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 energy and large-scale spacetime structure

I was reading an essay from Stephen Hawking's on the Space and Time warps and I was trying to make sense on some statements referring to the Casimir effect such as: The energy density of empty ...
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Impulse from absorbing a photon? Is there an increase in rest mass?

I'm going through A P French's special relativity. In one chapter (6) the following is set up: Suppose that a stationary particle of mass $M_0$ is struck by a photon of energy $Q$, which is ...
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Beta particle energy

what does it mean that a beta particle has a 1.6 MeV of energy , does it mean that it can ionize 120000 hydrogen particle with ionization energy of 13.6 MeV ?
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Where's the energy in a boosted capacitor?

Suppose I look at a parallel plate capacitor in its rest frame and calculate the electrostatic energy, $E$. Next, I look at the same capacitor in a primed frame boosted in the direction perpendicular ...
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How can you calculate (or convert) the Wh of a capacitor whose energy is given in Farads

When trying to compare the energy in a battery to the energy in a capacitor, the units don't match up. How can one compare a battery whose Ah are 10 and Voltage is 3 (for a total of 30 Wh) to a ...
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What's the difference between Capacitors, Ultra-Capacitors and Batteries

Capacitors are known to hold and release energy very quickly, unlike the slower release that batteries exhibit. If one were to bunch many (1000's of) capacitors together could they function as a ...
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Specific electron energy gap values $E_{i+1}-E_i$ vs. photons with arbitrary energy $\hbar \omega$

The energy levels of electrons in an atom are quantized $E_i$. A photon of a specific momentum $\vec p$ and energy $$\omega=(E_{i+1}-E_i)/\hbar$$ hits an atom and gets absorbed. Okay now say the ...
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How is energy transferred between atoms in a collision?

Consider two bare protons. One (A) is stationary (relative to some arbitrary massless observer); the other (B) is approaching A at 1 m/s. When they collide, I assume that they bounce. What is the ...
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324 views

How to guarantee that a kilogram of antimatter will quickly annihilate another kilogram of matter?

What I mean is, suppose we could somehow get a kilogram of matter and contain it safely. Now lets say we want to make a bomb using this kilogram, now, we have two ways, either store another kilogram ...
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668 views

Dynamics of a Vertical Mass-Spring Simple Harmonic Oscillator with Gravity

I am having some trouble obtaining the elastic potential energy and gravitational potential energy of a simple mass spring system. In this experiment, masses attached to a spring were dropped from a ...
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If human energy use were to increase by 2.3% per year, would we use all solar energy by 1400 years?

Assuming 2.3% per year exponential growth of human energy consumption (so, roughly speaking, that corresponds to multiplying consumption tenfold every century), it's argued that human annual energy ...
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Potential energy of a spring

I have a little problem with the potential energy of a spring... I hope you can help me! I have two coupled pendula, given by two masses $m$ fixed to two rigid bars (that haven't any mass) and with ...
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Kinetic energy with respect to different reference frames

I'm having problems understanding the following situation. Suppose two 1-tonne cars are going with the same orientations but opposite senses, each 50 km/h with respect to the road. Then the total ...
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Explanation of energy levels in molecules, atoms, nuclei and their relationship

Why are the energy levels of molecules, the atoms that form them and the nuclei inside the atoms considered separately? Or phrased in a different way- what is it that makes their energy levels so ...
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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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Why does a capacitor discharge a percentage of the original energy in the same time?

If I charge a capacitor ($220\mu{F}$) using a 6V battery, and then measure the time it takes to discharge 90% of the initial energy over a resistor (${100k}\Omega$), and then charge the same capacitor ...
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Potential energy in $E_f^2=(mc^2)^2+(pc)^2$?

Let's consider $$E_f^2=(mc^2)^2+(pc)^2$$ where the $mc^2$ is the rest energy due to the rest mass -- in Finnish "lepomassa". $$ \sqrt{(mc^2)^2+(pc)^2} - mc^2~=~(\gamma-1)mc^2$$ is the kinetic ...
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Can endergonic reactions occur outside of living organisms?

If the Gibbs free energy equation is defined as: ∆G = ∆H - T∆S And the amount of energy/work released from a reaction is: ...
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Is Keshe's technology for real?

http://www.youtube.com/watch?v=UrN99RELqwo (Video Title: "Keshe Foundation Promo Intro Video (english with multiple subtitles) === PLEASE SHARE ===") In this video they claim that they can create ...
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Lightning driven electrolysis

Could a lightning bolt be harnessed to perform electrolysis, would a useful amount of hydrogen be produced? I'm imagining some industrial scale device in the ocean with some hydrogen capture ...
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Do photons lose energy while travelling through space? Or why are planets closer to the sun warmer?

My train of thought was the following: The Earth orbiting the Sun is at times 5 million kilometers closer to it than others, but this is almost irrelevant to the seasons. Instead, the temperature ...
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Is the spin state of an atom related to the polarization of the photon it spontaneously emits?

From literature I've been reading, I find that scientists are able to "map" atomic states onto photon states. Are they talking about spin states and corresponding photon polarization states? Can ...
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free energy and entropy of 2D soap froth

This is a (exploratory) computational project. The soap froth was created by injecting bubbles into a chamber formed by two rectangular plates which are 0.16cm. From the moment the soap froth was ...
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Energy source of electrons?

I am aware that electrons are moving in an empty space so basically there is no friction to slow it down and its velocity stays the same. However where did the electron get its energy from in the ...
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Are physical probabilities also quantized?

