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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Negative temperature thermodynamics

I asked this question about thermodynamics. I considered $$T=\frac{\partial U}{\partial S}$$ at constant $V$ and $N$ (number of particles), but now I can't figure out the change of $U$ with respect ...
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Lagrangian under time transformation

Given a Lagrangian $$L(q,\dot{q},t)=\sum_{ij}a_{ij}(q)\dot{q}_i\dot{q}_j-V(q_1,q_2,\cdots,q_f)$$show that under a time transformation $t=\lambda T$ ($\lambda$ = constant), the invariance of ...
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How are the CPU power and temperature caculated/estimated?

From Wikipedia The power consumed by a CPU, is approximately proportional to CPU frequency, and to the square of the CPU voltage: $$ P = C V^2 f $$ (where C is capacitance, f is ...
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Is conservation of energy a set of principles that is inevitable in any 'possible world'? [duplicate]

It is usually stated in various books or stories 'Energy can't be created or destroyed , but is conserved'. I guess this is within a closed system. Are there any axioms or principles in physics that ...
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Gravity and its energy

Where does gravity get its energy from? So, I'm taking a high school environmental science class and we just started a unit on energy. We were talking about different types of energy and one was ...
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How can I determine density of a gas only given temperature?

I have a homework problem where exhaust is traveling through an exhaust system (assumed to be air for simplicity) from and engine and then released into the atmosphere. The exhaust is at a temperature ...
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Is it true that a processor with a higher temperature uses inevitably more power in comparison with a processor with a lower temperature?

If a processor has a higher temperature, can you be sure it it consumes more power than a processor with a lower temperature? And is the reverse relationship true? A processorchip with a higher mean ...
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Is kinetic energy in QM a state-property or is it distributed?

Suppose we have a quantum mechanical system, which is well described by its wave function in r-representation $\Psi$. We are interested in the properties of an observable, say the kinetic energy $T$. ...
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2answers
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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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71 views

Thermodynamics - internal energy

What would the curve that describes the change of internal energy ($U$) with the volume ($V$) be like if the change in the temperature is negative? My try: For a ideal monoatomic gas, we have ...
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1answer
51 views

How is heat and temperature related? How will a thermometer react in the following conditions? [closed]

Consider the cooking stove and LPG fuel for example, all my questions will be related to it. Assume I have a frying pan made of steel which is kept on the the stove with the knob set to moderate. ...
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Why do we need to know the shape of the slide to find the time to slide down it?

In my physics book after this solved example: A child of mass $m$ is initially at rest on top of a water slide at height h = 8.5m above the bottom of the slide. Assuming that the slide is ...
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1answer
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Why is it possible to choose an arbitrary zero energy level when dealing with frequencies of a wave function?

This is a followup of my previous Why don't the De Broglie dispersion relation contain a constant term? question. Answerers pointed out that only differences in energy matter I can understand ...
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1answer
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Why speed of block/box reduce when force is acted?

In my physic book there is a question which is as follows;- A food shopper pushes a wood crate of cabbage heads of mass $m$ =14kg across a concrete floor with a constant horizontal force $\vec{F}$ ...
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1answer
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Changing Earth angle of revolution

As a follow-up to the two previous questions What is the scientific reason for the inclination of earth's rotation axis? and How much effort would be required to fix the Earth's rotation? ...
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Does the energy of ground and/or excited states have uncertainty?

In this question about absorption of continuous energies by discrete atom states, one of the reasons given to explain the width of spectral lines is the uncertainty principle (natural broadening): the ...
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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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3answers
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How is mass reduced in a normal chemical reaction which releases energy like heat, sound or light?

I was reading this link. Just above the paragraph titled "OTHER CONSERVATION LAWS", it says that "This conversion of mass to energy happens well below the speed of light, in a very small way, when a ...
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Energy and work

I don't quite understand the concept of energy and work. We can define energy as the ability to do work. An object moving at constant speed has kinetic energy. Does the object have the ability to do ...
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What's the symbol behind Einstein's head? [closed]

What's the symbol called that's behind and to the right of Einstein's head in this picture? Bonus if you can tell me what the whole formula is!
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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
89 views

Why is kinetic energy defined as $(1/2)m v^2$? [duplicate]

What is special about $(1/2)m v^2$ that makes physicists believe that it is a representation of kinetic energy?
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2answers
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Is the curvature of space around mass independent of gravity?

Is the curvature of space caused by the local density of the energy in that area?Could gravity be a separate phenomenon only arising from the curvature of space? For instance if the density of energy ...
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1answer
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Is the change in cochlear potentials dependent on perilymph velocity? [closed]

context: This question may appear to be off topic because it is about the biological particulars of the human ear. Physics can solve abstract models inspired by living organisms, but it is biology ...
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Conservation of energy and Doppler effect?

