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.

learn more… | top users | synonyms

0
votes
3answers
2k views

How can there be heat in a vacuum?

I keep reading in the Physics World focus issue on vacuum technology about scientists creating high temperatures in the vacuums etc. If heat is caused by thermal energy being radiated from particles ...
2
votes
1answer
597 views

Winding Rubber Band

How could you store the kinetic energy of a soda-bottle using a rubber band so that it unwinds after being rolled across a flat surface? Basically, how can you make a bottle roll back after being ...
1
vote
0answers
51 views

Any examples of negative ADM energy solutions with WEC but not DEC satisfied?

Any examples of negative ADM energy solutions with weak energy condition (WEC) but not dominant energy condition (DEC) satisfied? Witten's proof of the positive energy theorem requires the dominant ...
1
vote
2answers
466 views

Maxwell-Boltzmann distribution and total energy per unit volume

We know that $$n(E) ~=~ \frac {2 \pi (N/V)}{(\pi k_B T)^{3/2}} E^{1/2} e^{-E/(k_B T)} dE,$$ where $V$ is total volume. If then, how do we derive total energy per unit volume from this equation?
0
votes
1answer
131 views

Why are there two ways to solve for energy of a spring?

I can find the energy of a spring using $F = -kx$, or by using the formula $e = 1/2mv^2 + 1/2I\omega^2 + mgh + 1/2kx^2$. The first way, I get $mg/k = x$, but the second way, I get $2mg/k = x$. Which ...
0
votes
2answers
497 views

Does the potential energy related to a particle determines its rest mass?

Would it be possible to determine the rest mass of a particle by computing the potential energy related to the presence (existence) of the particle, if this potential energy could be determined ...
2
votes
1answer
138 views

Where is the critical moment where the microcanonical ensemble enters the justification for the equilibium state?

As explained in many books, for the microscopic justification of the second law of thermodynamics (lets formulate it as the total entropy takes maximum among all possible exchanges of two systems), ...
7
votes
4answers
293 views

Is energy extensivity necessary in thermodynamics?

Given a partition of a system into two smaller systems, the energy $U$ is devided into $U_1$ and $U_2$, with $$U=\mathcal{P}(U_1,U_2):=U_1+U_2,$$ so that $U_2$ is given by $U-U_1$. Here the ...
3
votes
2answers
236 views

Inefficiency Comparison of Car Air Conditioning vs. Open Windows

On a recent long, hot journey in Spain, I was pondering which was the most efficient way of cooling the car. Which of these would be the most effective? Switching on the air conditioning, thereby ...
0
votes
2answers
183 views

What is the most energy-efficient way to crush the hardest bedrock on earth? [closed]

What is the most energy-efficient way to crush the hardest bedrock on earth while assuming it is impossible to use the chain reaction energy from that bedrock? How many energy is needed?
5
votes
1answer
1k views

The possibility of free electrical energy?

Please excuse my lack of knowledge/understanding. Question: Why Nikola Tesla's Free Energy concept was never worked upon? Even today. Context: Now that we know Nikola Tesla was a genius and did ...
0
votes
2answers
826 views

From where does the force/energy of action-reaction comes from? (Newton's Third Law of Motion)

I was wondering, from where does the opposite force and thus energy comes from when we apply force. For example, lets say there are two persons (P1 and P2) on the universe, and no force is applied to ...
2
votes
2answers
539 views

Is the potential energy in a compressed spring a Lorentz invariant?

The total energy of an object comes from the time part of the four-momentum, and so isn't a Lorentz invariant. On the other hand, is the potential energy of a compressed spring a Lorentz invariant?
0
votes
3answers
4k views

Relation between Newtons and Kilograms

Work is expressed as W=F*d, where the F is in Newton, d is in meters and result ...
0
votes
1answer
50 views

Which particle rolls faster? Is the speed of the particles which are rolling differs with their shape?

A solid disk and a solid sphere each of them has the same mass and same kinetic energies, which one is faster?
-1
votes
2answers
664 views

How we can find the kinetic energy? [closed]

A solid disk of mass m is rolling along a surface its center has velocity v what is the kinetic energy of disk?I cannot solve the problem,
3
votes
2answers
3k views

Are there devices which convert thermal energy to electric energy?

