# 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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### Thermal resistance [closed]

The problem is the following. Let a resistance be $R$, a surface be $S$, the tempeture be $C$ and the time be $T$. $S$ is a cylinder placed on the ground, vertically (stand up), and $R$ is positioned ...
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### Are hydrogen spectra guaranteed to be unique?

I was shown an interesting question that goes something like this: Consider the energy released by the transition in Hydrogen when the electron falls from $n=4$ to $n=3$. Explain how we know that ...
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### Question about ball collision in 2D and conservation of momentum & kinetic energy

I have a question regarding the collision of 2 balls (of the same unit mass m=1) in 2 dimension, please. We suppose that, right before the point of collision, the velocity of ball A was [1, 3]. The ...
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### Work done in a $pV$ diagram

From what I gather, work done in a $pV$ diagram is the area under the curve. I have this figure here: The solution says the quantity of work in A is greater than that in B. I am kinda confused, aren'...
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### Does $U_v=U_p$ where $U_v$ and $U_p$ is change in internal energy of system at constant volume and pressure, respectively?

Suppose a chemical reaction is happening once at constant pressure and another time at constant volume. For constant pressure, we can write $\Delta H=Q_p$ and For constant volume: $\Delta U_v=Q_v$ (...
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### Why do the bubbles in a champagne accelerate?

The bubbles in champagne are familiar to most people. They form almost exclusively at the specific points in the champagne glass, and from these they rise faster and faster. Why do the bubbles in ...
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### What is meant in condensed matter physics by a “gap” and why is it so important?

I come from a HEP background and moved to condensed matter physics. I keep seeing the word “gap” being thrown around a lot: this system has a gap, this is a gapless system, the spectrum is gapped, ...
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### What to make out of this.....? [closed]

Let's set up an experiment first. Suppose there is a body of rest mass $m$ at rest in an observers inertial frame and it is accelerated to almost $c$ (speed of light) at constant acceleration $a$ with ...
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### Is the law of conservation of energy broken by the side-effects of heating and cooling a liquid that evaporates and condenses?

Let's say there's a puddle of water on the ground. I use a magical device to give it enough thermal energy to vaporize into water vapor. The water vapor floats up into the sky. I then use the magical ...
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### If you thermally ionize cesium does the plasma have a net charge?

Cesium is a fairly easy to ionize metal. But it were thermally ionized would the ions be of any net charge?
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### Difference in energy required between flying Verically vs flying in the same horizontal plane

An insect of mass 150mg is hovering. What would be the energy required to fly straight up 10cm (against gravitational force) vs forward 10cm? Weight of insect:150mg Displacement:10cm I want to know ...
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### Conflict between Bra-Ket notation and Integration

Suppose, I have a wavefunction given by $\psi(x,t)$. This wavefunction, over time, becomes $\psi(\alpha x,t)$. I've been asked to compute the final kinetic energy of this new wavefunction, in terms of ...
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### How do electrons absorb energy?

I'm trying to understand how lamps work. I have read that the way this happens is when voltage is applied. The free electrons move due to the electromotive force (EMF), and when they collide with the ...
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### Why does the power series method result in discrete levels of energy in the case of the quantum harmonic oscillator

Working my way through Griffiths' "Introduction to Quantum Mechanics", I found the following excerpt: The result is the same as what I'd gotten using the operators' method, and yet in this ...
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### Vertical ideal spring, streched by weight, different results working with forces and energy [duplicate]

Imagine we have a vertical spring with constant $k$ and load on it (like a weighing machine) a mass $m$. Initially, the energy of the system is $0$, since the spring it's on it's original position and ...
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### Work done by force due to internal pressure of liquid drop

I am extremely puzzled in the situation mentioned below, which is a problem of Excessive Pressure in Liquid Drop. This is my question: A liquid drop exerts a pressure on walls of the drop, and the ...
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### Why the dispersion law have the form $\omega(\mathbf{k}_{1}+\mathbf{k}_{2})=\omega(\mathbf{k}_{1})+\omega(\mathbf{k}_{2})$?

In the book Mathematical Foundations of Quantum Field Theory by Albert Schwarz they define a particle corresponding to the Hamiltonian $H$, as a generalized vector function $\Phi(\mathrm{k})$, obeying ...