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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Kinetic energy of a three-body system in center of mass frame

The kinetic energy of a three-body system is defined as $$ T=\frac{p^2}{2\mu} + \frac{q^2}{2M} $$ where $p$ is the relative momentum for the two-body subsystem ($m_1$ and $m_2$) and $q$ is the ...
Raymond's user avatar
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Why don't the delocalised electrons in a metal emit light when they hit an atom and change their velocity very quickly (i.e. accelerate)

We know that in metals there is a sea of delocalised electrons which can freely travel around the lattice of metal ions and that these delocalised electrons move around at large speeds, sometimes ...
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Kinetic Energy in an acceleration and stop example [closed]

A vehicle of a mass m moves along a flat surface in a straight line. It accelerates - marked a - steadily from a stop to a certain velocity v in a given time t and then slows down steadily -marked b - ...
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Converting laser irradiance into volume energy density

If I want to convert a laser's irradiance of $10^{20} \,\text{W}/\text{cm}^2$ into a volume energy density, do I just divide it by the speed of light $c$?
Phil Bouchard's user avatar
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What is the difference between temperature and internal energy of a gas?

I came across this doubt multiple times while studying thermodynamics. Aren't both temperature and internal energy fundamentally due to the kinetic energy of the molecules? An even crazier doubt of ...
Vinay5101's user avatar
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Why escape peak positions are calculated using emission energy and not absorbing energy?

For a X-ray monocromatic source, escape peaks energy positions are described by the difference between the incident energy and the fluorescence ($K_{\alpha}$ for example), like $E_{Escape Peak} = E_0 -...
xor's user avatar
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Electromagnetic energy density in dielectric

Poynting's theorem can be interpreted to show that the total electromagnetic energy in a volume from the electric field is $$\frac{1}{2\epsilon_0} \int_\mathcal{V} E^2 dV$$ However, when using Maxwell'...
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Why is the general unit for energy (in terms of energy bills) $\rm kWh$? [closed]

Why is the unit for energy (in terms of energy bills) expressed as $$\text{time} \cdot \frac{\text{energy}}{\text{time}}$$ rather than just energy? Wouldn't it be better to express it in megajoules (...
flakpm's user avatar
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Why equation 5.29 in Schroeder's Thermal Physics is not equal to 0?

In Schroeder's "An Introduction to Thermal Physics" in section 5.2 (page 161), Schroeder considers the case of a system that is in thermal contact with a reservoir that is at a constant ...
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In what sense is $\int (u \cdot \nabla) u \cdot u dx$ an energy flux?

Due to the nature of this question I have have cross-listed it on mathSE. Let $u$ be either a solution to either the Euler equations or Navier-Stokes equations over a domain $\Omega$. In fluid ...
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Can a region of electromagnetic waves alone be considered a thermodynamic system? Can work and heat interactions be made sense for them?

Consider an ideal antenna producing coherent radio waves, and suppose the waves are not received by any body (they are sent off to space). Can we say a work interaction/transfer has been done? Or is ...
Maximal Ideal's user avatar
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Equipartition of Potential and Kinetic Energy in Linear Progressive Waves

While studying the properties of linear, progressive waves, a common property I've noticed among them is this: the energy in the wave is equally distributed, or equipartitioned between their two forms....
littlemissmosfet's user avatar
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Conservation of Momentum in particle splitting. Is it always conserved?

Let an intially stationary particle $A$ of mass $m$, if $A$ explodes such that it breaks into 2 equal pieces.(Assume no heat or sound energy loss) The red point is supposed be the point at which the ...
SHINU_MADE's user avatar
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Total Energy to Total Momentum

Suppose I have two particles with different masses and velocities. The momentums are as follows: $$\mathbf{p}_1 = m_1 \mathbf{v_1} $$ $$\mathbf{p}_2 = m_2 \mathbf{v_2}$$ The total momentum is: $$\...
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Calculating generator over-speed following sudden disconnection and delayed valve closure

I'm working on an exam question related to power systems and need help calculating the over-speed of a generator following a sudden disconnection from the grid. The question is: A 475 MVA, 2 pole, ...
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Time average of product of 2 function with same frequency

Hi I'm Reading a notebook on "light atom interactions" There is this calculation I don't understand of average power - $$P=\overline{-\boldsymbol{E} \cdot \frac{\mathrm{d} D}{\mathrm{~d} t}}=...
Yarden Sharabi's user avatar
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"Circular" motion with a changing radius - is mechanical energy conserved? [duplicate]

Suppose we have a frictionless table with a mass $M$ whirling over it, connected to a taut string which goes through a hole in the middle of the table. The mass $M$ is travelling in a circular motion ...
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Equations of motion for a force in special relativity seem to require a differential equation — in this context, what does Kinetic Energy *mean*?

