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Questions tagged [energy]

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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+250

Theoretical physics for better batteries?

It is Earth day, so I started thinking about the theoretical physics problems that could help reduce greenhouse-gas emissions and fight climate change. We actually have a reasonable range of ways to ...
3
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4answers
2k views

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 (as ...
23
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7answers
6k views

Why is there a $\frac 1 2$ in $\frac 1 2 mv^2$?

For elastic collisions of n particles, we know that momentum in the three orthogonal directions are independently conserved:$$ \frac{d}{dt}\sum\limits_i^n m_iv_{ij} =0,\quad j=1,2,3$$ From this, it ...
2
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1answer
42 views

Energy transfer and Negative work

Is energy always transferred from the source to the object when positive work is done by the source on the object ? If so, what energy is transferred from the earth to a freely falling body ? The ...
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2answers
585 views

Internal energy of a gas inside a moving box

Let there be a box filled with an ideal gas. Now apply force on it. Consider three cases: Acceleration from rest. Deceleration from some velocity. No acceleration at all. How will the internal ...
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1answer
57 views

what limitations stop a person from installing solar panels on his/her house? [on hold]

Solar energy is a great form of energy. sourced from the sun, it can save a great deal of electricity bill cost. My question is why doesn't everyone uses it and install panels on their roof. Of ...
4
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1answer
44 views

Why is the kinetic energy a fixpoint of the Legendre transformation?

Question: Why is (from an intuitive standpoint) the kinetic energy $T$ a fixpoint of the Legendre transformation, i.e. $\frac{\partial T}{\partial \dot q}\dot q-T = T$ for any general coordinate $q$? ...
-1
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1answer
92 views

Anatomy of combustion

I don't understand how energy is released in combustion. We have a reaction of a combustible material with oxygen and this releases energy. In what form is this energy released? It should be some ...
2
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2answers
289 views

Gravitational Binding Energy

Lets define the Gravitational Binding Energy of a set of point masses as the work required to separate all point masses by an infinite amount. First Question: Is the amount of energy required to do ...
0
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1answer
15 views

Why are number changing interactions inelastic?

Inelastic processes are those in which the kinetic energy of the initial particles and final particles are not the same. Is this true also for number changing processes such as pair creation or pair ...
1
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2answers
44 views

Change in Energy when placing an object on the ground

It seems like a simple question but I was wondering where does the energy go when I place an object from a height on the floor. Initially it's all stored as potential energy, and as I'm moving the ...
2
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2answers
34 views

In a NVE ensemble, can a particle access a state with a higher energy than the constraint?

I should start with saying that I know the answer is obviously no but I am trying to make sense of it mathematically. Let's say, for example, that each particle of a system with total energy $E$ in a ...
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1answer
51 views

If you could bottle a photon would it have mass? [duplicate]

Ok. A photon has no rest mass for the good reason that it has no rest energy. But what about a standing wave? There is fair belief that particles are standing waves, so perhaps nothing rests. So. ...
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0answers
23 views

If one if the masses is released [on hold]

Figure shows 4 identical masses of mass m , arranged on a cube as shown If one of the masses is released (A) it will collide with one of the masses (B) it execute an oscillatory motion (not Shm) (...
-1
votes
0answers
26 views

Will it be possible to generate electricity from a nuclear fusion reactor within the next 50 years? [on hold]

Will it be possible to generate electricity from a nuclear fusion reactor within the next 50 years? Please leave all supporting evidence for your answers as well.
0
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1answer
51 views

Net work done in lifting

(g is assumed as 10 $ms^{-2}$) We lift a stationary object of mass, $m$, to a height $s$, ensuring it has got a resultant upward acceleration of 2 $ms^{-2}$. The required force to be applied by us ...
0
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4answers
204 views

Electron in free space and Schrodinger's equation

Topic: Quantum Theory of Solids I have come across the following text: Electron in Free Space: Consider the motion of an electron in free space. If there is no force acting on the particle, ...
0
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1answer
20 views

Formula manipulation [on hold]

What is the potential energy of a metric ton (1000 kilograms), 3 meters above Earth's surface (at sea level), expressed in Kilowatt Hours? Assume that the mass will end travel when it contacts Earth. ...
0
votes
1answer
25 views

Why is the actual KE value smaller than what I calculated?

Problem The nuclear equation for the decay of calcium-47 into scandium-47 is given by: The following data are available: Mass of calcium-47 nucleus = 46.95455 u Mass of scandium-47 nucleus = 46....
2
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1answer
48 views

What is the information concept and how can it be measured,…?

According to the Wikipedia, ""Information of an entity is that which resolves the uncertainty of its properties"" source: Wikipedia, physical information Information contained in a physical ...
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1answer
657 views

What is the effect of volume on liquid temperature during evaporative cooling?

Imagine two evaporative cooling experiments: In the first, a beaker contains a known volume of liquid. The liquid starts at ambient temperature. The temperatue of the body of liquid remaining in the ...
-1
votes
0answers
44 views

Can the laws of conservation of energy and momentum be thought of as projections of matter wave onto the time and spatial axes? [closed]

I noticed that when we think of traveling or stationary particles (or objects) as lines on the ($x$, $t$) diagram, so that the velocity $v$ of a particle defines the slope, and assume they have ...
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0answers
21 views

Intensity, power and efficiency question help [on hold]

The solar Intensity in the Netherlands is $1.0e3\ W/m^2$ at a maximum sunshine and a perpendicular incidence. On average, about $10\%$ of this peak power can be utilized. The efficiency of the ...
0
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2answers
31 views

What is Work Done against the Atmosphere [on hold]

{Assume it's a Spherical Balloon} The answer on the book states (7pV), but I'm definitely missing something. I've looked up online on what 'Workdone against atmosphere' means but there is little to ...
3
votes
3answers
183 views

Why is there Net Work Done on a Pressure/Volume Cycle?

