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

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43 views

First law of thermodynamics clarification on internal energy

Starting from the experience of joules we know that the mass before falling has a potential energy $mgH$ where $H$ is height from floor. Once we reach the height $0$ we observe that the kinetic energy ...
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2answers
96 views

Why are certain quantities so fundamental to physics? [on hold]

Apart from having a qualitative description of quantities such as momentum, work and energy, why are these quantities considered so fundamental? What is the reason to define them in the first place? ...
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Black holes and positive/negative-energy particles

I was reading Brian Greene's "Hidden Reality" and came to the part about Hawking Radiation. Quantum jitters that occur near the event horizon of a black hole, which create both positive-energy ...
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19 views

Why is this way of deducig the 1 dimensional Maxwell-Boltzmann Distribution wrong?

I asked before how the 1D Maxwell-Boltzmann distribution can be derived. However, I did not get or find an answer to my own method explaining why it is wrong, which I am quite curious about. It is a ...
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2answers
265 views

Does ambient day/night temperature change have significant energy generation potential

Is there significant energy to capture in daily day/night temperature change? For instance, I see from posts such as the following that there are materials with varying coefficients of linear thermal ...
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1answer
391 views

Sum of energy for 2 solids in rotation

I would like to compute the sum of energy of the following case: Two solids are turning (disks). Yellow solid is turning at $w_1$ radians per second around its center of gravity and blue solid is ...
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0answers
35 views

Work a medium of energy conversion? [duplicate]

Is work a medium to convert one form of energy to other? i.e. to say without work you can't convert type of energy? Similar type of question pertains to heat.heat is also medium of conversion of ...
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1answer
75 views

Volume of a speaker needed to boil an egg [on hold]

I have a fun question to ask that me and my friends couldn't figure out. If I was to boil an egg using a speaker, how loud would that speaker need to be? (Yep, this is clearly what keeps us physicists ...
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1answer
49 views

Complex Scalar Fields and Killing Vectors

In a stationary and axisymmetric spacetime, there are two Killing vectors, say $\zeta^\mu$ and $\xi^\mu$, one timelike and one space like. I understand that for a real scalar field, $\phi$, that ...
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2answers
38 views

Is Kinetic energy always conserved in an elastic collision/impact?

While working out through some problems I encountered this problem : A ball moving with a velocity $v$ hits a massive wall moving towards the ball with a velocity $u$. An elastic impact lasts for ...
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1answer
17 views

Why potential energy is neglected in this problem for calculating the energy difference between two stationary Bohr Orbits?

Question An electron in a hydrogen like atom makes a transition from a state in which its de-Broglie wavelength is $\lambda_1$ to a state in which its de-Broglie wavelength is $\lambda_2$, then ...
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2answers
112 views

In the electrical circuit/fluid analogy is there an analogy for power loss across a fluid resistance?

For the fundamental passive $RLC$ electrical circuit, analogies are often used to describe the physics of fluid flow where $L$ represents the inertance analogous to inductance, $C$ is compliance ...
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1answer
704 views

How is internal energy related to degree of freedom?

I have a doubt in understanding the concept of internal energy. The internal energy of a system is equal to U=f/2nrT, where T is the temperature. Now, also we know that dU=Q-W Then if heat is zero, ...
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2answers
81 views

Why is particle physics called high energy physics? [duplicate]

The highest energy accelerator till date is the LHC which operates at an energy scale of perhaps 10-100 TeV. In SI units this is about $\sim 10^{-6}-10^{-5}$ Joule which is several orders of magnitude ...
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1answer
460 views

Friction energy dissipation calculation

In a moving block, a variable force pushes it. Friction is acting against the movement. Below equation specify the amount of energy could be used to give motion: $$ E_1 = \int F_T(t) .v(t) .dt $$ ...
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1answer
29 views

What is the “energy associated with torque?”