I was faced with a situation where I suddenly realized that although Kinetic Energy in Special Relativity is defined as $KE=\gamma m_0 c^2 - m_0 c^2$ The work energy theorem says $\Delta KE +\Delta ...
Steven Dorsher's user avatar
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Kinetic Energy equation: Is $K=\frac12mv^2$ a Definition, or a derived Theorem?

I am trying to understand classical physics as a mathematical model. I will first specify the trail of thoughts that led up to this question. (Please correct me if anything is wrong with the reasoning ...
jkuk5046's user avatar
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2 answers
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Why is velocity used for kinetic energy instead of speed? [duplicate]

This may seem like a self-explanatory question, but if kinetic energy is a scalar quantity, and squaring the velocity essentially removes its vector quality, why not use speed instead? I have a hunch ...
Ephemeralist's user avatar
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Joukowsky equation and cup of water collision with the ground

A water hammer is created when we close a valve of a flowing liquid, I was wondering if the same principle can be used for a cup filled with water hitting the ground. Can Joukowsky's equation explain ...
GoodApp23's user avatar
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Where did potential energy disappear in this case?

Just a new thought experiment. We know that the kinetic energy depends on frame of reference as do work and velocity. Now we can consider a falling ball. It converts potential energy to kinetic energy ...
Hazim Ahmed's user avatar
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With rigid bodies, can we compute work done be separate forces separately? If so, how is it calculated?

When calculating the work done on a point particle, we use the following formula: $W = \int \Sigma\vec{F} \cdot \vec{dr}$ Alternatively, we can also calculate the work done be each of the individual ...
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Mathematically solving for the energy loss in a resistor with ramping voltage over time [closed]

I am trying to calculate the total energy in a resistor where a constant voltage ramp is applied over time t. I am a little rusty on how to set it up. We know energy is power multiplied with time or E ...
Stonie's user avatar
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3 answers
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Does a single ball undergoing ideal simple harmonic motion count as being in thermodynamic equilibrium?

(Posting this and deleting a previous similar post to make the question more clear.) Suppose we have an isolated system inside a box consisting of a spring and a ball attached to the spring. The ball ...
Maximal Ideal's user avatar
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Trying to verify if forces and work match up with elastic energy [duplicate]

I am trying to prove if we can use two different methods with one singular problem regarding springs and elasticity, but it doesn't seem to be working. Would appreciate some input (since I'm finding ...
Maroun Tarabey's user avatar
1 vote
1 answer
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Does vacuum decay create/destroy energy?

From what I understand, vacuum decay involves the release of potential energy, but where does this energy come from? Is it created in the moment and how does it affect the energy already present in ...
KleinMoretti's user avatar
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What is the physical significance of momentum and energy in kinematics?

Thank you all for the insightful comments on my previous question Are the concepts of motion such as mass, momentum, impulse, work, energy, force etc. fictitious/abstract concepts or are they real &...
corrxn's user avatar
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Will the Intensity of parallel light rays decrease over distance?

As per what I have understood, the inverse square law applies to diverging beams of light. But what about parallel rays in a lossless medium? If its intensity does decrease, how?
Shristeerupa's user avatar
10 votes
1 answer
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What fraction of the universe's energy is contained in photons?

From each point in the universe, the light of billions of stars, galaxies, supernovae etc. can be detected. So there seems to be a lot of energy/momentum "in flight". Is it possible to ...
2080's user avatar
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8 votes
7 answers
313 views

Which block reaches the floor first?

There are two blocks, each starting at the top of an incline. The particular inclines are depicted in the image below. The height through which the blocks fall is the same, the table lengths are the ...
Relativisticcucumber's user avatar
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Struggling with question about non-increasing energy function in a symmetric weight neural network

I'm trying to work out problem 1.1 on page 13 in Statistical Mechanics of Learning where the authors ask you to prove the following claim. Consider a neural network model with neurons whose ...
thobanster's user avatar
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Up-tunneling of vacuum with high-energy events?

