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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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How much energy is wasted by a noisy refrigerator?

I recently bought a new refrigerator for my kitchen. The feet are adjustable but I've been lazy. Whenever the motor runs and the feet aren't all touching the floor there is a loud buzzing noise. As ...
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2answers
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Meaning of $E$ in time-independent Schrödinger's Equation (high school)

I've just learned the time-independent Schrödinger's equation as $$-\frac{\hbar^2}{2m}\cfrac{d^2\psi(x)}{dx^2}+V(x)\psi(x)=E\psi(x).$$ Does $E\psi(x)$ mean that $E$ is a constant (that the kinetic ...
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1answer
37 views

Potential energy of the oscillator

Is there a difference between the potential energy of an oscillator and the potential energy of the spring which is associated with? When we compress a spring horizontally without any object ...
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1answer
75 views

Finding an energy of a particle in an infinite potential well

This question arises from a discussion I recently had with my friend. We were talking about a particle in an infinite potential well. The particle is in an arbitrary wavefunction $\Psi$. When one ...
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1answer
81 views

Does a non-spherical black hole have distribution of mass like an empty body, solid body or pointed object?

Suppose a non-spherical (say, rotating or under distortion of another gravity source) black hole. Does it have its mass distributed as if all the mass was on its surface, or as if the mass were ...
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1answer
27 views

Which direction will this object move in at the end of this process?

1) Consider an object with no net force acting on it moving with a constant velocity v. It has a momentum mv. 2) Now let a force F1 act on the object. Let the direction of this force be opposite (to ...
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2answers
29 views

Gravity in classical energy

It's well understood that matter has mass, and more importantly gravity. I believe I am correct to say that gravity is more like an affect of matter. And not actually part of it. Ie mass bends space ...
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0answers
38 views

Energy of bound states in gauge theories

In gauge theories such as QED or QCD, are the rest energies, and therefore the invariant masses, of various bound states, such as those of positronium or charmonium, gauge-invariant observable ...
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4answers
65 views

In what context is enthalpy a convenient concept?

Internal energy $U$ is clearly an important concept; the first law of thermodynamics states that for an isolated system internal energy is constant $(\Delta U=0)$ and that for a closed system the ...
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1answer
44 views

Will Vacuum Energy persist at the Heat Death of the Universe?

By "vacuum energy" I mean the underlying background energy that exists in space throughout the entire Universe, codified in Heisenberg's energy–time uncertainty principle, a special case of zero-point ...
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0answers
40 views

Taylor expansion for a double well/perturbed infinite square well

I'm trying to estimate the ground state energy for a perturbed infinite square well directly. The potential is piecewise constant $$V(x)=\begin{cases}&\infty, \qquad x<-a/2\\ &0 \qquad -a/2&...
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2answers
97 views

What is more massive, Proton, standard Hydrogen?

If you consider the classical states of hydrogen, one in which the electron is rotating at an orbital distance of $r$ and then take $\lim_{r\to\infty}$ one obtains that the $r_\infty$ state has more ...
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2answers
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Can matter be described as the result of the curvature of space, instead of vice versa?

Can matter be described as the result of the curvature of space, rather than the curvature of space being the result of matter, and energy being the cause of the curvature of space?
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1answer
39 views

What is the incident power of the sun for different parts of the U.S over time?

I have seen number as low as $200\ \text{W}/\text{m}^2$ to $1000 \text{W}/\text{m}^2$. I'm in California and looking for information on how much power my solar panels will pick up. I have $4100\ \...
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2answers
102 views

Doesn’t gravity violate the first law of thermodynamics? [duplicate]

So gravity is a force, and when it pulls on an object the object gains kinetic energy. However, doesn’t this mean that energy is being “created”, since the object gains kinetic energy? Why doesn’t ...
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1answer
100 views

Special relativity: I arrive at a contradiction regarding the Lorentz invariance of certain quantity

I want to show the Lorentz invariance of $d^3 p/E$ (Eq. 8.11 of Mandl-Shaw), where $E$ is the relativistic energy. Peskin-Schroeder gives sort-of, a proof in section 2.3 which I am convinced of. But ...
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1answer
83 views

What is the difference between enthalpy and internal energy?

