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 express the interaction energy between two charged spheres?

Consider two identical insulating spheres each with radius $R$ and uniform charge $Q$ through their volume. They are separated from their centers by a distance of $d>2R$. Here is my general ...
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229 views

Wave Function of Particle in Nuclear Reaction

I was thinking and came up with the question of what happens to the wave function of a particle that decays into energy, say a neutron in a nuclear reaction. I know that conservation of probability ...
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227 views

How long should I leave the light on?

I remember my old physics teacher at school saying that if you switch a light on in a room and you're planning to return relatively quickly, it uses less electricity if you leave the light on until ...
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Energy transfers in boiling a liquid?

When you boil a liquid the KE stays the same (as temp is the same), so where does the extra energy go? I know the extra energy put in breaks bonds between the molecules/ atoms, but once it has broken ...
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1answer
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Solve a problem of work and energy

A man pulling sled of his daughter by a massless rope, climbing a snowy hill whose slope is equal to 15 °. Considering that the mass of the sled is $4Kg$, the girl's $26Kg$ and $\mu _c = 0,25$, ...
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A question about $E=pc$ for massless particles

Since photon has no (rest)mass and $$E^2=(pc)^2+(mc^2)^2$$ we derive that $E=pc$ for particle with no (rest)mass. However, if we transform the non-relativistic formula for kinetic energy ...
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1answer
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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
227 views

Ideal Gas Constant

Wikipedia states that the ideal gas constant relates the energy scale to the temperature scale. It serves as the constant of proportionality. This is obvious from the units. If temperature is a ...
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Why does the amount of energy transferred depend on distance rather than time?

The change in energy of an object can be determined by the work equation, where work is the change in energy: $$ W = F \cdot d $$ I conceptualize the transfer of energy as simply a series of small ...
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Can we generate infinite energy by successive fission and fusion reactions?

Fission divides one Helium atom into two Hydrogen (Deuterium) atoms. And fusion, once again, puts together those two Hydrogen atoms into one Helium atom. In both reactions, overall output energy is ...
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Entropy used to calculate energy?

I'm currently reading an online article, and below is a quote from that article: The thermodynamic entropy to change $n$ memory cells within $m$ states is $ΔS=k_B\ln(m^n)$, where $k_B$ is the ...
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281 views

Antimatter collision - Energy Released

I think it's about time for me to ask this question, as I've been contemplating this for a while. $$E = mc^2.$$ This is Einstein's most famous equation. But what does it mean? On my own, I had to ...
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If I switch on a generator connected to an open circuit, are the charges oscillating along wires?

If I switch on a generator connected to an open circuit, are the charges oscillating along wires? Where has the energy gone in the open circuit?
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Ball rolling into a bowl - where is its maximum KE (speed)… given there is friction. See diagram

Please examine this diagram and answer the apparently trivial questions. I am particularly interested in reasoned answers for part (a)(ii) - where is the maximum Kinetic energy? I say it is at B ...
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0answers
287 views

At what angle do billiard balls scatter if they collide off center?

The angle defined by joining a line from the centers of the balls must be important. But do they follow this angle when viewed in the rest frame of one of the balls or in the CM frame? The spheres ...
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Are Fresnel lenses widely used for solar electricity? If not, why not?

I was just wondering why Fresnel Lenses are not widely used in the production of solar electricity. Their use there would mean that you could produce heat within a fraction of a second, up to a few ...
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1answer
68 views

Minimum energy for turning over a cube [closed]

This is a strange question from the physics Olympiade I wish to get some explanation for: Using a certain type of wood, cubes are sawn with different edge lengths $R$. When they are on the ground, ...
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2answers
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Expression of kinetic energy in polar coordinates

Expression for kinetic energy in Cartesian coordinate: Expression for kinetic energy in polar coordinate (applying the transformation of coordinates): Why can't we express it in the following ...
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1answer
249 views

stimulated and spontaneous emission

I have a question regarding the dependence on energy density of the probability rate of emission/absorption while studying lasers in particular. It is given that,the probability rate of absorption as ...
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2answers
290 views

Why does a stationary force affect the conservation of momentum, but not the conservation of energy?

Let's say I have two positive charges approaching one another at the same speed with only their mutual forces acting on one another. Total momentum (= 0) and energy is conserved and the charges ...
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607 views

The time is a type of energy ?

You know energy is necessary for the occurrence of an event . Time is also exactly neccessary for the occurrence of an event. If there is no time , there will be no event. I mean time is a type of ...
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3answers
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Potential vs Kinetic Energy of Particles in Gas

"In the gas phase, the molecules are freely moving particles traveling through space, where the kinetic energy associated with each particle is greater than the potential energy of intermolecular ...
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3answers
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Simple harmonic oscillator: zero point energy?

Today we had a lecture on the simple harmonic oscillator and its quantum mechanical treatment. My teacher derived the equation for it and finally concluded it has some zero point energy. My ...
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1answer
447 views

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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0answers
190 views

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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3answers
680 views

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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2answers
309 views

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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1answer
69 views

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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583 views

“Correlation energy” using the pair correlation function

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 ...
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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 ...
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1answer
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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 ...
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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 ...
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3answers
615 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 ...
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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, ...
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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 ...
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1answer
144 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? ...
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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 ...
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1answer
478 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 ...
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53 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
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137 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 ...
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2answers
557 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?
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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 ...
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1answer
207 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? .
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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 ...
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1answer
303 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?
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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 ...