From what I understand, the frequency of light coming from the source moving towards an observer increases. From $ E=hv $ , this implies increase in energy of photon. What really is confusing, is ...
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a priori validity of $W=\int Fdx$ in relativity?

There are lots of different ways of arriving at the relativistic relations involving mass, energy, and momentum such as $E=mc^2$ and $m^2=E^2-p^2$ (the latter with $c=1$). One that I've seen in some ...
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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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How are the calories in food calculated?

This is intended to be a fun question. Calorimetry used for calculating the heat generated from chemical changes has been around for centuries, however, I suspect the process for calculating food ...
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1answer
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Work performed by a stationary object in a gravitational field “on Earth”

I was thinking about this problem: How much work is required to hold an object stationary in a gravitational field? or: How much energy is required to keep an object stationary in a ...
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4answers
358 views

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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Is energy content of a system different dependending on the observer?

For the sake of simplicity, let's imagine that the entire Universe is empty except for a single lump of (classical) matter with mass $m$. In its center of momentum frame, it is clear that the total ...
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What is the interpretation of pressure times volume as energy?

I know that pressure times volume has units of energy, but is there an intuitive explanation of how the pressure contributes to the total energy? It seems clear for ideal gases using the $PV=nRT$ ...
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Energy stored in hard disk [closed]

What is the type of energy stored in hard disk when data is stored? And Can we use it for electric power production?
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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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Calculating estimated HP from velocity, auto weight, and constant acceleration

I am working on a simulation program that runs theoretical performances of different cars and was wondering if there is a way to estimate the HP at any running RPM? the problem is (and this may be me ...
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Is the normal force a conservative force?

Most of the time the normal force doesn't do any work because it's perpendicular to the direction of motion but if it does do work, would it be conservative or non-conservative? For example, consider ...
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Is it possible for a moving body to have no kinetic energy? If yes, then how? [closed]

An object which is moving relative to your coordinate system has kinetic energy in that coordinate system; it does not have kinetic energy in a coordinate system which is stationary relative to the ...
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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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QFT and violation of Heisenberg uncertainty principle

In some QFT books is said that a free electron can emit a virtual photon as long as it reabsorbs the photon and returns to its original state within a time: $$\Delta t<\dfrac{\hbar}{2\Delta E}$$ ...
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Potential energy in Special Relativity

In Special Relativity, the energy of a free particle is $E^2=p^2c^2+m^2c^4$. But what would be the energy when there is potential energy? If it's something like $E=\sqrt{p^2c^2+m^2c^4}+U$, what ...
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Black holes and positive/negative-energy particles

I was reading Brian Greene's "Hidden Reality" and came to the part about Hawking Radiation. Quantum jitters that occur near the event horizon of a black hole, which create both positive-energy ...
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Force as gradient of scalar potential energy

My text book reads If a particle is acted upon by the forces which are conservative; that is, if the forces are derivable from a scalar potential energy function in manner $ F=-\nabla V $. I ...
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Catapult vs. Trebuchet

I have been looking at trebuchet designs lately, and I have noticed that most, if not all, have a sling attached to them. Without such a sling, the machine would be a catapult. In terms of the speed ...
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Conservation of kinetic energy in collision

Why is kinetic energy conserved in collision between glass balls while it is not conserved in collision between a ball and floor?
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Relation between linear momentum and translational kinetic energy

The momentum $m v$ of a particle is formally the same as the derivative its translational kinetic energy $\frac{1}{2} m v^2$ with respect to $v$. Similarly the angular momentum $I \omega$ is the ...
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What does the relation between mass and energy of a free particle mean?

What does the Hamiltonian for a free particle mean? Does it mean that the kinetic energy of the particle is in reverse relation with mass? $H$ or $E=\hbar^{2}k^{2}/2m$. Or better to ask: what's the ...
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Why does heat lose its energy as we get further away?

Why does heat lose its energy dramatically as I move back? Say I have a fire around 0.5 meters in front of me, I can clearly feel the heat, however, as I move even very slightly back, say 1 meter ...
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Energy Stored in a Capacitor with and without Dielectric

I have seen the equation $V = \frac {V_0}{K}$, but also the equation $V=\frac{1}{2}CV^2$. The values of C and V increase in the same linear ration with K (because $C=KC_0$). However, as the energy ...
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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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what property of steam makes it the preferable motive fluid in jet ejectors? why not air or nitrogen?

I want to know what properties of steam make it the fluid of choice in steam turbines jet ejectors. I want to understand the mechanism of energy conversion in these equipment and hence understand ...