What is pv in enthalpy's equation? if we transfer energy to a system in the form of work does it reflect in its internal energy fully and if it does why do we need a term called enthalpy which ...
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1answer
43 views

Goldstein's proof that $W = -\Delta V$: Why is $\nabla_i V_{ij} = \nabla_{ij} V_{ij}$?

On page 11 of the third edition of Goldstein's Classical Mechanics, the book claims that If the difference vector $\textbf{r}_i - \textbf{r}_j$ is denoted by $\textbf{r}_{ij}$, and $\nabla_{ij}$ ...
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2answers
225 views

An electron moving through a magnetic field [closed]

An electron, that has been accelerated from rest by a potential difference of $250\ \rm V$, enters a region of magnetic field of strength $0.12\ \rm T$. Show that speed of the electron after ...
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1answer
42 views

First law needs adiabat definition but adiabat definition needs heat definition in first law

It is usually said that the best formulation of the first law of thermodynamics is that work is path independent on adiabatic paths. From there, you can define the state function of internal energy ...
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2answers
137 views

Can the mass of a hydrogen atom be calculated in a gauge-invariant way?

Please excuse the lengthy question. It involves an interesting controversy which has arisen in discussions on this site: Energy/mass of Quantum Vacuum Relative potential energy vs Absolute potential ...
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0answers
43 views

What are the “basic things” we need in physics to define any kind of energy, including “mass”?

After having commented an answer there: Relative potential energy vs Absolute potential energy, I realised that the energy concept may be much more subtle than what we usually believe, even if we ...
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2answers
49 views

How to do dimensional analysis?

$$mgh = \frac{mc^2}{\sqrt{1-(v/c)^2}}-mc^2.$$ In dimensional analysis do we just ignore the square root? Or do we solve what’s inside first then we do the square root? Do we say $(v/c)^2$ is 1 as ...
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1answer
19 views

Defining internal energy for adiabatically inaccessible states

As is well known, a good way to define the internal energy in classical thermodynamics is to define the difference in internal energy of two points as the amount of adiabatic work from one point to ...
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5answers
2k views

Does a rock use up energy to maintain its shape?

A rock sitting on land, the ocean floor, or floating in space maintains its shape somehow. Gravity isn't keeping it together because it is too small, so I'm assuming it is chemical or nuclear bonds ...
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3answers
187 views

Relative potential energy vs Absolute potential energy

I have seen in many textbooks and sources which say that we can't experimentally measure potential energy but we can measure differences in potential energy. $$\Delta U_g=-W_g$$ Choosing zero ...
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1answer
30 views

Statics and dynamics in elasticity : how to add time in elasticity

In elasticity, there is are static relations of the form $R(\sigma,\epsilon)=0$. In fluid dynamics, there is a dynamics relation with the conservation of momentum leading to Navier-Stokes equation. ...
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1answer
57 views

Why particles of thermodynamic systems have a preference for low energy levels?

While reading Ken Dill's Molecular Driving Force, I noticed this paragraph when the author is trying to explain the Boltzmann distribution. Particles don’t have an intrinsic preference for lower ...
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3answers
76 views

How much electricity can a kilo of electrons produce?

I know how stupid this question sounds. I heard my grandma say something close to it in a phrase "It's a 1000€ aircondition, it's not gonna consume 200 kilos of electricity !" Made total no sence ...
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1answer
34 views

Why cant time-crystals form in thermal equilibrium?

This would make time-crystals a practically useless source of work, correct?
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1answer
105 views

Energy/mass of Quantum Vacuum

So far as we have measured the volume of the universe, has anyone calculated the virtual energy/mass of the Quantum Vacuum? If so, what? Or if not, a local volume such as 1 sq. meter? This relates to ...
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1answer
116 views

Conservation of mechanical energy proof

Using Newtonian mechanics, how does one prove that the change in mechanical energy of a classical system of particles is equal to the work done by non-conservative forces? What assumptions are made? ...
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1answer
52 views

How to calculate the ground state of Ising model at non-zero temperature

I'm studying the quantum Ising model, i.e. with Hamiltonian $H= -h\sum_{i}X_i-\sum_{\langle i,j\rangle}Z_iZ_j$. I know conceptually how to compute the ground state of the Ising model at zero ...
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1answer
68 views

Misunderstanding in definition of Enthalpy

Enthalpy is defined as $H = U + PV$, but internal energy is given by $U = \frac{3}{2} nk_B T = \frac{3}{2}PV$ for an ideal gas, so why don't we just write $$H = \frac{5}{2} PV$$ for enthalpy? Is ...
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1answer
45 views

Problem in derivation of Rydberg Equation

In deriving the Rydberg Equation I found that $\delta E=E_2-E_1=hf$ where $E_1$ is the energy the orbit in which the electron was and $E_2$ is the energy of the orbit in which the electron is ...
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1answer
56 views

What is a 'system'?

