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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Does brightness of the light remains constant with the variation of the distance with respect to the source in vaccum?

I have this question going in my mind from many days, i.e why brightness of light emitted from any light source around us decreases with distance? The brightness of light from tube light, streetlight, ...
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How can there be heat in a vacuum?

I keep reading in the Physics World focus issue on vacuum technology about scientists creating high temperatures in the vacuums etc. If heat is caused by thermal energy being radiated from particles ...
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Where does the energy required to stop a moving body come from and go?

For a long time, I have been thinking about this: In order to stop a moving body, I have to apply impulse to change the existing momentum to $0$. For the impulse I have to impart force on the moving ...
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Why does a $1/2$ appear in this simple capacitor equation?

The maximum energy stored by a capacitor is when it's (theoretically) fully charged or just before it reaches dielectric breakdown, if you will. It seems, then, that the maximum energy ($E$) $=QV$ ...
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Free energy device - QEG. Is this for real?

I'm not a physicist but I ran across reports on the web of a so called over unity or free energy device that has been created that supposedly works. It's called the QEG. Does this seem legit or is it ...
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Why isn't $E=mv^2$ a general formula for energy?

This is the usual derivation of the formula for kinetic energy: $$\begin{align} \mathrm{d}E &= Fv\,\mathrm{d}t \\ \mathrm{d}p &= m\frac{\mathrm{d}v}{\mathrm{d}t}\mathrm{d}t \\ \mathrm{d}E &...
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What are high energy photons?

I have often read the term High Energy photons, does this mean that there do exist photons with low energy? Aren't they supposed to have constant energy according to $E= h\nu$?
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Need help in identification of formula [closed]

i would greatly appreciate it if someone could provide me with the concept to learn about this equation
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What does it mean if a body has kinetic energy?

What does it mean if a body has kinetic energy? Does it mean that the momentum vectors of each particle of that body has the same direction? What about angular momentum?
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Can there be energy with no force or energy with no power?

I think that both force (number of newtons) and power (p=ui(?)) implies that there is energy so we can't have force without energy and we can't have power without energy(?) But can there be energy ...
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Lifting and putting down a weight

A man in a gym lifts a weight and then puts it down where it was before. 1) What can be say about the work done by the man to the weight? 2) Can it be equal in absolute value to the work made by ...
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Why is this specific definition of acceleration used in deriving a general equation for kinetic energy?

$$\sum_{i=0}^n m_i v_i$$ as well as $$\frac{1}{2}\sum_{i=1}^n m_i v^2_i$$ Have been proved to be conserved by Newton & Leibniz, respectively. (http://en.wikipedia.org/wiki/Vis_viva) I don't ...
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Why aren't we affected by radium?

1)We have radium clocks, watches, wrist bands and many things which glow because of radium but we know that radium is radioactive so why isn't it harmfull for us when in bands, watches etc. 2)Does it ...
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458 views

How to measure the energy consumption class of a home appliance? [closed]

Some of you may be familiar with the EU energy consumption classes for domestic appliances (or equivalents in other areas). Let's say we have a domestic appliance which was made before the energy ...
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1answer
80 views

Why the Lagrangian $L$ is KE - PE? Why not KE + PE!

With Lagrangian, is there any way to intuitively grasp why total energy equals the difference between the kinetic and potential energy? Seems counter-intuitive - whereas Hamiltonian calculation (sum ...
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143 views

Can a photon have little to no energy and/or speed?

Can a photon move more slowly than the speed of light and behave 'non-relativistically,' so to speak. Perhaps another way to express my thought is: could we stop a photon from moving?
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Can time be calculated without actually involving any matter?

Humans typically measure time by the change in material things. The movement of hands on a clock. The oscillations of a cesium atom. But can time also be measured by just changes in energy, and in ...
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299 views

Can matter be excited into energy and then be turned back into matter?

I was wondering the other day about teletransportation (human). And I had the idea that as far as I know, matter is energy. So I was wondering if it's possible to excite matter so it turns into energy,...
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485 views

does matter radiate energy? [duplicate]

Since we know that all accelerated charges radiate energy and we also know that all matter is made up of protons and electrons which are all the while doing accelerated motion.So from this can i ...
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239 views

Lightning Power?

What is the power source of a lightning? It doesn't consume fuel, nor an atomic fission/fusion, but it discharges a large amount of energy. Is it feasible to create an artificial lightning using the ...
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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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Bicycle Problem [closed]

A student watches two people coast down a hill on bikes. They both start from rest. Neither one of them peddles the bike. Gravity is the ONLY driving force. Friction and air resistance are negligible. ...
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A definition for relativistic energy without quantum mechanics

What are the minimal postulates to define energy in special relativity? Let us assume that momentum has not been defined yet (it can be defined from the energy in fact). I expect we are looking for ...
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1answer
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What is the origin of the quantum operators for $p$ and $E$? [closed]

It is always stated the quantum operators for p and E are the ones we´re familiar with (the operator for energy, $H=i\hbar\frac{\partial}{\partial t}$ and the momentum operator, $\mathbf{p}=-i\hbar\...
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160 views

Question about distillation

I have got a question about distillation. In distillation as we know the mixture is boiled and the mechanism then goes like substances with high boiling points condense at the bottom and substances ...
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90 views

Why do coal power stations not directly use combustion to turn the turbines?

