# Tagged Questions

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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### Why is kinetic energy only “often $(1/2)mv^2$”?

I am reading the first few pages of Nakahara and refreshing my memory on physics I learned a while ago as a physics math undergrad. Nakahara defines a field $F$ to be conservative if it's the gradient ...
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### Why is the receiver of this solar tower cream-colored?

I wonder, why the receiver of this solar tower is not black? Wouldn't a black receiver be more efficient? If not, why is it not white then, but is an intermediate color? It seems that the receiver ...
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### What are the physical limitations on energy density for a useful, safe battery, and are we anywhere near them?

Energy storage seems to be a fairly major bottle neck in technological development at the moment, especially for important things like clean-energy powered transport, but also for consumer goods. Are ...
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### 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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### Moving the plates of a charged capacitor to calculate energy density - where's the flaw in my argument?

For a charged air-dielectric capacitor, let the plates be parallel to the $xy$ plane, with the top carrying a positive charge $+Q$, the bottom a charge $-Q$. The force on an infinitesimal charge ...
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### Energy spent by motor in a period

Consider a single coil that is turning in a constant and uniform magnetic field thanks to a motor. The normal to the coil is given by: $${\bf u}(t)=\sin (\omega t){\bf u_x}+\cos(\omega t){\bf u_z}$$ ...
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### Proof of conservation of energy?

How is it proved to be always true? It's a fundamental principle in Physics, that is based on all of our currents observations of multiple systems in the universe, is it always true to all systems? ...
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### What are the forms of energy at fundamental level? [closed]

Most high school textbooks distinguish several forms of energy. Mechanical: -Kinetic -Potential Chemical Electromagnetic Nuclear Gravitational Thermal Hydraulic Electric Mass ... This ...
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### Is the total mass of the universe constant in time? [duplicate]