Is physics a full game of observation and after observing we frame law??Is it really true? My second query is what exactly is a system? When I was studying laws of motion i felt that system is ...
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3answers
139 views

Why is $E=mc^2$ and not $E=m\frac{c^2}{2}$?

Kinetic energy for a moving object is the integral of force with respect to distance, often given as: $$E=m\frac{v^2}{2}.$$ This would imply that for mass moving at the speed of light, the kinetic ...
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1answer
34 views

Utility of work and energy

Why do we study the scalars such as work and energy if we know how to solve daily classical mechanics problem with standard mathematics? My second query is about the main motive of defining ...
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5answers
151 views

Newton's laws vs energy for solving a problem

I have a problem I solved using kinematics/Newton's 2nd law. It gives the mass of a walker as 55kg. It then says she starts from rest and walks 20m is 7s. It wants to know the horizontal force ...
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3answers
79 views

Is system boundary part of the system or the environment or neither?

The book says: As an example, imagine a force applied to an object in empty space. We can define the object as the system and its surface as the system boundary. Okay, if the surface of that ...
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1answer
66 views

Does $f(x) = 1 $ belong to the Hilbert space of the infinitely-deep square well?

Let the square well be on the interval $(0,\pi )$. It is generally postulated that the wave function of this system should vanish at the end points, i.e., $g(0)= g(\pi) = 0$. The function $f(x) \...
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1answer
53 views

Mixture with a strange free energy curve

I have the following energy landscape for a binary mixture with a specie $1$ of volume fraction $\phi$ (it is composed for example of colloids) and a specie $2$ of volume fraction $1-\phi$ (for ...
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1answer
15 views

Overconstrained equilibrium binary mixture phase seggregation

Assume a binary mixture, with the component $A$ in concentration $c$ and the component $B$ in concentration $1-c$. The total energy $E(c)$ is thus given by a function of one variable $c$ (if we do not ...
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1answer
20 views

Interactions between “particles” to prevent diffusion and to cause phase segregation

The free energy of particles reads, if we assume no internal energy : $\tilde f(c)= \frac{f(c)}{k_bT}=-c \, ln(c)-(1-c)\, ln(1-c)$, with $c$ the concentration. If we had a term $F(c)$ for the ...
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1answer
118 views

Is it possible to create a limitless power by firing rocks from the moon to Earth? [closed]

Here is my idea for a limitless* power. A gun on the moon fires moon rocks towards the Earth. The moon rocks fall in Earths gravity picking up speed and smash into Earth's atmosphere generating huge ...
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2answers
71 views

Do empty orbitals have energy?

Suppose we have the Na (Sodium) atom. It has half-filled the 3s orbital and unfilled 3p orbital. Does the empty 3p orbital have energy? also does the energy of 3p change if an electron occupy it?
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1answer
52 views

Using coil gun make two mass of lithium deuteride collide and start nuclear fusion

We already have hydrogen bomb, why controllable nuclear fusion so difficult? And why controllable necessary? If we can detonate a tiny hydrogen bomb, we can collect the energy, like laser nuclear ...
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1answer
18 views

volume of liquid in a boiler based heating system

We're trying to improve energy efficiency for my dad's hothouse. The setup includes a gas boiler and a pump running the heated water thru special pipes to heat the grow space. My question is, leaving ...
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1answer
45 views

Energy loss in capacitor for slow charge transfer

From these questions: Energy loss in Capacitors What happens to half of the energy in a circuit with a capacitor? I come to the conclusion that energy loss in a capacitor is explained by the fact that ...
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1answer
67 views

About *Carnot's cycle* and *engine*

we know the Carnot's cycle diagram, so A, B, C, D are points regarding the state of the system. Consider a piston with weight on it. -----AB is just making the piston go up Q = W by providing heat no ...