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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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What is the energy loss when charging a phone battery with another battery

I'm curious to find out what the energy loss is when an external battery pack charges up another battery, say one in a mobile phone. Say my phone's battery has a capacity of 1000 mAh, and my ...
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42 views

How are these two definitions of energy related/derive from each other?

How are these two definitions of energy related/derive from each other: $$E = \frac{mc^2} {\sqrt{(1 - v^2/c^2)}}$$ vs. $$E = \sqrt{m^2c^4 + p^2c^2}~?$$ Both equations express the relation between ...
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1answer
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What percent of the Black Hole energy is in the gravitational field significantly outside the event horizon?

This question is sparked with an answer to another one. We know that fields have mass-energy and gravitational field is no exception, some share of BH mas should be contributed by its gravitational ...
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Does ABS increase the breaking distance of a car? [on hold]

Question: Can an ABS (Anti-lock Braking System) instead of decreasing the breaking distance actually increase it? If yes, is this its general behaviour, or only in specific circumstances? Wikipedia ...
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1k views

Dissipation of energy from 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
76 views

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
34 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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Simple pendulum and energy [on hold]

a simple pendulum with length l is known to oscillate with a period T seconds. If the length of the pendulum is increased to 2l and the maximum displacement remains the same, the total energy of the ...
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1answer
46 views

Horizontal work done by Projectile reaching maximum height [on hold]

When a projectile is launched, let's say at 30 degrees, when it reaches maximum height, are the vector components (x component and y component) related to the work done by the projectile? Was ...
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1answer
59 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
57 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
26 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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Clear formula for water penetration by wave frequency

I want a clear formula for clear water (and salty water) penetration by giving only the radio wave frequency . I searched the web , the formulas on the web are so complicated . Are there any simple ...
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How can we define internal energy with a limited set of adiabats?

To define internal energy in thermodynamics, we use the first law which asserts path independence of work on adiabatic paths but there does not necessarily exist an adiabatic path between any two ...
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2answers
25 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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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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0answers
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Equation to get the size of a meteorite impact [duplicate]

Im trying to find the equation to the the size of the resulting crater created by meteor strike when I have the mass, volume, diameter, density, velocity and kinetic energy of the meteor. This ...
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2answers
57 views

In theory does 1 Watt of energy accelerate a 1 KG object 1 meter faster each second it is input? [on hold]

Can 1 Watt of energy accelerate a 1 KG object 1 meter faster each second it is input? (Laterally in an ideal environment) (Basically wondering what the Watt equivalent standard to 1 Newton is when ...
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4answers
56 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
27 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
47 views

Do physical quantities really exists? [on hold]

I've always been suspicious that in the reality in which we live there are only 3 things: Matter: intended as all the physical particles that are potentially observable (up to the quantum level) ...
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0answers
31 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
91 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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An energy transformation flow diagram is shown [on hold]

(x on side, line pointing away from it. Y on top in middle of line) X is a type of energy and Y is the location where the energy transformation is taking place. What do X and Y represent? Select ...
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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
37 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
74 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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38 views

Energy imparted by particles at increasing Ev [on hold]

A 1 meter cube of iron is subject to an endless sequence of impacts from random directions by protons and neutrons, at a rate of 1 particle per second. Each particle in the sequence has 1eV more ...
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39 views

Do X-Waves carry Infinite Energy? [closed]

On this PDF file, which talks about X-waves energy, it talks about Infinite Energy on page 6, does that mean they do carry infinite energy? (theoretically at least?) https://www.osapublishing.org/...
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1answer
89 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
38 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
40 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
189 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
41 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
122 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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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
44 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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16 views

Pressure rise in heat pipes

How can I calculate the pressure in a heat pipe “duct” while it works? I need it to do know how much is the wall thickness required
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1answer
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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
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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
112 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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0answers
23 views

Link between microscopic quantities and macroscopic quantities in physics

I was wondering what are physical quantities that repeat themselves across all scales (from microscopic to macroscopic)? For example the sum of the microscopic kinetic energy of all the particles of ...
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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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0answers
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How is the laser frequency for doppler cooling determined?

How is the laser frequency for doppler cooling determined for a certain element? Will any frequency from the absorption lines of that element work? How is the amount of redshift in the laser ...
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1answer
55 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
73 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
33 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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Find the wavelength for the particular emission [duplicate]

The stopping potential for the photoelectrons emitted from a metal surface of work function 1.7 eV is 10.4 eV . Find the wavelength of the radiation used. Also identify the energy levels in hydrogen ...
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2answers
88 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
64 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? ...