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 positive work done by internal conservative forces $\implies$ decrease of potential energy?

Potential energy can be thought as the amount of work that the force can potentially do on the point because of its position. $$W=-\Delta U=U_{initial}-U_{final}$$ A positive work done by a force ...
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Are electrons packets of energies made up of strings? [on hold]

I always wondered that if electron was a packet of energy it would hold true for most of the experiments and we won't need to deal with wave particle duality anymore,but then matter came in.what if ...
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2answers
27 views

Work and energy example [duplicate]

The normal definition of Work, as far as I have read, is: Energy is transferred through work. But I just heard an educationist state this example, where there is a weightlifter who keeps up a huge ...
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2answers
23 views

Force applies to sphere not on center of mass?

Is this true: When force is applied to a sphere NOT on center of mass (COM) then the sphere will move the same way as when force is ON center of mass, because the sphere is symmetric in every ...
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1answer
33 views

How much realistically usable energy can be obtained from matter/antimatter interaction?

The high energy density of a matter/antimatter system is well known. However, depending on the nature of the material, most of the energy from the interaction is released in the form of photons (...
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0answers
32 views

Is it possible in this Universe to communicate a bit of information with energy that scales sub-linearly with distance?

If we look at all the ways that people do communicate information, they all seem to have a cost "at least linear in distance." For example, communicating over a wire has attenutation, so the energy ...
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0answers
39 views

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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1answer
49 views

Fermi Dirac distribution and degenerate energy states

In Quantum Mechanics and in semiconductor materials, the number of electrons $N$ in conduction band is usually computed as follows: $$N = \int_{E_c}^{+\infty} g_c(E)f(E)dE$$ where $g_c(E)$ is the ...
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1answer
248 views

Physical interpretation of distances

Suppose we have a universe with only two distinguishable particles, neglecting gravitation. These particles start off next to each-other, ostensibly "touching" each-other and at rest. If it takes ...
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1answer
331 views

How to calculate the heat transfer rate between the liquid and the gas?

Let water (liquid) be in a chamber (with no heat transfer to the walls). The gas is the residual space above the liquid, meaning that the contact surface is the surface of the water. Given the gas ...
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2answers
103 views

Work energy theorem explanation

So there is this problem in my head, it may seem very simple, but I need an answer. The problem is, we have some object, and we apply a force by our hands with a constant speed from a position (1) to ...
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1answer
59 views

Density of states from $k$ to $E$

Speaking about Quantum mechanics, considering the "particle in a box" condition as an approximation of the electrons condition in a semiconductor, let the material be represented by a volume $V$ with ...
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0answers
41 views

Kinetic energy of a suspended mass from an overhead crane [on hold]

I am using Lagrange's method to formulate the equation of motion for a 3D overhead crane that is shown below: For the formulation of the kinetic energy for the payload (suspended) mass. I have seen ...
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1answer
121 views

Do we have a better understanding of what energy is since Feynman's time?

When lecturing about conservation of energy in the 1960s, Richard Feynman remarked: It is important to realize that in physics today, we have no knowledge of what energy is. We do not have a ...
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1answer
38 views

Do energy levels such as $E_c$, $E_v$ have negative values in semiconductors?

In energy band diagram of a semiconductor, do energy levels such as $E_c$, $E_v$ have negative values? Also, why electrons in semiconductor have energy? What is the formula for energy of an ...
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3answers
479 views

Why can't we harness gravity? [closed]

We harness solar energy with constant supply of light but not with gravity. Why? What are the latest developments that can harness gravity? Why do Perpetual machines fail?
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1answer
17 views

How does the nature of bonding between individual atoms in a molecule affect the van der Waal interactions between molecules?

I am currently studying certain materials. Some of these materials have ionic bonds, while some others have covalent bonds. Moreover, each unit of these materials interacts with the adjacent unit via ...
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0answers
30 views

Why a positive voltage raises the potential energy of a positive charge and lowers the energy of a negative charge?

I am reading the relation between energy diagram and V, E (electric filed) in a semiconductor from the site (page 47-48). However, I don't quite understand this sentence below. Could anyone explain ...
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1answer
161 views

How to compute this change of Gibbs energy for a Van der Waals gas?

The change of Gibbs energy at constant temperature and species numbers, $\Delta G$, is given by an integral $\int_{p_1}^{p_2}V\,{\mathrm d}p$. For the ideal gas law $$p\,V=n\,RT,$$ this comes down to ...
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1answer
75 views

Does a measurement always require the exchange of energy?

Can you measure something about something without adding or removing energy from it? This comment has got me wondering if there is a way to measure a property or a quantity of something without some ...
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0answers
30 views

How to apply conservation of energy to active noise-cancelling headphones?

Active noise-cancelling headphones add the opposite waveforms of the ambient sound in order to cancel out this ambient sound by destructive interference. I do not understand why adding more power in ...
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1answer
211 views

Explain confinement energy for a particle in a box

What is a "particle in a box"? How does confinement energy equal kinetic energy? How does confinement energy relate to the spectrum energy (the absortion/emission between energy)?
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34 views

What physical observations led physicists to talk about the 'energy of light'? [closed]

Clarification edit: Why is EM energy is calculated differently than the way we calculate all other forms of energy? Thermal energy: Q=mcT where m is mass, c is specific heat, T is temp Kinetic energy ...
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2answers
57 views

Dissipation and first law of thermodynamics

Consider the following situation: a certain gas is contained in a well-insulated cylinder with a well-insulated piston head. Now, in this case the piston is not frictionless. In order for the piston ...
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4answers
227 views

Energy density with complex permittivity?