I was reading these papers by Sean Carroll (https://arxiv.org/abs/1405.0298 ; https://arxiv.org/abs/1505.02780) in which, among other things, he argues against vacuum up-tunneling occurring in the ...
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Shapes of waves in the surface of a pond when the breeze blows

When I throw a stone into a pond while a breeze is blowing across its surface and the water is at rest, what would be the shapes of the waves? Before the wind, the shape of the waves is circular. Then ...
Majid's user avatar
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2 answers
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Shape of graph of energy in S.H.M

I'm confused to whether the graph of KE/PE of a simple harmonic motion system is sinusoidal or not those are my best sketches but if unclear, the blue one is in a shape of a sine wave. this question ...
Safa yousif's user avatar
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2 answers
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Question related to conservation of energy

Here is a textbook passage that I have trouble understanding. It is taken from Classical Mechanics by Tom WB Kibble and Frank H Berkshire. We consider first a particle moving along a line, under a ...
Hazim Ahmed's user avatar
9 votes
1 answer
824 views

For a charged hollow sphere: why is the energy calculated from the electric field different to its total energy?

I'll first calculate from its electric field, the energy of a charged hollow sphere of radius R carrying charge $Q$. I'll then show that this is different to calculating the total energy directly from ...
Physiks lover's user avatar
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1 answer
31 views

Confusion about Momentum and Energy and their conservation

I've heard that momentum is always transferred macroscopically while kinetic energy of an object can be transferred microscopically, as far as I know Why? And how does it make sense, how do we know ...
Saad's user avatar
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4 votes
1 answer
108 views

Internal energy of a gas under motion

Suppose, there a gas in a cylinder which itself is placed on a fast moving car, will there be any change in its internal energy due to the motion of the car? Do you think it may gain some kinetic ...
aadim sapkota's user avatar
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1 answer
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Are there very high voltage very low amperage electromagnetic/electrostatic launchers?

Electromagnetic launchers (coilguns or railguns) use extremely high amperages and voltages, but I was wondering if are there very high voltage very low amperage electromagnetic/electrostatic launchers....
Fulano's user avatar
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How to understand the relationship between Weinhold geometry and Ruppeiner geometry in thermodynamic geometry? [closed]

According to the content of the following paper On the relation between entropy and energy versions of thermodynamic length The second derivative matrix $D^2 U$ of the internal energy may be used to ...
000 666's user avatar
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8 votes
5 answers
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How does energy become sound?

From my understanding energy and matter are interchangeable ($E=mc^2$). Also, motion requires energy, but energy does not require motion. Now, for my question, as an example, I will use the chain of ...
Emotion's user avatar
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Why is there a $\mu_0$ term in the potential energy derivation of Stoner ferromagnetism?

I am reading Blundell's book of Magnetism in Condensed Matter, and I couldn't convince myself why a $\mu_0$ term pops out in the potential energy expression in Stoner ferromagnetism, i.e. ...
Burnie Barren da Brunette's user avatar
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Modeling an Electrolyzer System

In this study, the authors provide an equation for operating cell voltage: $V_{cell} = V_{rev} + V_{act} + V_{ohm}$, the sum of the reversible overvoltage, the activation overvoltage and the ohmic ...
Lucien Jaccon's user avatar
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1 answer
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Where does the energy density of black holes fit in a figure with the different energy densities of the universe like matter and radiation? [closed]

Based on the black hole mass, giving that lower mass black holes have higher mass densities than higher mass black holes. Energy Densities `The density is dependent only upon the mass of the black ...
New's user avatar
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-1 votes
1 answer
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How one should interpret the line that "an object/system carries energy or has energy"? [closed]

Energy is not really defined formally in what it is but in how it changes Energy is as a number we can assign to a system using certain carefully chosen formulas and we state that this number is ...
Anuj's user avatar
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2 votes
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What is the connection between energy in classical mechanics and thermodynamics

In classical mechanics the concept of energy is very simple. If I have a bunch of particles $r_1$...$r_n$. Then the total energy is: $$E=\frac{1}{2}m(\dot r_1^2+...\dot r_n^2)+U(r_1...r_n)$$ Now in ...
Robin's user avatar
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6 votes
2 answers
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The partition function of a particle in a magnetic field diverges. Why?

Using the symetric gauge $\mathbf{A} = \tfrac{B}{2} (-y, x, 0)$, the stationary states wave functions of a quantum particle in a constant and homogeneous magnetic field are $$\tag{1} \psi_{n m}(r, \...
Cham's user avatar
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5 votes
1 answer
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Does the notion of temperature depend on the zeroth law of thermodynamics?

In Thermodynamics (1st ed.) by James Luscombe (2018, p. 12), the zeroth law is used to show the existence of empirical temperature (of fluids in thermal equilibrium) as a function of pressure and ...
Dumb Koala's user avatar
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1 answer
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Why does more rotational inertia result in a smaller velocity down an incline?

In my physics class we are learning about how objects with greater rotational inertia result in less translational velocity when "rolling without slipping" down an incline. When explained, ...
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