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 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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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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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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Energy of a standing wave

I've been taught that energy of a standing waves remains between the nodes of the wave or in other words energy remains within the wave. I wanted to know how that's possible. Also if energy of a ...
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21 views

Klein-Gordon quantum relativistic equation negative energy [duplicate]

Interpretation of solutions with negative energies was such that charge, rather then probability, density was assumed. When inserting charge of a particle negative sign is then obviously due to ...
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2answers
15 views

X-ray radiation - What does h * fmax = e * U mean?

I'm trying to figure out this formula for X-ray radiation $$h \times f_{max} = e \times U$$ $h$ = Planck's constant $f_{max}$ = maximum frequency, in hertz (?), that the photons can have because ...
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41 views

Total Mass in Universe [duplicate]

Currently (2015), what is the total mass of the universe? How can we say with mathematical certainty that 69% is dark energy; 25% is dark matter; 5% is atomic matter; and 1% is neutrino, photon, and ...
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20 views

Is energy discrete [duplicate]

It is often stated that because the energy in an em wave is $E=hf$, the energy comes only as multiples of $h$, ie quantized. But we know that $f$ is a real number, and you could have fractions of one ...
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Hydro-electric gravity problem - Is there a air-resistance-like force affecting water flow down a mountain making placing hydro-electric ?

Say I have a theoretical mountain with a river starting exactly at the peak. Gravity would act on the water, accelerating it down the mountain. There would also be a force limiting the water's ...
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1answer
54 views

why doesn't liquid metal vaporize in a vacuum?

I am wondering why molten metal in a vacuum of electron beam and machines never turns to gas like liquid water does when exposed to a vacuum.
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1answer
26 views

Explanation of force amplification inside a solenoid

For a system being actuated by a motor, the force can be amplified by gearing. The energy is being used for force instead of distance, so it produces more torque but moves slower. For a system being ...
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1answer
34 views

Why does negative energy imply that a system is bounded? [duplicate]

I wanted to know why "negative energy" of a two particle system implies that it is bounded. That is what happens in the case of a hydrogen atom; my textbooks say so, but they do not give any reason ...
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23 views

Work done against a physical field: positive or negative?

My understanding of signing conventions contextual to physical fields: With frame of reference as object: Work done by an object against a physical field is positive. Work done by a physical field ...
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23 views

Non-linearity of energy conversion efficiency

I have a very general question to all of you. I am wondering if there is any basic physics principle that would state that energy conversion efficiency will be always non-linear (for the practical ...
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1answer
22 views

What happens to energy used by reaction force?

Suppose a person pushes a rock. The person is exerting a force and his energy is used in burning up of fats and converted to thermal energy. But which energy is used by the rock's reaction force.?
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1answer
47 views

What observations would be needed to falsify the law of conservation of energy?

I've been doing some thinking, and began to wonder: What observations have led us to the conclusion that ‘energy can neither be created nor destroyed’? Essentially, this means that the big bang ...
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1answer
36 views

Dielectric heating - does EM field transfer energy to polar molecules?

In AC field, polar molecule are trying to align with external field. At high freguencies, changes of field are so quict, that molecules cannot align with it. This leads to increasing kinetic energy of ...
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1answer
38 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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1answer
41 views

Thermodynamics vs Kinetics

As a chemical reaction approaches equilibrium, one of forward or backward reactions dominate the other. According to thermodynamics, this is because the gibbs free energy change for one is negative. ...
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1answer
47 views

Isolation of a Physical System and Energy

I'm trying to grasp basic concepts of energy and i have a little doubt regarding conservation of energy. According to Feynman book and Wikipedia, the Law of Conservation of Energy states that ...
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1answer
52 views

Wondering about Energy [duplicate]

Me and Energy I'm trying to move along with my study of non-advanced physics but not grasping what energy really is, is driving me nuts. Whenever i see anything about energy ( Kinetic, ...
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Energy density in electrostatics

Imagine two hollow spheres with radius $R$ with charges $q$ and $-q$. $L>2R$ is distance between them. Potential energy of the each sphere is \begin{equation*} W_1=\frac 1{8\pi \epsilon_0}\frac ...
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22 views

Calculation of energy deposit of photon beam

Let's say I shoot a 1 MeV photon beam onto a volume. I want to know the ratio of deposited dose per path length for different materials (water and air) at a certain position (e.g. x=0) in the volume. ...
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Solving the quantum well gives you eigen energies gives $E_n$, are these energies in conduction band or valence band?

I wonder if the energies $E_n$ that is derived from solving the SWE for the quantum well can be considered as energies in the conduction band or the valence band. In other words is $E_1$ is lowest ...
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1answer
19 views

How to calculate the electric energy caused by eletrostatic field stored in a region, given $V$?

I have problem in apply the rules to find the energy stored in free space here is the problem Find the energy stored in free space for the region $$0.002<r<0.003m,\quad ...
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1answer
21 views

Why reduced atoms are more energetic

For example, in glycolysis, NAD+ is reduced to NADH and then it's more energetic. Why adding electrons to an atom gives it more energy?
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Possible Lack of Invariance in Two Inertial Reference Frame Scenarios

Two objects are connected together via a ideal spring which is compressed initially. Then the spring is released, the two balls are no longer connected and go their separate ways. Assume no other ...
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55 views

Conservation of energy during rolling without slipping?