In physics there is quanta and energy occurs per this unit. Is it it then reasonable that probability also is quantized since energy is?
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Understanding mathematically the free expansion process of an ideal gas

I'm trying to understand mathematically that for the free expansion of an ideal gas the internal energy $E$ just depends on temperature $T$ and not volume $V$. In the free expansion process the ...
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Elastic potential

I have a doubt: elastic potential energy is given by: $U=\frac{k}{2}x^2+K$ but does elastic potential exist? (for example: potential gravitational energy is given by $U=mgz+K$ and gravitational ...
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Can the vibrational energy of a engine be used to increase efficiency?

In whole mechanics, we never talk about wasted vibrational energy! As we see in our daily life, a lot of energy is wasted as vibrational energy in every engine, for example as we fire our car's ...
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Mechanics Question: Energy, Work and Power

I'm a pure mathematician by trade, and have been trying to teach myself A-level mechanics. (This is not homework, it is purely self-study.) I've been working through the exercises and have come up ...
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The building blocks of energy

I have a couple of related questions that have been bothering me for a while. They might sound unscientific, but here is goes: What are the building blocks of energy? What does energy consist of? Is ...
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How does rest mass become energy?

I know that there's a difference between relativistic rest mass. Relativistic mass is "acquired" when an object is moving at speeds comparable to the speed of light.Rest mass is the inherent mass that ...
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Mass-Energy Equivalency

We call $E=mc^2$ the Mass-Energy Equivalency because it equates mass and energy together. But, by that same logic, shouldn't we call $E=\frac{1}{2}(mv^2)$, the equation of kinetic energy in Newtonian ...
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Why does the nature always prefer low energy and maximum entropy?

Why does the nature always prefer low energy and maximum entropy? I've just learned electrostatics and I still have no idea why like charges repel each other. ...
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Ball Rolling in a Parabolic Bowl

I encountered a physics problem which inquired about a ball rolling inside a parabolic bowl (i.e. a bowl where any cross section through the vertex would make a parabolic shape given by $y = kx^2$). ...
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Amplitude of an electromagnetic wave containing a single photon

Given a light pulse in vacuum containing a single photon with an energy $E=h\nu$, what is the peak value of the electric / magnetic field?
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How efficient is the human body?

This question sort of comes to mind when hearing how efficient an internal combustion engine is turning chemical energy in mechanical energy (something like 20-40%) with lots of excess heat. As an ...
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Internal energy according to the van der Waals equation

I am trying to derive the internal energy of a gas which obeys the van der Waals equation. I have however encountered some problems. I calculate the integral of $dU$ from $V=0,T=0$ to $V=V, T=\infty$ ...
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Is $E=mc^2$ reserved to nuclear physics?

I was wondering, while putting a log in my fireplace, how much energy the piece of wood would give. The most famous formula poped into my head: $E=m \cdot c ^ 2$! Is this formula applicable to a ...
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Two expressions for expectation value of energy

I was looking up expectation value of energy for a free particle on the following webpage: http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/expect.html It says that $E=\frac{p^2}{2m}$ and ...
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What matter in the original atom bomb is converted to energy?

When an atom bomb goes off some matter is converted to energy according to $E = m c^2$. I'd like to know exactly what matter in the original atom bomb is converted to energy. Is it protons, neutrons, ...
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Relatively how much energy do fluorescent light tubes take to turn on?

Fluorescent lights are already efficient when they’re running but I’ve heard that it takes a lot of energy to turn a fluorescent light tube on. So is it more efficient to turn off a fluorescent tube ...
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Why is an electric motor more efficient at higher loads?

My question is driven by the plot below. We see that acceptable operating range of a motor is between 50-100% of the rated load. Below 40% or so the efficiency of the motor drops off dramatically. ...
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Not working brakes: just another energy conservation problem

A car is driving down a mountain ($v=90 km/h=25 m/s$, when the driver realizes that brakes aren't working. He try to lose velocity going up an inclined ($20°$) plane, with a friction coefficient of ...
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Is energy the ability to do work?

Here was my argument against this, the second law of thermodynamics, in effect says that, there is no heat engine that can take all of some energy that was transferred to it by heat and do work on ...
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Energy problem: What's wrong here?

A car ($m=540\,\text{kg}$) engine, has a power of $60\,\text{kW}$. The static friction coefficient between wheels and road is $k=0.6$. How long does it take to reach the speed of ...
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What kind of energy gravitates, and why?

When listing energies for the purposes of keeping track of conservation, or when writing down a Laplacian for a given system, we blithely intermix mass-energy, kinetic energy and potential energy; ...
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How to deduce E=(3/2)kT?

It says in my course notes that a particle has so-called "kinetic energy" $E=\frac{3}{2}kT=\frac{1}{2}mv^²$ Where does this formula come from? What is k?
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Ice skater increase of energy

This may be a very basic question but I am not seeing how it works. Consider the standard example of an ice skate rotating about his/her center of mass and pulling in his/her arms. The torque is zero ...
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What is the difference of work $W$ and thermal energy $Q$ in thermodynamic Stirling-process for ideal gas?

What is the difference of work $W$ and thermal energy $Q$ in thermodynamic Stirling-process (in simple form) for ideal gas? I think that you need work to preserve this process and you bring thermal ...
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Calculating partition function of ultra-relativistic 1D gas

This is a problem (Problem 3.16) from the book Statistical Mechanics 2nd Ed. by Pathria. In the problem I have to calculate the partition function of an ultra-relativistic 1D gas ($E_i=cp_i$) ...