Converting optical energy to electric energy is a huge business based on the photovoltaic effect. Is there an analogous effect for phonons? Are ther devices which convert phonon energy to electric ...
0
votes
1answer
369 views

Faraday's law and superconductivity

According to Faraday's law of induction, volts = -Number of coils in a solenoid * change in strength of magnet / change in time. This doesn't take into account distance or speed, only time. If amps = ...
0
votes
0answers
684 views

Error Analysis, Change in momentum, Kinetic Energy

If momentum of an object is increased by 10%,then its kinetic energy will be increased by? Well, I don't need the solution logically by calculating the change in Kinetic Energy etc, but my question ...
-2
votes
2answers
99 views

Painting wine botles [closed]

Yes, it's a physics related question. Read on. I know from general knowledge that in order to produce a decent wine, you must keep it in a dark place. Therefore, from my knowledge of physics I guess ...
2
votes
1answer
212 views

Why don't cancelling wavefunctions for two different particles give zero total wavefunction?

Let $\left|a\right>=e^{i(kx-\omega t)}$, $\left|b\right>=-e^{i(kx-\omega t)}$ be two neutral particles in the 1D free space without any interaction. Then ...
8
votes
2answers
5k views

What is the relationship between Energy, Entropy, and Information?

What is the relationship between Energy, Entropy, and Information? I read this - What Is Energy? Where did it come from? - and the top answer says that 'energy' is an abstract number that is a ...
35
votes
7answers
17k views

Is a hard drive heavier when it is full?

Browsing Quora, I saw the following question with contradicting answers. For the highest voted answer: The bits are represented by certain orientations of magnetic fields which shouldn't have ...
2
votes
1answer
292 views

Man on treadmill and energy/mass conservation

Suppose we put a very obese man inside a large air tight isolated room with a treadmill and he runs on the treadmill for 24 hours and at the end of the day he sees his weight has come down (mostly ...
2
votes
2answers
106 views

Is there any correlation between the energy density fluctuations of two separate systems in a vacuum state?

I think the title says it all. What I am curious to find out is if there are any observable changes in the fluctuations of zero-point energy in a vacuum state system that are the consequence of ...
7
votes
5answers
21k views

What's the difference between Fermi Energy and Fermi Level?

I'm a bit confused about the difference between these two concepts. According to Wikipedia the Fermi energy and Fermi level are closely related concepts. From my understanding, the Fermi energy is the ...
1
vote
2answers
340 views

Are the higher-order terms in the series for energy really negligible?

To show that energy in special relativity reduces to $E=m+mv^2/2$ for low velocities, if we make a Taylor expansion of $m\gamma$ around $v=0$ we get $$E=m+mv^2/2+3mv^4/8+\cdots$$ But why can we cutoff ...
1
vote
0answers
58 views

Energy efficiency of antimatter producion [duplicate]

Possible Duplicate: Matter - Antimatter Reactory Practicality Certain reactions in particle accelerators lead to production of antimatter, which can then be collected in noticeable ...
18
votes
2answers
3k views

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 ...
0
votes
1answer
134 views

Do you think it’s possible to make a power plant of the described kind?

We are working on a science project and try to engineer a power plant of a new kind. It is called Air HES (air hydroelectric station). The idea is described on our website. Do you think it is ...
4
votes
1answer
629 views

Wind Turbine Impact

OK, I'm not sure this is not a silly question, but here goes. I've often wondered what the impact is of wind turbines on the weather. My question is, since wind turbines transform wind energy into ...
1
vote
1answer
584 views

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 ...
3
votes
1answer
247 views

Hamiltonians and Lagrangians, Euclidean and Hyperbolic: Are they related?

