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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7
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4answers
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Newton's 3rd law… hitting drywall (which I break) vs hitting a brick (which breaks me)?

According to the Third Newton's law of motion: For every action there is an equal and opposite reaction. So, I understand that if I hit a brick wall with $50\, \mathrm{lbs}$ of force, the ...
1
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0answers
34 views

Are all kinds of energy quantized? [duplicate]

I was reading this answer of David Z about meaning of "Quantum" that this question came in mind: If light is quantized (i.e. is a integer multiples of some unit $\hbar c/k$), are other kinds of ...
2
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0answers
36 views

When we say particle in a box has quantized energy, is that kinetic or total energy?

In quantum mechanics, it is usually said that energy of the bound (constrained) systems such as particle in a box (infinite potential well) is quantized. It confuses me exactly what type of energy is ...
5
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1answer
71 views

Energy Spectrum and Gauge Transformations

There is a question in SE about the fact that the Hamiltonian isn't invariant under the EM gauge transformations. I wanted to ask about its consequences here. I know that in general, the Hamiltonian ...
0
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0answers
26 views

Is there a maximum temperature? [duplicate]

I have read about absolute zero. I understand that it is impossible to cool a system below absolute zero (or to absolute zero for that matter). However, is there a maximum temperature? My intuition ...
3
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2answers
42 views

Does shortening the path length of an excited etalon do work? What about LIGO?

Start with a high-Q etalon excited on-resonance with a laser for a time long enough that it has built up an essentially stable standing wave. There is a constant outwards-directed force on each mirror,...
1
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0answers
30 views

Minimum gap between consecutive energy levels?

Assume a standard one-particle, non-relativistic Hamiltonian of the form \begin{equation} H=\frac{p^2}{2m}+V(r) \end{equation} and denote its eigenvalues as $E_{n,\tau}$, where $n$ is the principal ...
-7
votes
1answer
57 views

Doesn't quantum uncertainty disprove string theory? [duplicate]

String theory states that the oscillations of little strings are responsible for all the particles in and the evolution of the universe. The specific type of particle created by a string depends on ...
0
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1answer
29 views

Is the energy cost of desalination linear vs salinity? [on hold]

There is a practical limit to how salty water can be and still have a useful potential for desalination. For example, a recent study on the potential for water recover from known sources on Mars ...
0
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0answers
24 views

Stuck with 2D kinetics problem [on hold]

I've been trying to solve this exercise for the last four hours and I'm totally stuck. The problems goes like this: Given the mechanism in the image, located in the vertical plane, the OB cord ...
0
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1answer
20 views

how does an induced draft fan work in a coal

I have been studying a coal-fired power plant, and the power plant cycle has an induced draft fan that is used to create negative pressure to help get the flue gas travel to the stack. I am confused ...
2
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1answer
25 views

What is the root mean square energy?

How the root mean square energy of nucleon in a nucleous of atomic number A in ground state is depend on A ?
-3
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1answer
59 views

In relativity theory could we talk energy rather than frames of reference? [on hold]

In relativity theory, the space geometry, of space and time, is no longer absolute, if one observer is accelerating compared to the other. Could I say in other words that space geometry is changed if,...
0
votes
1answer
58 views

Proper definition of Internal energy

In Internal energy of a gas ($U$), we have to include ALL types of energies possible like $KE$, bond energies, nuclear interaction energies, etc.., However we have derived that $U = (f/2)nRT$ from ...
0
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1answer
28 views

Conservation of Energy with Rotation Caused by Friction

A book that I am using to study poses the following problem: Suppose you wish to use a spring with a force constant $k$ to launch a spherical ball or radius $r$ and mass $m$ up a ramp of inclination ...
2
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0answers
26 views

If a wormhole connects between ground and sky, does an object require extra energy to pass through it? [duplicate]

As we know, if we bring an object from ground to sky (higher position) by normal methods, extra energy is required to overcome the gravitational potential energy. Then if a wormhole connects between ...
0
votes
0answers
24 views

How would I go about calculating the frequency and amplitude of a sound produced by a collision?

