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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Mechanical energy in an harmonic wave and in normal modes

I think I miss something about energy of a mechanical wave. In absence of dissipation the mechanical energy transported by an harmonic wave is constant. $$E=\frac{1}{2} A^2 \omega^2 m$$ But, while ...
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54 views

Proof for fluctuations in vacuum

I'm not a physicist. My understanding of Heisenberg's uncertainty principle and its proof (that is given by an imaginary microscope) is that for example: at a specified time determining the exact ...
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53 views

Energy of a particle as measured by an observer at infinity

I'm wondering if it is possible to make a definition for the energy of a particle as measured by an observer at infinity. I've looked through Wald for this but wasn't able to find anything - I may be ...
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change of energy in changing frame of reference

Let's imagine a car that can jump onto, or off a moving train. The train moves at 10m/s. The car, on a road next to the train, accelerates to the same 10m/s, jumps off a ramp and lands on the train, ...
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How can removing energy from a system do work? (ice)

Water freezing into ice expands, damaging roads, breaking rocks, etc. To my understanding, this is doing work (in a physics sense) by removing energy from the system. How is this possible?
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how to calculate enthalpy and internal energy in relation to heat capacities?

I remember that $\mathrm du=c\mathrm dT$ and $\mathrm dh=c\mathrm dT$ with $c$ being two different heat capacities, but I can't find anything about it, any help?
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2answers
62 views

What is beyond Gamma Rays and Radiowaves in the Electromagnetic Spectrum?

The electromagnetic spectrum is commonly refered to as consisting of; Radio-waves, Microwaves, Infrared, Visible Light, Ultraviolet, X-rays, Gamma Rays - of increasing frequency from left to right. ...
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1answer
61 views

2 times the velocity, 4 times the energy, 8 times the heat

In SpaceX's webcast today they talked about the first stage getting back to Earth with twice the velocity as usual. I quote: "So 4 times as much energy means harder on the stage, and 8 times as much ...
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What is the energy density of a spinning orbiting ball of uniform mass with surface charge?

What is the energy density of a spinning orbiting ball of uniform mass with surface charge? I know that all the energies involved are mass-energy, spinning energy, orbiting energy, electric energy ...
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1answer
60 views

Work done in bringing down a body [closed]

A block of mass $30$ kg is being brought down by a chain. If the block acquires a speed of 40.0 cm/s in dropping down $2.00$ m, find the work done by the chain during the process. I have calculated ...
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3answers
67 views

how could Kinetic energy increase if no energy was there in the system?

i was reading Physics-Halliday, Resnick, Krane(fifth edition) there on page 638 it is written: if the external agent does positive work in assembling the charge ...
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1answer
53 views

Why is standing still easier than walking? [closed]

Both the works basically push force into the ground.So why does walking require more work?
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1answer
55 views

Understanding potential energy

I am self-studying the classical mechanics using the book by Taylor, and I have a question about the potential energy. The book (pg 111) says: If all forces on an object are conservative, we can ...
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1answer
101 views

Why does holding an object cost energy while no work is being done? [duplicate]

I was reading the discussion here: Why does holding something up cost energy while no work is being done? I feel as though the question is being avoided. Suppose instead of holding an object by hand ...
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3answers
200 views

How do electromagnetic waves carry energy?

Its said that electromagnetic waves carry energy. Is this because these waves are made up of electric and magnetic fields which can cause changes to the stuff that falls with in their range? Is that ...
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1answer
28 views

Gravitational collapse (conservation of energy)

During gravitational collapse, gravitational potential energy of the gas is converted to its internal kinetic energy so the internal energy of the clous of gas is said to be increased But isn't ...
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0answers
27 views

Interaction of atoms of the same material

So I've been reading a section about energy levels and came across this statement: "As the atoms of a material are brought closer together to form the crystal lattice structure, there is an ...
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0answers
15 views

Rugged Energy Landcapes (Free Energy vs Potential Energy Questions)

A spin glass has what is called a "rugged energy landscape." That is, when you cool down below a certain temperature, the system divides into many wells, all corresponding to slightly different ...
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1answer
52 views

Why is the energy operator special?

Only the energy operator controls the time dependence of a quantum system, but not the others, why is that?
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1answer
39 views

Why is the energy shift due to a 'sagging' potential negative and independent of box size?

Consider a box of width $L$ and the composed of the following potential $$V(x)=\frac{V_0x(x-L)}{L^2}, x\in[0,L]$$ and $V(x)=\infty$ elsewhere. Using perturbation theory - with a square box as the ...
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1answer
59 views

Can we have $E=0$ in Schrödinger's Equation?

I've read a little bit about zero-energy states, but I just don't get it. I'm just starting to study quantum mechanics and, at least for all the potentials I've seen until now (the most popular ones, ...
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3answers
62 views

How do Calories (kcal) relate to watts?

In thinking about exercise and "burning calories" it occurred to me that there should be some fairly intuitive correlation between the biological energy conversion going on in a person's body with the ...
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0answers
66 views

Zeroth component of 4-momentum and relativistic energy-momentum relation

As I understand it one is forced to use 4-vectors since we require objects that transform as vectors under application of Lorentz transformations and 3-vectors do not (technically they do under ...
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1answer
20 views

The energy of de-excitation

I am 10th class student and what i dont get is when electron dexcites it produces energy but what is main phenomenon which produces energy is it the motion of electron or something like disturbing ...
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1answer
35 views

Variation of Veritasium bullet block experiment

I'm new here. this is in reference to the video posted by veritasium on the bullet block experiment. i realise there is already a thread on this but i want to ask a variation of the question. so ...
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2answers
47 views

Particle in a box, quantization of energy

I'm learning about how the energy of matter is quantized like how the energy of light is. My textbook illustrates the concept of quantization with the particle in a box: "A particle of mass $m$ ...
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3answers
9k views

How is 6W equivalent to 40W, as claimed by adverts for LED light bulbs?