Say there's a cylinder laying on a flat surface, and the surface is rested on frictionless ice. Attached to the surface is an engine that is attached via a string to the center of the cylinder, and ...
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1answer
49 views

Heat capacity $C$ at low temperature

The internal energy, $U = Nk_bT$ where $N$ is particle number, $k_b$ is Boltzmann constant and $T$ is temperature. Therefore, the heat capacity $C$ is given by $C=\frac{dU}{dT}=k_b$. However, in ...
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40 views

How much energy and momentum is carried by a mechanical wave?

Let's assume there is a wave spreading across in a substance. Described by some partial differential equation. How can I calculate the momentum and energy carried by this wave? Or maybe the energy and ...
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39 views

How to compute minimum shallowness of quantum well to have at least one bound state?

Given a potential $V$, how does one compute how shallow the potential can be such that it allows at least one bound state?
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1answer
210 views

What happens to kinetic energy of an object when i stop its motion? [duplicate]

Assume an object is moving and it is stopped at a point . Now kinetic energy will transform into some other energy . what can be those energies and on which cases these will happen respectively ? ...
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2answers
165 views

How does friction help the car to move forward? [duplicate]

Below is the extract from my book. There is a car with four wheel drive system and during acceleration, the engine causes the tires to push backward on the road surface. This push produces ...
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26 views

Using theater lamp for heating [duplicate]

I'm asking myself a question about how to warm my house up, and the efficiency of such a thing : I live in a theater, and we got plenty of 1000W lighting fixture, mostly Tungsten incandescence ...
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1answer
100 views

Kinetic energy (KE) in atomic orbital

Within an atomic orbital, electrons must obviously have relative differences between points in space due to potential gradient. But there is kinetic energy as well. If we choose a particular point as ...
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1answer
44 views

What is the equivalent of the “amount of fuel” for a battery?

I am trying to make a performance diagram of an electrical powered device vs range, speed, etc. I want to compare it using the equivalent of fuel for a battery? I am not sure if it is just voltage... ...
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1answer
58 views

Conservation of energy in a multiverse

When I was studying about classical mechanics, I learned about Hamiltonian mechanics. In a example problem, we saw that in a system of just one capacitor, energy seemed to flow in and out and the ...
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1answer
33 views

does electron - positron pair annihilation occur when E is negative?

Suppose I release an electron-positron pair from rest at a distance of $r$. Then the particles attract each other and collide. The total energy $E$ is $$E = 2m_ec^2-\frac{e^2}{4\pi\varepsilon_0r},$$ ...
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22 views

Dependence of kinetic friction on relative velocity

While I was reading my book I came across a line that tells that there is slight dependence of kinetic friction on velocity of relative motion of the bodies. Can anyone tell me how is this so?
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1answer
38 views

Too many antennas around a single energy transmitting antenna?

I am not a physicist. So I am not even sure, if I am phrasing this question properly. Lets say there is a wireless transmitter transmitting energy. Let us say I place $N$ number of antennas around it ...
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what factors cause the “energy gap” to change between different materials?

To my understanding, every electron wants to be "excited" its next energy level. Depending on the material, the electron will need a different amount of energy to reach the next energy level. This ...
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1answer
40 views

Battery charging using a windmill?

i want to charge 4*(12V/100AH battery) using a 3*( 12V/5AMP GENERATOR ) which is drawing its power from a windmill. these battery have to heat 500 litre of water from 30 celcius to 50 celcius how ...
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23 views

Different ways of converting solar energy to other forms of energy

Okay so after receiving our Physics papers today I decided to tell the teacher why I lost marks on the last question: "Mention the energy conversions undergone here Sun --> Coal-fired boiler --> ...
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temperature and velocity exchange

So there is a relation between total temperature and velocity of a fluid. In an adiabatic flow what determines how much one property (velocity or temperature will change)? I know the equation. But ...
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0answers
34 views

Is my representation of $\varphi$ Work function correct?

I am a middle-school so my understanding of physics may not be as solid as you professional physicists but never the less thought its worth a try to learn more. I read about photo-electric effect by ...
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37 views

How to determine speed/energy of something dropped from a given height?

Say I dropped a ball from some height, which is measurable, is it possible to determine the speed of the ball when it hits the ground and thus the force exerted on the ball my the ground (or vice ...
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27 views

What does the Equation $E_s = -dF$ mean in terms of a Rydberg Atoms Dipole Moment and its Energy?

Could someone explain what the equation $E_s = -d$•$F$: Where, '$d$' is the Rydberg Atoms Dipole Moment, '$F$' is The Electric Field Strength and '$E_s$' is the Energy of the Rydberg Atom. In terms ...
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1answer
92 views

Conservation of momentum 1-D problem: track is not level

This problem is a standard kind of 1-D problem. You have 2 equal carts approaching each other and collide on linear air track. BUT the person setting up the track did NOT do a good job at making the ...
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1answer
48 views

Will a heavier or lighter tank shell / bullet/ etc. travel further given the same starting energy?

Let's say I have for example two tank shells, both being the same size and having the same shape. Let's say the starting energy is 100kg*m/s^2, and the one shell is 1kg, the other 25kg. 1kg * ...
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1answer
43 views

Out of phase AC transmission lines in parallel

I've been reading up on power transmission networks and came across this thing. http://en.wikipedia.org/wiki/Quadrature_booster Basically it balances the load between two parallel power lines by ...