How would I go about calculating the sound pressure level, frequency and amplitude of a sound caused by a collision, given the velocities of the two objects colliding, their acceleration, densities, ...
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0answers
64 views

What is the difference between a “thing” and an “abstract”? [closed]

My terminology may be screwed up here, but bear with me, please. A "thing" is something palpable: it has Mass. Mass itself is an abstract whose purpose is to measure Matter. We are told that speed, ...
1
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2answers
52 views

Is energy only transmitted through electromagnetic and particle radiations? [closed]

Which are the other ways of transmission if any? If energy does not require any medium for transmission(as for sunlight reaching earth, the heat too), is it transmitted in quanta in particle radiation ...
0
votes
1answer
52 views

What is the capacity of an electron to absorb energy? [closed]

When an electron get excited we find that electron absorbs energy and jumps to the required shell. This means that electron can absorb an infinite amount of energy and can get excited to infinity ...
3
votes
1answer
93 views

How does eating a cold ice cream actually heat up your body? [closed]

The question is the title itself. During winter one of my friend told we'll have an ice cream. And rest of us where like are you nuts. He was like cold ice-cream actually heats up your body. Even I ...
4
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2answers
47 views

Do radio waves from the Sun reach Earth?

Do radio waves from the Sun come into contact with Earth? If so, do they penetrate the atmosphere or are they reflected, absorbed, or scattered?
3
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1answer
32 views

Black body radiation and number of photons emitted

Usually the black body radiation (at a certain temperature $T$) is given by $$\rho ( \nu ) = \frac{8 \pi h \nu^3}{c^3 \left( e^{h \nu / (k_B T)} - 1 \right)}$$ This quantity $\rho ( \nu )$ should be ...
-1
votes
0answers
16 views

Bernoulli equation: energy conservation in moving reference frames

I get a bit confuse with the use of Bernoulli equation in moving (inertial) frames. Consider the following example. The tube in picture is moved with velocity $u$ inside a tube where fluid moves with ...
0
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2answers
20 views

Vertical and Horizontal Oscillations With the same period and speeds

Why do vertical and horizontal springs with the same masses attached oscillate with the same period and the same speeds at matching positions? Assume that the horizontal surface is frictionless and ...
-1
votes
1answer
41 views

What is the maximum amount of electricity I can draw in 48 hours from a domestic supply? [closed]

An electricity company is offering free electricity for a 48 hour period every week. The rate the rest of the time is very expensive. How much power can be drawn from a domestic UK supply? Would it ...
1
vote
1answer
35 views

What is the average range of voltage of lightning in sky?

I wondered how much energy is required to produce , such a loud sound? What is the average range of voltage in sky? Is there any possible way to store it ,and use when needed?
14
votes
4answers
679 views

Increasing a potential causes increase in energy levels

Suppose a potential $V(x)$, and suppose a bound particle so the allowed energy levels are discrete. Suppose a second potential $\widetilde{V}(x)$ such that $\widetilde{V}(x) \geq V(x)$ for all $x$ (...
29
votes
3answers
4k views

Can we run out of gravitational (tidal) energy?

I read an article on energy forms and sources that made me think. Energy comes from somewhere and is limited in various senses. It's most obvious for fuels: we burn coal and oil and at some point we'...
2
votes
1answer
113 views

Why are general wave functions expressed in terms of energy eigenfunctions?

I have read that the eigenfunctions of any hermitian operator can be used as a basis to express any function, but I have only ever really seen the eigenfunctions of the Hamiltonian used. Why is this? ...
-2
votes
2answers
90 views

Can Light Really be Matter? [closed]

I remember during my early education being taught that light is a form of matter. This has always seemed counter-intuitive to me.
3
votes
2answers
38 views

A question on the redshift of photons due to cosmic expansion

Given that the universe is expanding over time, in the sense that the (spatial) metric is changing over time, corresponding to the physical distance between objects increasing, naïve intuition leads ...
0
votes
4answers
104 views

Why don't high pressure gases stored in containers lose energy?

Containers holding gas at a high pressure don't slowly lose the internal energy of the gas. It seems like the high speed particles would collide with the metal walls and slowly transfer their energy ...
1
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0answers
41 views

Can energy go through higher dimensions?

It is hypothetically predicted that gravitons can escape from the 4th dimension or brane and "leak" or go through higher dimensions. I wonder if energy can do the same thing. Is there any explanation ...
0
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0answers
13 views

Is there a way to calculate the change in temperature of water in a vacuum?

