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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Total Energy of Inverted Pendulum

I have to find the total energy $V$ of an inverted pendulum (rod). The following parameters and their values are given: Mass $m$ and length $l$ of the pendulum $\theta$ as the angle of the ...
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“Definition” of internal energy

Conversation of energy implies that if we have a thermally insulated system which goes from state 1 to state 2: $$\Delta E_{12}=E(2)-E(1)=\Delta W_{12}$$ and the 1st law of thermodynamics ...
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Why does moving through time not require energy?

Moving through the other three dimensions necessitates energy. But why doesn't moving through time necessitate energy?
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Entropy: Disorder or energy dispersal?

The first definition of entropy given by Clausius is I believe this $$S=Q/T$$ It is as I understand a common fact to understand entropy and maybe often teach it as a measure of disorder through the ...
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Experiment validating existence of virtual particle

I'm curious about virtual particle whether do they really exists, therefore I dug out an experimental theory called lamb shift by measuring the difference between the two energy levels of a hydrogen ...
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Absortion of light by a plasma

Considering a plasma generated in a fixed volume by increasing the temperature of Helium gas from the environment temperature (this temperature is not stated) and an initial presure of 0.001 Torr, to ...
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Does it take more energy to open a door when applying force close to the hinge?

Assuming an ordinary hinged door (without any springs), would it take more energy to open it when applying force in the middle of the door (point b), rather than at the end of the door (point a), ...
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Closed-system power-generation using waterwheels

I'm not a physicist, and this question is bugging me .. :) Is it possible to create a closed power-generation system, using upper and lower water-reservoirs where a continuous flow from the upper to ...
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4-momentum of massless photons

The time component of a particle's 4-momentum is $$ E'=\frac{E-up}{\sqrt{1-u^2/c^2}}\tag{1} $$ Now suppose it is photons that you are observing from a system moving at $u$ on a line towards ...
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Why doesn't the evening/morning sun feel much hotter on your face?

I understand that places on the Earth's surface get hotter in summer, and in the middle of the day rather than morning or evening, because the surface of the Earth is presented 'face-on' to the Sun at ...
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23 views

Series of parallel thermal resistance connection for this enclosure?

Let's say that I have an 2D enclosure in the shape of a pyramid with unequal areas for the faces and I am looking to determine in the inside air temperature. The parameters I know are the outer and ...
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49 views

If we hit an electron will it go to an excited state?

For example i have a block of silicon doped with phosphorous and i hit it hard with a hammer will the energy get transfered to the block and make the electrons excited?
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How does a photon drive out the electrons in a solar cell?

We know that solar cells work when a photon hits the n-type the photon's energy drives free the electrons in the n-type to generate a current. But we also know that when a photon hits the atoms it ...
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Energy conservation $\iff \frac{dE}{dt} = 0\ $?

If I'm asked to prove that a system is/ isn't conservative and compare it to whether or not the Hamiltonian is conserved, does that mean I need to compute the time derivative of energy $(T+U)$? Doing ...
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How did Planck derive his formula $E=hf$?

Some time ago I asked my quantum physics lecturer the question: How did Planck derive his formula, the Planck–Einstein relation $$E=hf$$ with constant of proportionality $h$, the Planck ...
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91 views

Maximum Extension of a Spring [closed]

In the given figure: m= 5kg, F = 30N, K = 700N/m In the figure shown above. the surfaces are friction-less. The blocks are initially at rest and the spring is initially in its natural length. What ...
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Identity operator in terms of the energy eigenstates in case of continuous spectrum

Let us confine ourselves to the 1d case. If we define the momentum eigenvector $|k\rangle $ as $$ \langle x |k\rangle = \frac{1}{\sqrt{2\pi}} e^{i k x} ,$$ we have the identity operator decomposed ...
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Potential and kinetic energy

If a ball stays on the ground, it has no height and no velocity. So what energy does it have kinetic or potential?
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Calculate energy-density of known plasma in microwave?

Let's assume I want to create a plasma in a regular household microwave similar to this home-made experiment. Although I am dealing with a small amount of mass, I assume that the addition of microwave ...
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62 views

Why can “slow” neutrons trigger fission?

My understanding of nuclear fission is that some massive isotopes such as uranium-235 are unstable and when split via fission there will be a "slow" neutron. This slow neutron will hit another ...
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How much energy does a PV cell need to work?

How much energy does a $1m^2$ photovolatic/solar cell need to work? Can it work using a bunch of Laser lights? Edit: Ok so this PV cell is $1m^2$ is dimesions. The N-type has silicon doped with ...
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2600mAh Battery Charging Time

I've spent a very long time trying to work this out, but some websites seem to conflict the information - or I'm completely missing the point. I have a rechargeable battery: 2600mAh 4.2V max charge ...
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Relativistic Momentum and Energy - Lorentz Transformations [closed]

The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Lab is colliding fully ionized gold (Au) nuclei accelerated to an energy of 200 GeV per nucleon. Each Au Nucleus contains 197 ...
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Constant 2 in kinetic energy equation [duplicate]

Trying to understand where the constant 2 comes from in the kinetic energy equation, $mv^2/2$. Why 2 and not another number?
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2answers
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Calculating average energy stored in Li-Po battery

I'm currently sourcing Li-Po batteries and need to find an average mAh per cm3 to implement it into my product. From what I read (wikipedia), the energy density of Li-Po ranges from 250 to 730 W.h/L ...
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How can a photon have energy when its mass is zero? [duplicate]

How can a photon have energy when its mass is zero? According to Einstein's equation $E = mc^2$ energy depends on $mass*c^2$ Light has zero mass so the energy would be zero too but solar cells use ...
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117 views

Stone vs. Atomic Bomb

What would do more damage, a B53, a 9 megaton uranium bomb, or a medium sized stone (2 kg) stone throw at 90% the speed of light. One thing that I know that will have to be considered is that fact ...
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69 views

How do solar panels generate infinite electricity?

We know that solar cells generate electricity utilizing the energy of the photon, but how can they generate electricity forever? In a n-type terminal we have the bond of silicon and phosphorous so ...
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How does energy get transfered from destructive interference point to constructive one?

If there are two pressure waves (like sound waves) that travel in opposite directions and have the same amplitude then destructive interference occurs: one wave will compress the air particle (here ...
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How did the concept of energy evolve with the years?

The concept of energy is quite common today but this was not the case before. As far as I know, even Newton, for example, didn't use this concept and Liebnitz just noticed that $mv^2$ was conserved ...
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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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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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47 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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36 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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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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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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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
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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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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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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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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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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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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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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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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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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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How does antarctica stay frozen? [migrated]

Antarctica is receiving heat from the Sun everyday, so how does it stay frozen? I have made some thoughts about the answer but unsure of whether it is correct I hope people here can clarify. The ...
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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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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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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 ...