Every advert I come across for LED bulbs advertise them as the equivalent of a higher W incandescent bulbs. This makes no sense to me, if the room requires 40W to lighten it up then it'll always ...
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1answer
101 views

Is Phone-Charging Plant Product Even Possible?

Bioo, a new company making the rounds in the press, claims to produce "electricity from plants' photosynthesis." Specifically, their website claims that a single potted plant with their integrated ...
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2answers
407 views

Is gravity the current limitation to laser cooling?

This is something I've been wondering for a while. I've read about the experiment with NASA to reach unprecedented cold temperatures in space because of the microgravity environment not being such a ...
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1answer
63 views

Calculating kinetic energy without Mass [closed]

Is it possible to calculate the kinetic energy of a body if its velocity is given its direction is vertical and its hight is 0 at moment t=0
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2answers
44 views

how can energy conservation equation be different in different situations?

Neglecting potential and kinetic energy: energy conservation equation in a closed system is dU=Q-W and in an open system with steady flow is dU+d(PressureEnergyPV)=Q-W or in other words dH=Q-W. How ...
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1answer
27 views

How does an object with gravitational energy exhaust that energy as work?

To my understanding (which is pretty basic) energy is the ability to do work. The higher objects are above a point - more gravitational energy they have relatively to that point, which means that they ...
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0answers
23 views

What is happening to energy that is not used as work? [duplicate]

To my understanding (which is pretty basic), energy is the ability to do work. But if for example, I push a wall, I do exhaust my energy, but no work is done since the wall is not moving. So what is ...
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0answers
31 views

Thermal de Broglie wavelength - definition

The thermal de Broglie wavelenght is often defined by the formula $\lambda=\frac{h}{\sqrt{2\pi mkT}}$ but equally frequently is it defined as de Broglie wavelength for a free ideal gas of massive ...
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1answer
64 views

How does the energy of a supernova compare to that of its parent star?

How can the energy produced in a supernova be quantified? And, if the energy of the supernova is not equal to the energy of the parent star, how do we account for this difference?
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2answers
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How does kinetic energy do work? [closed]

To my understanding, work is done on object A when object B is applying a force on object A, causing object A to displace. Since energy is the ability to do work, what work does a moving object do, ...
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2answers
43 views

Hourglass fluid mechanics issue with Bernoulli's equation

I know that I'm thinking about this problem wrong, but I've been pondering it for a while and can't understand something. On my fluid mech. exam I had a problem that asked to determine the radius of ...
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Energy efficiency in modern houses [closed]

What is an energy efficient way of heating modern houses in a winter scenario and how could we experiment it? For example, my group tried heating a beaker of water with a hot plate, microwave,and an ...
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1answer
43 views

What are the assumptions behind the Lagrangian derivation of energy?

What are the assumptions behind the Lagrangian derivation of energy? I understand that we're searching for a function $L$ that describes a set of physics so that solving the energy minimization ...
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2answers
62 views

Utilising Black Holes as a potential energy source

I'm aware of the Penrose process and the basic physics behind that. Also, I know that the Blandford-Zjanek process (That is potentially responsible for the relativistic jets). Aside from these two, ...
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2answers
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Is the energy change $\delta Q$ form 16C-15C the same as 72C to 73C?

As the tempreature change is the same. so the amount of energy change should also be same. According to the foumula $$\Delta Q=m s \Delta T$$ So no matter form what temperature to temperature change ...
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Is expectation value of the Hamiltonian always the energy? [duplicate]

There are time dependent & space dependent systems (magnetic fields) and time independent (particle in a box or harmonic oscillator). In the latter the expectation value is the 'average' energy ...
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1answer
27 views

Finding Intensity of light incident on a photodiode

Given a halogen lamp(9500lumen,500W) at a distance d from a photodiode. How can I find the intensity of light incident on the photodiode?
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Energy density of a transverse

I'm finding it hard to understand the concept of energy density of a transverse wave. I know the formula, but I can't quite get my head around it. I know energy density is energy over volume, but ...
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1answer
95 views

How did gravity exist before mass?

I'm brushing up on my Big Bang, and I noticed that, apparently, although gravity is the first force to split from the original "superforce", mass does not appear on the scene until after the second ...
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1answer
16 views

The difference between Gibbs and grand canonical potentials

To my understanding, Helmholtz free energy is a measure of the amount of work a thermodynamical system can perform. Following the same reasoning Gibbs and Grand-canonical free energies measure the the ...
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1answer
31 views

How can I get 100% fuel efficiency from an IC Engine in a car?

I understand that the current technology of internal combustion (IC) engines gives about 20% for a normal car. I want to understand what I can do to achieve 100% or near 100% efficiency in the same ...
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2answers
29 views

Trouble understanding non-conservative force and work

I am having some trouble defining whether a force is conservative or not. In the example that we are working on a mass on a pulley system is used to cause a slender bar to rotate from rest to a ...
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0answers
23 views

Light bulbs Flashing on when disconnected

For the 2nd time I've had a lightbulb flash on in my hand after removing from the power socket. The first time the overhead light in my hallway was out for a couple of days before I got around to ...
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1answer
54 views

Is fire more harmful than a phone?

I have read somewhere that the higher the frequency of electromagnetic radiation, the higher the damage it causes to your body, and visible light has a very high frequency in comparison to microwave ...