Why do coal power stations run by creating steam rather than from the expansion due to combustion? I understand that in order to generate electricity in a coal power plant, steam is usually produced ...
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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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299 views

What is internal energy?

My physics book says: The total potential energy and kinetic energy possessed by the particles that make up an object, but excludes the potential and kinetic energies of the system as a whole. I ...
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3answers
229 views

How can the bodies attain same velocities during the max. deformation during collision?

My book writes: Collision between two bodies is almost similar to the case where there is a spring between the two colliding bodies. At the instant they come into contact , the rear ball has a ...
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1answer
3k views

Relationship between temperature and energy

What is the definition of temperature in relation to energy? I'm mostly interested in general dimensional terms. Is temperature the kinetic energy per mass? Or kinetic energy per volume?
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1answer
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Potential energy of the human body if all its atoms were split

I am working on a sci-fi story in which a character has control over matter on the atomic level (think Dr. Manhattan from Watchmen). In thinking up potential tragedies for this character I thought, "...
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384 views

Does our existence cost us energy?

Anything when it needs to inform its presense such as electromagnetic presense of charged particles and gravitational presense of particles due to their mass does so by sending information of its ...
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14k views

How do you determine the heat transfer from a P-V diagram?

I doubt this question has been addressed properly before, but if there are similar answers, do direct them to me. I am currently studying the First Law of Thermodynamics, which includes the p-V ...
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1answer
837 views

Can negative energy explain black holes?

I think black holes are peculiar and I want to know if there is a relation to negative energy. Does negative energy have positive mass and therefore can explain the nature of black holes? IS there any ...
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1answer
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Experiment to measure initial speed of high speed tennis ball?

I want to devise a way to measure the initial speed of a tennis ball fired from a tennis ball cannon, but without using any speed-measuring devices. Just plan distance-measuring and physics formulas. ...
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221 views

How much time can I power my laptop by eating one dessert?

Random question that popped into my mind after a 4-hours power outage. Let us assume that I am eating an extra dessert (250 kcal) and that I am using a bike and a generator to power my laptop (it ...
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255 views

Equation $H(q,p)=E$ is the equation of motion or energy-conservation law?

I do not completely understand, why do we consider Hamilton–Jacobi equation $H(q,p)=E$ as equation of motion, whereas it is looks like energy-conservation law?
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1answer
57 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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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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2answers
44 views

What is the sign of the energy exerted by a man sitting in the back of a boat pulling a canoe?

A man sits on the back of a boat pulling a canoe while the boat travels at a constant velocity a certain distance in the positive X direction. The man feels a constant tension in the rope during this ...
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4answers
99 views

Prove that gravitational potential energy of a body of mass m at a height h is mgh

Many introductory physics books just write that potential energy of a body of mass $m$ at a height $h$ as $U_g=mgh$. However, they never show how this was derived. I'm interested in knowing this ...
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1answer
59 views

Why doesn't frequency of a sound wave reduce with time?

I understand how energy depends on amplitude as this source says but this source mentions that the energy of a sound wave depends (maybe "also on") on frequency of the sound wave. If energy of a wave ...
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2answers
40 views

Energy of a waterfall

Why cannot a waterfall be considered to be never ending source of energy? A waterfall is converting it's potential into kinetic energy and if we "take out" some of it's kinetic energy while the water ...
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3answers
105 views

is the potential energy of a black hole infinity

It appears like black holes posses an escape velocity greater than the velocity of light. It takes an atom infinity energy to travel at speed of light.Do black holes posses that energy to retain light ...
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1answer
97 views

A Confusion About Energy-Time Uncertainty Relations

In all the textbooks that I have seen, energy-time relation is written in the following way:$$\Delta E \cdot \Delta t \geqslant \frac{\hbar}{2}$$ Here is my interpretation of this principle: The ...
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1answer
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Why is there no $1S_{-1/2} $ state of the hydrogen atom?

Heyho, i found this term scheme for the hydrogen atom: https://upload.wikimedia.org/wikipedia/commons/9/94/Wasserstoff_Aufspaltung.svg I was wondering why there is only a $1S_\frac{1}{2}$ state and ...
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204 views

Gravitational force in a building

Suppose that a person is standing on the top of a multi storey building and at an instant earth's gravitational pull becomes zero.What will happen in the next instant? (Consider only earth's gravity ...
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641 views

Definition of “first excitation energy”?

I was solving a problem but I didn't know what the term "first excitation energy" means. The first excitation energy of the hydrogen atom is $10.2\text{eV}$. Calculate the speed of the slowest ...
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Is possible to generate energy from gravity?

When the wind goes through the vanes of an wind turbine, that energy is harnessed to generate electricity. When the water goes through the water turbines, that energy is harnessed to generate ...
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339 views

Reconciling Units in Classical System Analogies: Why Does Torque Have Units of Energy?

In classical physics we often cast an analogy between translational and rotational systems Force < > Torque Energy < > Rotational Energy Momentum < > Angular Momentum and considering SI ...