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 do I describe and calculate the effect of an impacting object?

My lab studies the physiology of impact injury on biological tissues. I use a pneumatic cylinder to impart injury into a biological sample and then assess the molecular and physiological changes in ...
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1answer
2k views

Where does the energy of a light bulb come from?

Where does the energy of a light bulb come from? Is it from the coil of wire, magnet in the generator, mechanical input to the generator or plug where the generator is connected to the wall?
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2answers
169 views

When do we see particles to be in a superposition of energy states?

I have two doubts: Exactly when does this happen? and If we are in a superposition of states (lets say E1 and E2) and the particle absorbs a photon, what will happen? If E3-E1 = hf, will it go to E3? ...
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0answers
38 views

Is it possible to formulate a Hamiltonian for a damped system?

I recently found out that it is possible to formulate a Hamiltonian for a system with time-dependent coordinates such that the Hamiltonian is not the same as the energy When is the Hamiltonian of a ...
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1answer
134 views

What happens to gas molecules after ionization?

I know that gas molecules conduct electricity after they get ionized but what will happen if we keep increasing the voltage even after ionization? Will it explode? If it will then how much energy ...
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0answers
11 views

Pressure change in throttling & flow rate reduction in a water circulation

If the pressure in a water pipe line is reduced from 7kgf/cm2 to 4 kgf/cm2, what is the % reduction in flow rate?
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52 views

Do solar panels work forever?

Do solar panels generate electricity infinitely? IF YES then how do the electrons in the n-type which go to p-type then how do those electrons restore themselves. I mean in n-type terminal we have the ...
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1answer
38 views

Determining the force of a spring

How can I determine the maximum force a spring can release when it pops up? Is there even a formula for this?
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1answer
27 views

Big knot in my head, please help. (Nuclear fission and binding energy)

Please look at these two links: http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/imgnuk/bcurv.gif ...
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1answer
221 views

Why potential energy of external force on particle attached to spring is equal to full force multiplied by full displacement?

I am having quite difficult time to understand the meaning of total potential energy on spring. All references I read say that the total potential energy on particle attached to spring is: ...
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2answers
67 views

Are black holes an infinite source of energy? [duplicate]

Black holes are considered particularities because in a determined point in time they are pointless, as consequence there are some mass in a null space so the density become infinite. Finally if ...
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2answers
49 views

Is it possible to make an electromagnetic shield?

Is it possible to make an electromagnetic shield to protect something (or ourselves) against things like bullets, human attacks, metals thrown on us or simple dust particles to protect a spaceship ...
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1answer
30 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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3answers
70 views

What is $E$ in the Planck-Einstein relation?

The Planck-Einstein relation was first given for photons $$E = h\nu$$ But later, de Broglie extended it to matter waves, and showed that it would hold for all particles as well. The $E$ for a photon ...
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1answer
27 views

The potential profile of two batteries in series

I would like to know the potential profile in two batteries in series, like the battery in the attached image.
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2answers
118 views

Does radiating heat to outer space through the atmospheric transparency window have any impact on heat loss?

I was reading this article: Passive radiative cooling below ambient air temperature under direct sunlight The authors are improving upon the age-old idea of reflecting incoming sunlight while ...
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1answer
25 views

What is the source of energy in this system? (resonance realated)

This is curiosity question, sorry if description is too vague. Suppose we have a string attached to some small frame on both sides Then we apply a force by plucking it with a finger, lets say 5.0 N ...
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1answer
95 views

When (or what is the meaning of) $I \propto E^2$?

For a monochromatic plane wave: $$\mathbf E = \mathbf E _0e^{i(\mathbf k \cdot \mathbf r -\omega t)},\qquad \mathbf H = \dfrac{\mathbf B}{\mu _0}= \mathbf H_0e^{i(\mathbf k \cdot \mathbf r-\omega ...
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1answer
349 views

Sufficient conditions for the energy to be not conserved?

I'm almost embarrased to ask this question because I thought I was by now very confident with classical mechanics. Someone has stated that given a mechanical system with a Lagrangian $L$ s.t. ...
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1answer
152 views

Thermodynamic Internal Energy Reference States

The internal energy, $U$, of a given thermodynamic state, $\boldsymbol{R}$, as introduced in thermodynamics textbooks like Callen seems to be defined as: $U (\boldsymbol{R}) = \Delta U ...
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1answer
25 views

What is the physical basis behind burnt calories estimates?

we see on treadmills, bikes, fitness trackers, etc., estimations of calories burnt. These are clearly based on correlations to weight, age, hear rate. How were these values correlated? Is there a ...
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5answers
726 views

Noether Theorem and Energy conservation in classical mechanics

I have a problem deriving the conservation of energy from time translation invariance. The invariance of the Lagrangian under infinitesimal time displacements $t \rightarrow t' = t + \epsilon$ can be ...
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1answer
38 views

How do particles in a wave make the next particles move?

i only know few things about waves,but this question just popped up in my mind. I already know that the particles transfer energy to each other,but i do not know HOW they do it! The truth is that i ...
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0answers
31 views

Photons under pressure

Does it make sense for photons to be put under pressure, if so how would this be measured? I'm wondering because I would like to plug in $E = h\nu$ into the formula for enthalpy.
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1answer
83 views

Is possible to generate energy from gravity?

When the wind goes through the vanes of an wind turbine, it's generate energy. When the water goes through the water turbines, it's generate energy. So my question is short and simple, is possible ...
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0answers
92 views

Fusion - A List of Experimentally Achieved Beta Numbers in Magnetic Confinement [closed]

I want a list of all the beta numbers achieved in across all magnetic fusion energy devices. This is not theory, but the actual number measured experimentally. No list like this exists. The beta ...
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0answers
23 views

Threshold energy, does this indicate particle annihilation?