Is the total mass of the universe constant in time?
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In this paper on the Quantum Hall effect the authors refer to something called the correlation energy of electrons. It is defined at the top of page 5 as $E=\frac{n}{2}\int (g(r)-1)V(r)dA\ ,$ where $... 2answers 615 views ### How much faster is the fusion we make on earth compared to the fusion that happens in the sun? I have come across many websites that states that the proton-proton fusion which is the dominant type of fusion that powers the suns, is extremely slow and that is why the sun is still burning to that ... 1answer 80 views ### We create systems with different values of Energy? [closed] I understand that Energy is a conserved quantity in a system. A number, that's always the same to the system. However, don't we determine such a number? I mean, we can create systems(Or study them ... 1answer 118 views ### Can it require different amounts of energy to generate the same impulse? According to impulse principle the impulse is the same as the change in the object's momentum:$\bar I = \delta p$Because the momentum can be calculated like this:$\bar p = m\bar v$. If we solve ... 3answers 634 views ### Shouldn't the sign of generated entropy always be positive? I have a process where 10 g of liquid lead at 400 C is dropped into a water bath that is at 25 C. The lead solidifies over time and comes to thermal equilibrium with the water bath. The bath is so ... 0answers 516 views ### Making an equal amount of positive and negative energy? I was wondering if it would be possible to create an amount of positive energy out of a vacuum, in addition to an equal amount of negative energy, thus not violating the first law of thermodynamics, ... 1answer 47 views ### Propulsion Energy in Special Relativity I have a question regarding how to calculate the energy required to move objects within the frame of special relativity. To that extent, I understand the mass of an object as it approaches the speed ... 1answer 151 views ### How is the energy of a lightning calculated? I was wondering how is the energy of a lightning measured? Is it based on the luminous intensity and/or the generated sound? What about the generated heat and energy spent in the production of ozone? ... 0answers 59 views ### Kaon spontaneously splits into two pions (just need an confirmation) I just need a confirmation on a problem that I am dealing. In my problem I have a statioary kaon which spontaenously splits into two pions which are headed in different directions. Is it possible to ... 1answer 483 views ### A dielectric table is being inserted between a plate capacitor and$\triangle U<0$how to deduce the table is attracted to the plates? I am practicing for an exam in my Physics$2$course. One of a previews exam questions described a plate capacitor and asked to calculate the initial energy$U_{0}$, then a dielectric table was ... 0answers 55 views ### How do you determine the energy of an isotope? How do you determine the energy of an isotope? I there a calculation for it? Sorry for the confusion 2answers 138 views ### what is “work”? (of a force) I would like to know why two forces (of different magnitudes but same direction) that we'd use sequentially on an object to move it by the same amount twice, will produce two different values for the ... 2answers 569 views ### Elastic potential energy of compressed spring? [duplicate] If a compressed spring is dissolved in acid what happens to the elastic potential energy of the spring? 2answers 7k views ### Why can't we destroy energy? From a wikipedia article: In physics, the law of conservation of energy states that the total energy of an isolated system cannot change—it is said to be conserved over time. Energy can be neither ... 1answer 210 views ### Vacuum energy extraction: What happens to the cosmological constant? If the vacuum energy is extracted, then what will happen to the cosmological constant? Will it remain the same? . 8answers 21k views ### Where does gravity get its energy from? I would like to know where gravity gets its energy to attract physical bodies? I know that the law of conservation states that total energy of an isolated system cannot change. So gravity has to be ... 1answer 306 views ### Can magnetic energy be used to create a magnetic motor which can power vehicles and machines? [closed] Can magnetic energy be used to create a magnetic motor which can power vehicles and machines? 2answers 269 views ### How much more energy would I spend if my bicycle was 2 kilos heavier? [closed] I have a bicycle weighing 8 kilos. An 10 kilometer ride averaging 20 km/h requires z kWh's of energy. How much more energy would I spend if I added 2 kilos to my ... 3answers 7k views ### Work, Energy & Power - Body slides down a hemisphere A small body of mass$m$slides down from the top of a hemisphere of radius$r$. There is no friction between the surface of the block and the hemisphere. The height at which the body loses contact ... 0answers 66 views ### what are the main factors holding back magnetic energy storage systems? Perhaps is it the magnet's own magnetic field that causes it to lose its superconductive state? It seems as though these systems could become incredible energy dense. 1answer 226 views ### Does a 27 hp engine output the same amount of energy as lifting a 1 ton stone block almost 3 meters per second? I’m trying to get a sense of how much energy a 27 horsepower engine outputs. 27 hp$=$20 133 watts (joules/second). Potential energy can be calculated as$E = mgh$where$g = ~9.8\ m/s^2$on earth. ... 1answer 468 views ### How is the energy distributed in a proton-antiprotion annihilation? I know the products of the annihilation, but I don't know how much energy each particle has or gets. For example I know that 1876 MeV is released for each annihilation. Now, this energy is distributed ... 0answers 189 views ### What causes an electron to return from a higher orbit to a lower orbit Ultraviolet light can cause electrons from Hydrogen gas to jump from a lower energy orbit to a higher energy orbit. What causes the electrons to return from the higher energy orbit to the lower energy ... 3answers 1k views ### Energy needed for Superman to take off and fly at the speed of sound I just watched "Man of Steel", and I'm wondering if my logic is correct. Let's assume Superman is 80 kg. The energy required to take off from the rest to reach the speed of sound in air (if I neglect ... 2answers 4k views ### How to transform mechanical work into electrical energy without using piezoelectricity? can someone help me with the following issue. I need a method for transforming mechanical work into electrical energy without using piezoelectricity. I have such kind of mechanical forces (like on the ... 0answers 79 views ### Where does the relation$u=h_f-RT+(c_p-R)(T-T_0)$come from? I'm working on a college project for a subject called Motorcycle Engineering, and in this project I'm supposed to do some reverse engineering on a MATLAB program to model the engine of the motorcycle. ... 2answers 665 views ### What is the percentage of useful energy do we get from matter-antimatter annihilation? This is a theoretical question since we haven't made enough antimatter to try it in reality of course. But I am asking about the physics part in this. Also, by "useful energy" I mean the energy we ... 3answers 37k views ### Why is Energy = Voltage x Charge, and how to prove that? As you know the equation$\mathbf{E=V\times Q}$. Where:$\mathbf E$is the energy measured in joules,$\mathbf V$is potential difference (Voltage),$\mathbf Q$is the charge. So my qustion is: ... 3answers 3k views ### Can antimatter be used as fuel for nuclear reactors? I completely understand the difficulties of making and storing antimatter, so I am not talking about the mechanism or the way of doing it here, I am just talking about the concept. As far as I know, ... 2answers 1k views ### The Quantization of Photon Energies [duplicate] Despite Planck's constant being in$E=hf$, it would appear to me that energy is still not discrete, as frequency can be an fraction of a Hertz that one wants. How does this imply that electromagnetic ... 1answer 779 views ### Diagonalization of a hamiltonian for a quantum wire with proximity-induced superconductivity I'm trying to diagonalize the Hamiltonian for a 1D wire with proximity-induced superconductivity. In the case without a superconductor it's all fine. However, with a superconductor I don't get the ... 1answer 117 views ### energy efficiency, transformation from chemical to thermal energy, for district heating plants If I have a system in which I introduce 100e.u.F (energy units) derived from chemical energy (i.e. fuel) and generate thermal energy to heat water what are e.u. absorbed by water? I.e. the efficiency ... 0answers 26 views ### What are physical effects that could be employed to emulate this system? This is a simple system consisting of a tree of numbers such as ((1 2 (3 4)) (2 6) 1 6) and a rule of application, that states that a tree A applied to B is a copy ... 2answers 5k views ### The Energy Eigenvalue of a Wavefunction I've been reading an introduction to quantum mechanics online, and while constructing the Schrodinger equation for a free particle, the equation$i\hbar \frac{d \Psi}{dt}=\hbar\omega\Psi$is obtained. ... 3answers 5k views ### In nuclear fusion reaction, what is the percentage of mass converted to energy? I read somewhere that it is about one percent of the mass, but I find this too high. Also I have done some calculations, for example, the Tsar Bomba was 50 MT bomb and weighed about 27 tons. Although ... 0answers 60 views ### What highest amount of energy can we get from a laser? Can we get high energy from laser like fusion and fission reactions ? what is highest energy we can get from laser ? 2answers 287 views ### Can an Incandescent Bulb be considered as a heater? Due to an experiment, I need a small heater (around 70 to 100 watts). I intend to use an incandescent bulb so it can act as a heater. What I wonder here is will a 70 watts Incandescent Bulb be equal ... 1answer 1k views ### Can electromagnetic radiation (i.e. photons) produce gravity? [duplicate] I don't want to play with physical laws in a frivolous way. Assuming that the nature of matter and energy is the same, can a high density of highly energetic photons produce a gravity force? We do ... 3answers 1k views ### Connection between$\Delta x \Delta p \geq \frac{\hbar}{2}$and$\Delta E \Delta t \geq \frac{\hbar}{2}\$

Is there a way to derive second equation from the first one? I mean is there a connection between those two uncertainty relations? \begin{align} \Delta x \Delta p &\geq \frac{\hbar}{2}\\ \Delta ...