The Lagrangian of a system is the difference between its kinetic energy $T$ and potential energy $V$, and is relativistically invariant: $L = T - V$ The Hamiltonian of the same system is the sum ...
3
votes
2answers
2k views

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 ...
1
vote
0answers
334 views

Slackline Jump Tension

So a slackline is basically a bouncy tight rope. I found a site that has a calculator for the tension of a static slackline ...
5
votes
2answers
3k views

Amount of energy required to hover.

I've noticed a motionless kingfisher over a lake looking for prey and wondered what amount of energy does a bird, weighing 0.15kg, require to hover for 15s?
1
vote
2answers
334 views

Why is “space-time” not fully explained as the observational effect of the casimir effect seen from a human perspective?

Simply stated the casimir effect can be observed bringing two plates together microscopically and in vaccum Is this analogous to the observation of gravity? Question: What is casimir effect? ...
1
vote
3answers
135 views

Carrying water on person, or on the frame when bicycling

So, the question is as follows: What is the difference in work exerted by the rider in the two following scenarios? a) Rider + bike. Water carried in a holder on the frame b) Rider + bike. Water ...
2
votes
1answer
161 views

How is/do they plan on having fusion energy collected at the National Ignition Facility?

I was just watching a program on television showing how the method they use at the National Ignition Facility to attempt to achieve fusion is to fire extremely powerful lasers at a small pellet of ...
0
votes
2answers
226 views

Calculating kinetic energy?

Would this be a valid equation to calculate kinetic energy created from a drop from a height: $$E_{kinetic} ~=~ v_{vertical}tmg$$ Velocity multiplied by time gives distance. Distance multiplied by ...
0
votes
2answers
189 views

Does using batteries in a series raise the overall Wh's of the batteries used? Why/Why Not?

If one has 2 batteries of 6 Volts each with 500 mAh each, then separately the batteries should provide 6 Volts * 500 mAh = 3 Wh each or 6 Wh total. However would it be the case that if someone where ...
-1
votes
3answers
932 views

From knowing just the change in kinetic energy, can we find the friction force and engine power?

I understand this topic well enough to get all the task done because they aren't very creative. But for my exam I think I should have this clear. During the acceleration the force from the engine is ...
0
votes
1answer
250 views

Decay of particle (for example $\pi^+$); Find energy [closed]

I don't understand how to solve this: A $\pi^+$ decays into a muon and neutrino. Find the pion's energy if max $E_\nu$ / min $E_\nu$ = 100/1; $m_\nu = 0$ $m_\pi*c^2 = 140\text{ peta-eV}$
1
vote
1answer
187 views

Is energy “destroyed” when walking?

Conservation of energy states energy can't be destroyed, but isn't energy used up when walking in a straight line? If your not walking up a slope, kinetic energy isn't converted to gravitational ...
4
votes
2answers
898 views

I have a slight problem understanding the concept of “work”?

What I understand is that work is not the same as a car using gas or a crane lifting a car high up into the air. Let's use the crane as an example. And let me write out a few lines from the book. ...
0
votes
1answer
115 views

Is it safe for astronaut to do this “dangerous” movement?

So, if the astronaut is working outside the space station in thew space, could he not tie himself onto the space station and if he is going further away of the space station, i think he could just ...
2
votes
2answers
2k views

What's the relationship between an object's color and it's energy?

All else held equal, which has more energy: a violet-painted teacup or a red-painted teacup? I know that violet light has more energy than red light, but does this mean that the inverse holds true ...
1
vote
1answer
248 views

Could ions emitted by an ion thruster represent any realistic danger?

As ion thurster designs improve, the ions emitted could have a velocity (relative to the spacecraft) of well above 10^5 or even 10^6 m/s. It the likelihood of any such ions ever hitting a human ...
2
votes
1answer
2k views

What does the Fermi Energy really signify in a Semiconductor?

In understanding the behavior of semiconductors, I'm coming across a description of the Fermi Energy here and at Wikipedia's page (Fermi Energy, Fermi Level). If I understand correctly, the Fermi ...
4
votes
4answers
227 views

Why does it require such little energy to create the fastest thing in the universe?

I have noticed when I turn on the light switch in my house light comes from the bulb. How is this light created?(process occurring in the bulb) and why is this small amount of electricity enough to ...