My understanding is that water boils when the vapor pressure is reached and then, at some point, freezes. Is there a way to relate the vacuum pressure to the temperature of the water until it changes ...
0
votes
1answer
28 views

Photon frequency [duplicate]

I understand frequency depicted as an oscillation over time, (sound pressure, pendulum swing, etc.). What then can be meant by the frequency of a photon, which we think of as traveling in a straight ...
1
vote
1answer
38 views

Einstein coefficients for emission/absorption and energy density

When dealing with a system of two energy levels $E_1$ and $E_2$ in laser theory, the following rate equations are written: $$\frac{d N_1}{dt} = A_{21}N_2 - B_{12} \rho ( \nu ) N_1 + B_{21} \rho ( \nu ...
4
votes
1answer
95 views

Energy Spectrum of Star Products

We have that a property of the transition operator defining c-equivalence (or star equivalence from equation 1 in Bertelson) is \begin{align*} T(f\star_Mg)=T(f)\star'T(g)\,, \end{align*} where $\...
17
votes
2answers
466 views
+100

trouble reconciling doppler shift for energy and momentum of light

[See late addition at bottom for what currently remains unresolved in this question.] I have seen the assertion in a couple of places that if you trap light in a reflective resonator, or any other ...
3
votes
1answer
74 views

Mass dropped on a spring

I thought this would be a particularly simple problem but it is turning out to be quite the opposite. I am sure I am doing a very simple mistake. The problem statement is that there is a mass which ...
-3
votes
1answer
100 views

How is hydrogen fine structure currently accounted for? [closed]

At a usually reliable site, Hyperphysics, the fine structure of Hydrogen is accounted for by the interaction between the B-field generated by the orbit of the electron, $0.4\:\mathrm{T}$ at $1s$ and $...
1
vote
1answer
66 views

is the sun going to make an electromagnetic pulse in the next 50 years?

EMP is very damaging and can cause blackouts and other bad results specially on electronic devices , so i heard that our sun is going to make an electromagnetic pulse during the next 50 years ..is ...
1
vote
1answer
16 views

Why is it that only thermal energy is available in a particle propagating throughout a space-charge layer in a plasma (not kinetic)?

In this PDF on page 5 the author explains that a particle moving through a space-charge (electron density in a plasma) gains only thermal energy. Should there not also be kinetic energy?
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2answers
44 views

Do induction chargers waste electricity?

Do induction chargers use more energy than traditional chargers while plugged in to the mains but not charging a device?
2
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1answer
52 views

Is there a theorem which specifies the state (with energy $E$) with maximum entanglement? [closed]

Does there exist any theorem in quantum information which states that the particular state with energy $E$ is the state with maximum entanglement (or discord) among all possible state with the same ...
-1
votes
0answers
19 views

Efficiency of engine in the case non-boundary work done on the system

The engine efficiency is defined as $$\eta = \frac{W_{\mathrm{produced}}}{Q_{\mathrm{absorbed}}}\tag{1}$$ But suppose that, in one of the processes that takes place in the cycle of an engine that ...
0
votes
1answer
44 views

Kinetic energy of a Tesla model S discrepancy

According to Tesla, the model S weighs 1961 kg and uses 237.5 watt hours (855000 Joules) to drive one km. However, when I plug this amount of energy and 1000 meters into the formula W=FD, I get the ...
0
votes
1answer
51 views

Number of electrons in conduction band

As mentioned in a previous question, the number of electrons in conduction band in a semiconductor can be computed as follows: $$N = \int_{E_c}^{+\infty} g_c(E)f(E)dE$$ where $g_c(E)$ is the density ...
1
vote
1answer
55 views

How is the energy efficiency in bicycles accounted for? [duplicate]

Bikes are more efficient than walking, but that's only because roads are flat so wheels make sense, right? Does it make sense to say that this fact is simply gained by putting a lot of energy into ...
3
votes
1answer
28 views

What happens to the energy of fermions when a degenerate gas forms?

For example, when an electron degenerate gas forms, two electrons (of opposite spins) occupy each of the lowest possible energy states up to the Fermi energy. This is because of the Pauli exclusion ...