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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Why does the phenomenon of localized energy producing particles and antiparticles lead to the creation of an infinite number of particles?

Penrose says the following (paraphrased) in "Road to Reality" on pg 611 of the first American edition A particle and anti particle may combine to produce energy, as given by general relativity. ...
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Why is there a 1/2 in the expression for electrostatic energy U?

The expression for electrostatic energy is $$U= \dfrac{1}{2} \times \int \rho\,\phi\, dV$$ where $\rho$ is the charge density and $\phi$ is the potential at that point in ($dV$) Let me explain what I ...
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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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102 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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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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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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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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79 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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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38 views

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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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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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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Does the kinetic energy always decrease with gain in potential energy even if the system is being provided energy externally

A car accelerates uphill does its kinetic energy increase or decrease?
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Why and in what cases we use $E$ instead of $\Delta E$ in energy-time uncertainity relation?

sometimes in problems of energy or time uncertainity we use the value of energy of the particle/system to calculate the delta t. Now what are those cases specific examples, where we can apply this?
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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-...
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Can an exothermic reaction be modified to alter how its energy is released?

As I understand it, exothermic reactions release energy. This energy can be released through the generation of heat, sound, light, or electricity. Questions: Is it true that exothermic reactions ...
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Why is more than one ripple created when a rock is thrown onto the still surface of a pond?

I have tried making an analogy with a simple pendulum: if you give it a push it will probably make several oscillations. But why isn't there just one ripple that would carry the energy of the several ...
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How to find an equivalent for the partial derivatives of internal energy?

Actually, I am trying to relate force-temperature results from one test (hydrothermal isometric tension) to heat-temperature results from another test (Differential Scanning Calorimetry). I started ...
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What is free energy in the context of a quantum field theory?

I was reading the papers Large $N$ behavior of mass deformed ABJM theory and New 3D ${\cal N}=2$ SCFT's with $N^{3/2}$ scaling. These papers talk about the free energy in the context of quantum field ...
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Solving a lever problem using Newton's laws, without energy conservation?

Context: I have no background in physics, and I'm trying to understand the role of the concepts of energy, momentum and other invariants/symmetries in physics. In particular, I'm trying to get a feel ...
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Does all energy of moving car converts to thermal energy?

I argue with my friend and cannot say that he is wrong although I feel it somehow. But I don't have good arguments against him. I want to know who is right and why. The problem is Let's imagine we ...
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Energy is in vacuum or not?

Is energy present in vaccum is zero, infinity or particular value depending on volume, length, height etc of container containing vaccum or some other parameter.
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69 views

What is the most general conception/formalization of “energy”? [duplicate]

(disclaimer: I am not a physicist). I have been taught about the concept of energy in a fairly ad-hoc way. i.e. I was simply "told" that kinetic energy equals $\frac 1 2 m v^2$, and similarly I was ...
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Why is a grand partition function defined as the sum of total energies?

In Eq. 22.19, why is this partition function defined as not the sum of internal energy, $U_i$, but the sum of total energy, $E_i$?
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Why can't we use air pressure to recharge an electric cars battery

If we use energy to accelerate a vehicle, a lot of that energy is lost to drag and friction, especially in an electric vehicle. Why can't we harvest some of that energy using an alternator and some ...
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36 views

Earth's Magnetic Field and Rotation used to generate electricity in huge longitudinal coil

If we wrapped the Earth in a longitudinal copper coil, would the planet's magnetic field and rotation give current through the coil? Is this the energy solution?
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57 views

Conflicting models about how energy transferred in an electric circuit [duplicate]

I've been studying DC circuits recently, but I've run into some confusion concerning how batteries deliver energy to circuit elements. From what I've read online, it seems that energy in circuits ...
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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
45 views

Elastic energy of graphene: new idea for (non-chemical) battery

Is it possible to build a battery using the elastic property of graphene? Here is the stress-strain curve of graphene. Image source: https://www.researchgate.net/figure/Stress-strain-curves-of-...
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1answer
26 views

Interaction energy of two brownian spherical particle in liquid [closed]

Let us consider two hard sphere in finite volume. Their motion is Brownian. What can we say about interaction energy? Is it less then $kT$? I know that we can describe this system by Langevin ...
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1answer
51 views

Different statements of the Heisenberg uncertainty principle [duplicate]

I know that from the Heisenberg uncertainty principle, ∆x∆p=ℏ/2 . And I know that this equation can be rewritten as ∆t∆E=ℏ/2. From QED I also know that the equation ∆t∆E=ℏ/2 claims that some energy ...