Question What is the correct form of the energy density when we have a complex permittivity (such as in a dielectric)? Additional information A complex permittivity means that we have: $$\vec ...
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0answers
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constants to be assumed before designing a mechanical spring [closed]

what are the constants to be assumed before designing a spring? Under what criteria those constants should be assumed?
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0answers
26 views

Ratio of de Broglie wave length [closed]

Electron A has twice the kinetic energy as electron B. What is the ratio of de Broglie wavelengths of electron A and B? - this was a question given to me in advanced higher physics but I got a bit ...
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3answers
94 views

Explanation for the spectrum of the sun?

My Question is, why exactely does the intensity vanish below 240 nm ? If i look at the plank's law, obviously the intensity for $\lambda \rightarrow 0$ and $\lambda \rightarrow \infty$ will vanish, ...
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2answers
97 views

Ignoring constant potential energy

The reason why constant potential energy terms are dropped is because the derivative of a constant is zero. And since force is the gradient of potential energy, constant terms won't change what the ...
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3answers
4k views

How much energy was consumed when we turn on/off light?

My parents told me to turn off the light when I am not using it. But I remember my physics teacher told me that the action of turning on/off a light can cause huge energy. I am wondering how much is ...
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1answer
61 views

Where does energy go in the death of the Universe?

So I thought that energy can't be destroyed or created but can only be transformed into another kind of energy. I read something about the Universe dying because all the stars will burn out and the ...
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0answers
28 views

Joule's “Work-Heat Equivalent” Experiment

I have been reading about the Joule's experiment that is supposed to have shown that work is the equivalent of heat. I can't really find the original paper and I am finding trouble understanding how ...
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1answer
105 views

how do electrons in 2 separate atoms change its energy level when atoms come close together to form molecule?

The question is just as the title. It's said that electron must receive a specific amount of energy in order to go or drop to another energy level. So how can electrons of separate atoms which have ...
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2answers
143 views

what happened finally to the photons emitted by the atoms cooled in laser cooling process?

I mean, do these emitted photons go out of the system without any other happening. Because they are in higher frequency and so more energy, can they be used as a source of energy?
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1answer
58 views

Why do particles moving in circular motion emit em radiation? [duplicate]

For example particles moving in a synchrotron . Which energy of the particle is converted to the energy of the radiation ? What is the relation between this energy and the change in direction of the ...
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2answers
42 views

Relation between field and Potential energy of a body

I have read that if a body is in a field and is 1. moved in a direction opposite to the direction of a field, its potential energy increases.But why does it increase? 2.Also, if we move the body in a ...
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1answer
33 views

Work done relation to potential energy

I know work done is negative of change in potential energy, I.e., $W=-(∆U)$. It means that Work done against a force (or work done on a system) increases its potential energy. And Work done by a ...
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2answers
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Nuclear bomb power - myth?

I'm not experienced in physics yet (if it helps I've covered as much as acceleration, momentum and energy transfer/chemistry ionic and covalent bonding) but I've heard that the way people compare ...
6
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2answers
232 views

does a rocking kettle cause the water to boil more slowly?

When I boil water in the kettle on my electric stove, sometimes it rocks back and forth making an annoying sound at a frequency of about 6Hz. When that happens, I move the kettle slightly to make it ...
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1answer
43 views

If a flywheel is spinning at say 1,000 rpm, would an input of 500 rpm would actually slow it down? [closed]

When a flywheel is stationery, an input of 500 rpm would translate to less than 500 rpm in the flywheel considering entropy, but what happens if the flywheel is already running at 1000 rpm, wouldn't ...
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1answer
524 views

Could ions emitted by an ion thruster represent any realistic danger?

As ion thurster designs improve, the ions emitted could have a velocity (relative to the spacecraft) of well above 10^5 or even 10^6 m/s. It the likelihood of any such ions ever hitting a human ...
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1answer
26 views

Calculating the work done by friction on a rigid body due to the piston

I have a uniform rod of length 0.5m and mass 0.1kg, and on this rod are two balls of mass 0.03kg both at a distance of 9.997cm away from the center of the rod. There is a pin about the center of the ...
3
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1answer
318 views

Marvin the Martian vs. the Death Star: how much energy will they actually need to disintegrate the Earth?

According to a detailed analysis by Dave Typinski, Marvin the Martian’s Illudium Q-36 Explosive Space Modulator will require $1.711 \cdot 10^{32}~\text{J}$ to shatter the Earth into a gravitationally ...
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2answers
45 views

Negative work leading us to negative energy [closed]

We know that energy is the ability to do work and we know that the work can be negative as cos theta of the angle in which force is applied can be negative. Moreover we take the units of work and ...
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1answer
128 views

Pressure, Momentum and Energy: A 'Peculiar' Relation

Is the following possible? $$ P = \frac{p^2}{3E} $$ where $P$ is pressure, $p$ is momentum and $E$ is energy. In what convention is the above relation acceptable? EDIT: The source paper can be found ...
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0answers
26 views

Scanning Electron Microscopy on a smaller scale

Is it possible to sufficiently focus a ~1nm beam of electrons into a sample chamber in less than 10cm of vertical space, therefore reducing the volume of the entire vacuum chamber, and subsequently ...
2
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4answers
243 views

How to calculate the work of the electrostatic forces in a parallel-plate capacitor?

The expression of the energy stored in a parallel-plate capacitor is: $$U = \frac{e_0\cdot A \cdot V^2}{2d}$$ with $e_0$ the vacuum permittivity, $A$ the surface of the capacitor, $V$ the applied ...