I'm told that when a magnetic dipole is placed in a magnetic field $\vec{B}$, it experiences a torque $\vec{\tau} = \vec\mu \times \vec{B}$ and an associated energy $ H = -\vec{\mu} \cdot \vec{B} $, ...
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1answer
142 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 ...
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0answers
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Did I manage to quantize any energy? [closed]

Recently I was doodling with some equations and managed to quantize the energy of any particle. I don't exactly know what I've done, can someone take a look at it? Final equation: E=1/n(3hc/(2*...
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1answer
30 views

Energy consumption by body

i think the energy consumed by vertical jumping is too high when compared to the "2000 calorie a day requirement". Say i weigh 60kg, i have studied my centre of mass from a video with wall markings, ...
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2answers
107 views

Can I use the expansion of the universe to create or destroy energy?

Imagine I have a setup like this: (The objects are not attached to each other. The red objects are much heaver than the black one, and the setup is balanced.) This setup has a non-zero ...
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0answers
143 views

Making An Energy Momentum Plot For A Rashba Model (Using Discretization)

I want to make a plot of the Energy versus the Momentum of the Rashba model, using discrete matrices. First Ill show how I did this for the free particle. Subsequently I will show what goes wrong for ...
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2answers
107 views

Is vacuum energy transient or permanent?

Vacuum energy is based on Heisenberg's energy–time uncertainty principle which says that $\Delta E$ can exist for $\Delta t$. This $\Delta E$ can produce electron positron pair for $\Delta t$ by ...
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3answers
129 views

Is kinetic energy relative? [duplicate]

Assume there is a rocket with 10 kg of fuel in a large empty space without any external forces such as gravity. Rocket burns 1 kg of fuel and gets a $v_1$ velocity gain. Now it is moving in $v_1$ ...
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2answers
75 views

Calculate work done by a hovering helicopter over time

This is likely to be very simple, but... How does one calculate work done by a hovering unmoving aircraft over time? As in work in Joules. In this scenario, to remain hovering the aircraft has to ...
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1answer
57 views

How much kinetic energy does a helicopter use in a hover? [duplicate]

A helicopter just circulates air in a hover and maintains a stable altitude. So, how much energy is used to do this? Using the standard equation $KE = \frac12 mv^2$; then the kinetic energy used would ...
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1answer
607 views

Rest energy and kinetic energy

Eureka! I have got it. $$E-mc^2= \frac12mv^2$$ The interpretation goes like this. Total energy- rest energy= kinetic energy. But we all know Total Energy = Potential Energy + Kinetic Energy. Since $$...
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1answer
120 views

Relating Gibbs free energy to spontanity

can we say that $$\Delta G =\Delta H-T\Delta S$$ is spontaneity criteria only if $$\tag{$t,p =$ const}T_\text{system} = T_\text{surrounding}$$ please see the image attached. In gibs function what ...
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2answers
115 views

Newton's Universal Law of Gravitation doubt

The Universal Law of Gravitation states that the module of the force, $F$ is $$F = \frac{GmM}{r^2},$$ where $m$ and $M$ are the mass of the two objects and $r$ is the distance between the two objects....
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1answer
295 views

Angular momentum of the center of gravity of yo-yo

Sorry for my broken English. I am a physics undergrad, and I know only a basic stuff about the subject. Yesterday I was taught about the angular momentum of yo-yo and my prof said that when the yo-...
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5answers
5k views

What causes damage, kinetic energy or momentum?

Let's assume we have 2 bullets. The first bullet has $450$ $J$ of KE and $3$ $kg.m/s$ of momentum, while the second bullet has $250$ $J$ of KE and $5$ $kg.m/s$. Now if they are both shot at ballistic ...
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0answers
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If energy exists today, wouldn't it have had to be created somewhere? [duplicate]

Energy exists today in different forms, and the 1st law of thermodynamics states that energy cannot be created or destroyed, only changed into different forms. However, since there is energy today, ...
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30 views

Physics of Athleticism for Dunks

I was curious of the athleticism I'd need to dunk a standard rim at $10$ ft above the ground. Using an online calculator, given that I am $5$ feet $6$ inches tall with a standing reach of $7$ feet $1$ ...
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3answers
69 views

Is “physical damage” proportional to momentum or kinetic energy? [duplicate]

If a rock is thrown at someone's head, or a car drives against a wall, or a meteor hits a planet, is the physical damage done to the object proportional to the projectile's momentum, or to its kinetic ...
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0answers
40 views

Canonical or mechanical 4-momentum to measure energy?

