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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Expression deduction for energy density per wave length

Energy density per frequency is defined by Planck formula as: $$u(\nu,T)=\frac{8\pi h}{c^3} \frac{\nu^3}{e^{\frac{h\nu}{kT}}-1}$$ The relation between wave length, $\lambda$, and frequency, $\nu$, ...
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158 views

When i tear a paper, can i accidentally create nuclear fission?

I know it's a stupid question... but when I tear a paper i may coincidentally split it and create nuclear fission. When i tried experimenting, by tearing the paper for HELLKNOWS how many times, I ...
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86 views

Entropy reversal in magnets

Entropy is an irreversible phenomenon without any energy supplied to reverse it. I was reading about paramagnetic substances and how dipoles align inside them on application of magnetic field. My ...
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241 views

conversion of units

I have a quantity that I need to convert to different units. First, I need to note that the value is in the natural units system. The quantity is in cm^-1 (or, to be accurate, $cm^{-1}\hbar c /k$, ...
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150 views

How is energy dissipated in a travelling em wave

How is energy dissipated in a travelling em wave. Will there be any dissipation if it were to travel trough vaccum ?
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165 views

How does The work done in going around any path in a gravitational field is zero implies conservation of energy?

In The Feynman Lectures, Feynman states: The work done in going around any path in a gravitational field is zero. This is a very remarkable result. It tells us something we did not previously ...
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137 views

Work done by friction on a body?

I know that when a body slides over a surface, the work done by friction is not stored as potential energy in the body. It is dissipated in the form of heat. But why is it not stored as potential ...
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137 views

Preventing Heat Escape

Is is possible to completely prevent heat from escaping from a closed container? Here is a diagram of vacuum flask, which tries to implement the design - Vacuum Flask prevents heat from escaping ...
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262 views

Relationship between Fermi Energy and Average Energy

Electrons are in a one dimensional box of length 2L. What is the relationship between Ef, the fermi energy and E, the average energy? The answer is Ef/3 I looked up the formula for the relationship: ...
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85 views

Is possible to synthesize diamond at relatively lower pressures (6 to 7 GPa vs 12 GPa)?

Pure binderless nanopolycrystalline diamond can be produced from graphite when the graphite is treated with more than 12 GPa pressure and at least 2100°C temperature . If we treated graphite or ...
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50 views

Interpretation for negative energy of curves

Let $(M,g)$ be a Lorentz manifold and $\gamma :[a,b] \to M$ a differentiable curve. I understand we define the energy of $\gamma $ as: $$E[\gamma] = \frac{1}{2} \int_a^b ...
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47 views

Elements of particle mass

From what I can tell, it seems that particles have two kinds of mass, the mass inherent in a fundamental particle itself, or for composite particles, additional mass associated with the Higgs field. ...
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71 views

What does it mean for a quantum particle to have energy $E_n$? And what is its general normalised state?

In this particular case, I have found the energy to be quantised with energy levels $\frac{h^2n^2}{2m} >0 $ where $n$ is an integer. Suppose a particle has energy $E=\frac{4h^2}{2m}$, then this ...
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619 views

What happens when an electron collides with an atom?

I was solving this question: Here is part of the energy level diagram of hydrogen: n=4 --> -0.85eV n=3 --> -1.50eV n=2 --> -3.40eV n=1 --> -13.6eV When an electron of ...
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275 views

Evaporate water using compressed air [closed]

I want to evaporate water by forming air bubbles which are formed by flow of compresses air through perforations. What is the energy (in KWh) input to compress 1 cubic meter volume of atmospheric ...
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68 views

Are nuclear processes the only processes that release more energy than is input?

Are the nuclear processes of fission and fusion the only known processes or events that release more energy than is input to create those processes? I know I am being a little vague; I do not know the ...
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335 views

Does the speed of electrons depend on energy?

I would like to know whether the speed of an electron depends on energy. If yes then in a circuit when electrons flow out of a resistor the energy decreases by a considerable amount, leading to the ...
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67 views

Do transferring energy and applying force to a body imply same?

Do transferring energy and applying force to a body imply same meaning? When we say, "I throw a ball using my pushing force so on the other hand, can I say that I transferred my kinetic energy to the ...
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268 views

Kinetic energy of a variable mass particle

If a particle's mass is a continuous differentiable function of time, $m(t)$, and its position is also a continuous differentiable function of time, $x(t)$, what is the expression of its kinetic ...
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142 views

A problem related to Work done by falling bodies : Expert's attention much needed! [closed]

I'm having a lot of trouble with this question, that I've found in my textbook. I've solved it in my own way and it's very simple! But the solution in the book is totally different. It doesn't make ...
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1answer
110 views

Where does energy goes when I work out?

Let's say I am using a machine to lift weight by pulling a bar. When I do that, the state of the machine is exactly the same at the beginning and the end of the workout; no heat has been generated ...
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678 views

What is the interpretation of pressure times volume as energy?

I know that pressure times volume has units of energy, but is there an intuitive explanation of how the pressure contributes to the total energy? It seems clear for ideal gases using the $PV=nRT$ ...
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65 views

Do I need to convert units to be compatible with constants?

I want to calculate the wavelength of radiation given its energy. I know I need to use $E=h f$ and $f = c / \lambda$. All I'm given is $E = 20 \text{ keV}$, now my true question is: Do I use $E = ...
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122 views

Internal energy and difference in volume at constant temperature

If the volume of a cylinder of a gas is decreased so that the work done on the gas is $W$ and the temperature is kept constant by taking away heat energy $H$. $\Delta U = -H + W$. Someone told me ...
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78 views

What's diamond's melting point?