If we have a question on threshold energy like the following: Two particles A and B collide to create C and D. Find the threshold energy required for particle A given that particle B is at rest. ...
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1answer
60 views

When to use h = Cp∆T or u = Cv∆T

I'm getting myself confused on when to use h = cp∆T or u = cv∆T where cp is the specific ...
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0answers
36 views

What will happen to this fan in this situation?

Lets say I have a pipe with a fan at the end of it and it is drawing ambient air from the other end of the pipe. Once I account for some blockages and looking at the fan curve, it was found the ...
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0answers
26 views

How does a synchronous motor know to increase current when the mechanical load is increased?

Consider a very simple singe phase synchronous motor, such as the one shown in figure 1. This motor will not be self starting, but if an AC voltage is applied and the permanent magnet is given an ...
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1answer
8k views

Water Electrolysis Calculations

From Wikipedia (http://en.wikipedia.org/wiki/Water_electrolysis#Efficiency): The electrolysis of water requires a minimum of 237.13 kJ of electrical energy input to dissociate each mole. Each ...
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1answer
27 views

How much power does it take to keep a massive particle suspended in a gravitational field?

For instance if I have a rocket of mass $m$ in a uniform gravitational field $g$, and I want to keep it floating in the air via thrust alone, then how much power in the form of (say) chemical energy ...
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6answers
679 views

Why is electric potential 0 in this case?

On a test, we had a question where there are 4 point charges at the vertices of a square. The 2 charges at the upper vertices have charges of +q and the 2 charges ...
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0answers
21 views

Calculate Total Energy Of Universe [duplicate]

Physicist using $mc^2$ to calculate positive energy and $-MmG/R$ to calculate negative energy to calculate total nergy of the universe but I heard that one of them is relativly invariant and other ...
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0answers
22 views

How is the Joule normalised?

Apologies if this question is a duplicate, I tried searching for this question both on Google and here, but was unable to find an answer. A Joule is defined in various ways, some of them being: ...
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4answers
313 views

Is any energy required to deflect an asteroid, with force always perpendicular to its trajectory?

Let's assume there is an asteroid traveling on a straight line (far from any gravitational source), and we need to deflect it from its actual trajectory, so we build a rocket motor on the surface and ...
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1answer
14 views

Energy lost is a percentage or flat rate?

If I drop a ball onto the floor, there are a few possible outcomes. Firstly, that the collision is perfectly elastic, and no energy is lost from the ball into the surroundings (ie it bounces to the ...
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1answer
35 views

Is it possible to calculate the information content of matter? How?

I know the Bekestein bound is the upper bound for the information content of a region of space, but is it possible to actually calculate that information content (number of bits, not the bits ...
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0answers
53 views

how does V divided by EMF shows efficiency

I get that emf = energy per coulomb supplied and that V= energy per coulomb used but i dont get what it means when is says "how"
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1answer
36 views

What observation(s)--if any--confirm that the types & concentrations of energy, which are influenced by gravity, are the same ones that cause gravity?

General relativity allows various forms of energy to participate in the gravitational force. What observation(s)--if any--confirm general relativity's notion that the various types & ...
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1answer
41 views

How do I find the average kinetic energy and average potential energy of a hydrogen electron in the ground state?

In my modern physics class, we are wrapping up the 3D Schrödinger equation, and I am more than a little lost. A few chapters ago, we learned about operators, and I have an equation for both these ...
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1answer
82 views

To what does energy belong in an energy diagram?

I was reading this answer and I saw an energy diagram: Does the energy belong to the system or to the outside?
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57 views

How reflective are typical mirrors in the visible part of the spectrum?

I was wanting to perhaps grow some plants using grow lights. For the sake of efficiency, however, I would like to use some mirrors to reflect this light, ie, keep less of it from uselessly ...
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1answer
51 views

Dissipated Energy from Falling Object using Lagrangian [closed]

A plate of mass $M$ moves horizontally with initial speed $v$ on a frictionless table. An object of mass $m$ is dropped vertically onto it from the height $h$ and smashes. How much energy is ...
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1answer
84 views

How to heat water to 80°C? [closed]

I'm trying to figure out an easy water to get 80°c to consistently make the perfect cup of coffee. Let's assume room temperature water is 22°c. What is the ratio of boiling water to room temp water ...
0
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1answer
65 views

Why do resistors always cause a voltage drop equal to the voltage of the battery

Consider a wire connected to a battery. Now,potential is analogous to the energy of the particles.And potential in a resistor drops because of the friction inside the resistor(considering there is no ...
2
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1answer
28 views

Explanation for electrostatic energy expression

My question is about the electrostatic energy $We$ expression for $n$ point charges. I just can't figure out where the factor "$\frac{1}{2}$" came from: $We=\frac{1}{2} \sum_{i=1}^{n} q_iV_i$
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3answers
69 views

Shouldn't solar cells be nonreflective?

If solar cells take energy from light, then why do they reflect it? How come they can take energy without actually affecting the source? Shouldn't reflected light (as a minimum) have a lower ...
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0answers
93 views

Why 'Force x displacement', and not momentum to measure energy? [closed]

[I have changed the title and also edited the question in other ways to make it clearer what I am asking. I hope it no more reads like a duplicate. Nor it remains any more a history question.] I have ...
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2answers
178 views

“Rocket in a box” thought experiment

Here's a thought experiment that I came up with a while ago. It might be hard to visualize, so I'll describe it as best I can. Take a rocket in a vacuum, in outer space. Attach a large "box" to the ...