I've often seen that the energy of a particle $p$ with 4-momentum $p^\mu$ measured by an observer $O$ with 4-velocity $u^\mu = dx^\mu/d\tau |_O$ is given by $$E=-p_\mu u^\mu.$$ In general, what ...
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1answer
143 views

How local is local in the Hypothesis of Locality?

A while back I asked this question Freely falling frame and the use of special relativity? which I later answered using the Hypothesis of Locality which I used to state that $E=-p_\mu u^\mu$ in any ...
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1answer
27 views

How to reproduce free energy in canonical ensemble in stat mech

Given a quantum mechanical density matrix $\rho$, the internal energy and entropy of the system is given by: \begin{align} E &= \text{Tr}[H \rho] \\ S &= -k_B \, \text{Tr}[\rho \ln \rho] \end{...
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1answer
69 views

Total energy of a two particle system [closed]

Question: Velocity of center of mass of two particles is $~v~$ and the sum of the masses of two particles is $~m~$. Kinetic energy of the system is: A$)~~=\frac{1}{2}mv^2~$ B$)~~\ge \frac{1}{2}mv^2~$...
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2answers
168 views

How general is the relation between entropy and energy?

I am not a physicist, and my understanding of entropy is based on information theory and probability theory, rather than on physics. I am somewhat confused by how physicists think of entropy, and ...
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27 views

Confinement energy and energy conservation

When particles are confined, they gain confinement kinetic energy. Where does this energy come from in terms of energy conservation?
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1answer
34 views

Determine/measure energy of neutral particles

I wonder that nobody came up with this question in advance - maybe it is too obvious? However.. How can neutral particles be measured (with respect to their energy in a particle detector)? I know ...
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4answers
95 views

A spacecraft orbiting the sun charges a battery which becomes a tiny bit more massive. How does this effect the velocity of the spacecraft?

According to $E=mc²$ the spacecraft which stores the Energy from the sun becomes a tiny bit more massive. how does this effect the impulse of this spacecraft. Would it grow because P = (m + extra mass)...
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2answers
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Where does the energy go when punching a ball in the air causing it to “float”?

Here is a link to a video of a people punching a ball so it appears to float: https://www.youtube.com/watch?v=v7wqrUFREK0 When the arms are continuously punching the ball, it does not change the ...
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2answers
122 views

How can the 1 dimensional Maxwell-Boltzmann Distribution be derived?

In terms of energy, the 3 dimensional MB Distribution is giving the probability for a particle to have an energy $E \geq E + dE$ is: $$f(E) = \frac{2}{\sqrt \pi} \cdot \bigg(\frac{1}{k_BT}\bigg)^{\...
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2answers
126 views

Frictions in a slowing car

When you apply the brakes to a car and it slows down, there are two kinds of friction and I am getting confused. On the one hand, there is friction between tyres and road. This friction a) Is the ...
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1answer
51 views

In electrostatics, why can't we tell how much energy is located in a volume inside the $E$-field?

In Purcell's Electricity and Magnetism, page 32, it is stated that the potential energy $~U$ of a system of charges is given by (using cgs units): $$U=\frac{1}{8\pi}\int_{\mathrm{Entire \\field}} E^{...
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2answers
22 views
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2answers
205 views

Phase transitions and Helmholtz vs Gibbs Free Energy

Gibbs phase rule for Gibbs free energy says that phases during a phase transition must be in mechanical, chemical, and thermal equilibrium, e.g. $T_1=T_2$,$G_1=G_2$, and $P_1=P_2$, where the ...
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1answer
119 views

Velocity of two water slides sliding down from the same height

If two water slides are released from the same height (let's assume it's $h$), and the endpoint is $a$ meters away in term of the $x$-axis and $b$ meters away in term of the $y$-axis, one slide takes ...
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1answer
140 views

Conservation of heat equation: what represent heat, enthalpy or internal energy?

I'm trying to write the heat transfer equation in an arbitrary fluid (compressible and viscous). Consider an adiabatic system where the only heat generated is due to the internal friction/viscosity. ...
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2answers
50 views

Observing a system in an energy eigenstate when the eigenstate is not normalized

In the following notes from an MIT OCW course, Zweibach claims that energy eigenstates are not necessarily normalized. https://ocw.mit.edu/courses/physics/8-05-quantum-physics-ii-fall-2013/lecture-...