
Considering how low the power density is at the sun's core, I seem not to be able to expect what would happen to matter in case it was thrown inside the sun's core.
For example, let's assume an Earth-...
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What exactly is work?
What exactly is work? My book confuses me:
a force can lift an object to a height h, or it can accelerate an object through gravity. In all these cases, a force displaces an object and change the ...
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Why does blowing on hot coffee cool it down?
And will it cool off faster if you blow across the top of the cup or directly into the coffee?
Does it have to do with the fact that when you blow across the top of the cup the velocity of the air ...
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Does electron absorb energy? [closed]
According to my teacher, an electron is point sized and it does not absorb or release energy. Moreover, my teacher says their orbital absorbs energy rather than the electron. If that is the case, then ...
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What is negative Energy/Exotic Energy?
So I have been researching around a little as I am highly interested in Astrophysics and I came across an energy I have never heard of before; negative energy also commonly known as exotic energy. Now ...
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Strength of moonlight
So I read this xkcd, and thought it was kind of cool. However, I showed it to my engineer father and physics undergrad brother and neither seem to find the explanation very convincing (though they ...
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Can the kinetic energy be a function of the position vector?
,I got one confusion when reading Goldstein's Classical Mechanics (page 20, third edition). After getting the equation
$$
\sum \left\{\left[\frac{\mathrm{d}}{\mathrm{d}t}{\left(\frac{\partial T}{\...
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Entropy: Disorder or energy dispersal?
The first definition of entropy given by Clausius is I believe this
$$S=Q/T$$
It is as I understand a common fact to understand entropy and maybe often teach it as a measure of disorder through the ...
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Understanding relationship between work and energy
I've read over 10 books about work and energy, and I just simply can't understand it.
First of all, they go ahead and randomly define that work is force times distance:
$$W=F X \cos\theta$$
Okay, ...
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Particle with spin in uniform magnetic field
In the text "Introduction to Quantum Mechanics" by Griffiths the following is stated: The magnetic dipole moment $\vec{\mu}$ is proportional to its spin angular momentum $\vec{S}$: $$\vec{\mu} = \...
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Why does the work-energy theorem need to include internal forces?
Can anyone kindly explain me why work energy theorem must also include internal forces?
The proof of work energy theorem is derived from Newton's laws of motion, but Newton's laws of motion don't ...
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Why is everything floating in space so cold?
Of course there is pretty hot stuff too. For example Suns.
But isn't the vacuum of space the perfect containment for heat? And shouldn't the rare collision with particles even heat up an object that ...
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How does laser cooling work?
While asking this question: physics.stackexchange.com/questions/273119/is-there-a-way-to-hold-a-molecule-still I learned about laser cooling. I wanted to know how it works. Does it have anything to do ...
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Empirical equivalence of shifted chemical potential $\mu_i$
It is often said that, in classical thermodynamics, entropy $S$ and energy $U$ are defined only up to an additive constant proportional to the total amount of substance $N=\sum_i N_i$ (where the sum ...
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Positive and Negative works [duplicate]
When we displace a body from ground to a height we do positive work against gravity and gravity too does work but negative. Do they cancel each other? Or does the external force do extra work?
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Is the molecule of hot water heavier than that of cold water?
We know that the molecule of hot water($H_2O$) has more energy than that of cold water (temperature = energy)
and according to Einstein relation $E=mc^2$ ,this extra energy of the hot molecule has a ...
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Veritasium Electricity videos: where does the majority of energy really flow? [duplicate]
After watching Veritasium second video on electricity (references at the end), I have some doubts about where the majority of the energy flow actually happens. The reference experiment is the simple ...
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Why is photon energy no function of transition time?
An electronic transition in an atom produces a photon, and the photon's energy is determined by the difference between upper and lower electronic state. For the final photon, its energy is described ...
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Where does the energy of the expansion of frozen water come from?
As you cool a system, you are removing energy, yet as water transitions to a solid, it expands, exerting sufficient force to rip through metal, for example in residential copper water pipes that ...
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What are the forms of energy at fundamental level? [closed]
Most high school textbooks distinguish several forms of energy.
Mechanical:
-Kinetic
-Potential
Chemical
Electromagnetic
Nuclear
Gravitational
Thermal
Hydraulic
Electric
Mass
...
This ...
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Work done while compressing an ideal gas (the physical significance of $\int \mathrm dp\,\mathrm dV$)
Today in our chemistry class we derived the pressure-volume work done on an ideal gas. Our assumption was that $$p_\mathrm{ext}=p_\mathrm{int}+\mathrm dp$$ so that all the time the system remains (...
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Refraction: Energy flow, tunneling and Evanescent EM waves
In the full internal reflection case where we have a refracted evanescent wave, If another object is nearby, then we could have wave tunneling phenomenon(frustrated total internal reflection). I am ...
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In layman's terms why is the time component of the stress-energy tensor associated with energy density?
Why is the time component of the stress energy tensor $T^{00}$ associated as energy density?
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What does the work on a current carrying wire in a Magnetic Field?
We consider that the force acting on a current carrying wire placed in a uniform magnetic field perpendicular to the length of the wire is given by $IBl$. If the wire moves by a distance $x$ in a ...
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