I've been looking around but I've found no clear answer. The most I could find was: "around 4000K" Has it not been determined and it's just an estimation?
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163 views

Space curvature based on net energy = 0

In Neil DeGrass Tyson's epic video, at 2:26:50 ...
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59 views

What physics considerations direct the charging of cars with wireless?

Is there any frequency at which cars can be charged with using wireless? Surely, wireless transmission can be safely assumed to be a form of energy transfer, and there can be charging of cars without ...
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50 views

Power delivered by a plug

Ok so i know that (in the UK) the voltage of a mains supply is 230v. But when we plug a item e.g. a tv in to it what determines how much power is drawn from this supply. I presume it is the current ...
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237 views

Does the Alcubierre drive use negative energy to compress space infront of it?

The contraction in most images looks like a gravity well. Is it that you use negative energy to BOTH contract and expand space or positive and negative to contract (positive energy) and then expand ...
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76 views

Kinetic energy of a mass rotating about two axes

I'm working on a question where I have to find an equation for the kinetic energy of a mass being rotated by a fold-able arm. The equation for $m_1$ is obvious, however, the equation I derived for ...
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2answers
48 views

water pressure in the oceans or great lakes

Is there a way to create energy from water depth pressure? Convert the pressure to any type of energy? Does there need to be movement? Can someone comment on this process? Thanks
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71 views

What becomes of the massive clouds expelled from Quasars?

What becomes of the massive clouds expelled from Quasar jets? Do they simply disperse into intergalactic diffuse clouds? Or do they on occasion have enough gravity to hold together and or collapse? If ...
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4answers
406 views

How much up will an object of mass $m$ will be vertically lifted if a constant acceleration $a$ is given so that resulting force is more than weight?

Yesterday My friend asked me a question which put me into a confused state. If a body of mass m is given a vertical thrust of $F$ such that $F > mg$, and the $F$ is allowed to remain applied on ...
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209 views

Exertion from swinging on a playground swing

I've read about how by tilting one's body one changes one's center of mass while swinging on a playground swing, and thereby increases the energy of the swing. But I would like to get a better sense ...
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Conservation of mass energy and kinetic energy in different reference frames

With a little work it's easy to show that kinetic energy by itself is not necessarily preserved when switching between frames of reference. And it is my understanding that energy should be preserved ...
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799 views

Is energy released during change of state?

When a substance changes state from solid to liquid, the temperature of the substance remains the same and the heat energy from the external source is converted to potential energy within the ...
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231 views

Potential Energy of a Particle in One Dimension?

Since a particle in one dimension can only move in a straight line. Is it possible to have potential energy? And how would the kinetic energy and potential energy differ in higher dimensions?
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170 views

Gravitational interactions by energy or by mass?

The mass-energy equivalence, first established by Einstein is an important and highly discussed phenomenon in physics. Without claiming much knowledge about high-end discussions on this topic, I would ...
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685 views

What is the energy of a standing EM wave? Is it probabilistic?

In a cavity, the standing wave will constructively interfere with itself, so its energy gets higher while the oscillator is still vibrating. Since the vibration time is not a constant value, and ...
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1k views

Can electromagnetic radiation (i.e. photons) produce gravity? [duplicate]

I don't want to play with physical laws in a frivolous way. Assuming that the nature of matter and energy is the same, can a high density of highly energetic photons produce a gravity force? We do ...
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128 views

potential energy of a dipole?

The very popular from of potential energy of the dipole is $-P.E$. But in the derivation of it, we have negelected the potential energy of the pair of charges constituting the dipole. will this not ...
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11k views

Relation between work, kinetic energy and potential energy

We derived two equations in class. The work done between two points $A$, $B$ is equal to the difference between the kinetic energy at the last point and the one at the first point. The work done ...
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How is the energy of an electron-shell related to the speed of electrons in that shell?

I am trying to gain an intuitive picture of what is referred to by "electron-shell energy". I have read that outer electron shells have higher energy than inner electron shells, and this seemed to ...
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How is it possible to equate the internal energy at constant volume with the internal energy of an adiabatic process?

I hope my question makes sense. My problem is that, I have read through numerous textbooks that $$nCdT = -PdV$$ (where $NC$ is constant volume) when deriving the relationship between $T$ and $V$ for ...
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449 views

Energy of particle in electric field

I'm taking a physics class and the professor teaches us really basic things in lecture and then gives homework way beyond what he taught in lecture. Obviously I need to find some resource other than ...
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354 views

calculating work done by friction

I want to calculate the work done by friction if the length $L$ of uniform rope on the table slides off. There is friction between the cord and the table with coefficient of kinetic friction $\mu_k$. ...
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1k views

Is lattice enthalpy positive or negative?

I've learnt that the lattice enthalpy (defined as the energy change from a solid ionic lattice to separate gaseous ions) is always positive, obviously. However, I've seen it explained as the opposite ...
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263 views

Definition of energy

What is the definition of energy $E$ given a dispersion relation $\omega=\omega(k)$ where $k=|\vec k|$ and $\omega$ is not necessarily linearly proportional to $k$? What about momentum $\vec p$? This ...
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95 views

kinetic energy of the stone

Suppose we have a man traveling in an open car (roof open) with speed $v$ towards right (man faces right). He throws a stone (mass $m$) towards right, in his frame-forward with speed $V$. In the ...
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115 views

Static Friction work and Energy [closed]

I have this problem: A 4.0 kg block is given an initial speed of 8.0 m/s at the bottom of a 20° incline. The frictional force that retards its motion is 15.0 N. (a) If the block is ...