The area enclosed by the rectangle is said to be the Net Work Done. My question is why would there be any net workdone if it returns to its initial position (W). Shouldn't it act like a force-...
1
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2answers
424 views

Definition of internal energy

I have read some thermodynamical and statistical physics books but it feels like they always gloss over the definition of internal energy. In wikipedia they included terms like Kinetic energy. But ...
0
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1answer
19 views

Why does a drop in gravitational energy result in star contraction

Consider a gas cloud that is radiating away heat. From virial theorem we see that if the cloud radiates away heat and loses energy this will cause its gravitational energy to drop. Why does this mean ...
8
votes
1answer
323 views

Is there an equivalence between information, energy and matter?

Consider the physical world as made of information, with energy and matter as incidentals. This statement must be placed in the context of the holographic principle and the works of Bekenstein ... ...
1
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1answer
41 views

Work from string wrapped around cylinder [on hold]

A cylinder, on a level surface angularly accelerates as a string wrapped around is put under tension. Because this string causes the cylinder to roll without slipping, the tension force cannot to any ...
1
vote
2answers
39 views

Why do we define Energy density

So we define the electrical energy density $(\bar U)$ between a parallel plate capacitor as $$ (\bar U) = \frac{1}{2} \epsilon_0 E^2$$ where $E$ is the electric field between the plates usually given ...
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0answers
18 views

Composite objects, mass, and internal energy in relativity

Why do we define the free Hamiltonian of composite particles as $$H_0 = \sqrt{M(t)^2c^4 + P^2c^2}$$ with $M(t)\neq\sum_im_i$ instead $$H_0 = \sqrt{M^2c^4 + P^2c^2} + U(t)$$ with $M=\sum_im_i$ and $...
2
votes
1answer
453 views

Electron energy from beta decay

I read in my IB-physics book that the average energy for an electron in the beta decay of Potassium-40 is 0.44 MeV. However this would imply the electron have a velocity of 3.9E8 m/s, i.e. faster than ...
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0answers
519 views

What is the link between free energy and lagrangian?

Free energy is a generalization of energy when the system exchanges heat with the environment. Energy, in its turn, can be extracted from lagrangian under the symmetry of time $(\frac{\partial L}{\...
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1answer
35 views

Kinetic energy of a yoyo

Consider the yoyo above: The yoyo is constructed from two heavy disks of radius R connected by a light axle of radius r , as shown in the figure below. The total mass of the yoyo is M and its moment ...
2
votes
1answer
42 views

contradiction between total photon energy in the universe and early matter antimatter ratio

When looking at the energy count in the Universe, on one hand we are told dark energy 68%, dark matter 27%, regular matter 5%, neutrinos and photons a very small fraction (trace like). https://en....
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0answers
34 views

Calculating the work done on a rotating cylinder by friction [closed]

There's a cylinder on top of a incline with constant friction, and it both rolls and slides down when let go of. How would I calculate the rotational work and the negative friction (heat, sound, etc.) ...
3
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2answers
1k views

What was the initial source of energy?

According to law of conservation of energy; "Energy can neither be created nor be destroyed but may change it's form" Where did the energy come from which led to the creation of universe by big bang?
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4answers
71 views

Mechanical Energy Concept Problem

When an object is at rest on a smooth horizontal "surface" (floor) its potential energy $P_E$ equals zero and, as the object is at rest, its kinetic energy $K_E$ is also zero. When the same object is ...
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1answer
62 views

What exactly makes negative energy negative?

Antimatter is the opposite of matter since it has opposite electric charge (e.g. proton -> positive whilst anti-proton -> negative). So what exactly makes negative energy negative? What property does ...
2
votes
1answer
893 views

What is the inverse of Energy?

Is there a name for the inverse of Energy $1/E$ or the inverse of the Joule $1/Joule$? It comes up if you divide the mass-energy relation by energy (so you get a constant of 1).
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2answers
367 views

Gravitational Potential Energy and Zero Points

I have two questions. 1) The first one has to do with the formula for deriving Gravitational Potential Energy. I learned that, for the derivation of Gravitational Potential Energy given large ...
3
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1answer
547 views

Entropy: Disorder or energy dispersal?

The first definition of entropy given by Clausius is I believe this $$S=Q/T$$ It is as I understand a common fact to understand entropy and maybe often teach it as a measure of disorder through the ...
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3answers
48 views

In a collision, what causes solids to break?

example: A car crashes into a wall, breaking the wall into pieces at collision. Was it the transfer of of KE from the car to the wall particles that did this? Or the force exerted onto the wall by ...
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2answers
414 views

What is an isolated system in Newtonian mechanics?

What is an isolated system in Newtonian mechanics? Is it a system on which the net external forces and torques are $0$? Consider a man lifting a heavy suitcase from the floor and placing it on an ...
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1answer
39 views

What is the energy of one photon radiation? (Probably super basic for you)

So, this is given question: The radio station emits radiation power of 6 kW, wavelength 3 m. a) What is the energy of one photon radiation? b) How many photons are emitted in 5s? I tried ...
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0answers
37 views

1st law of thermodynamics

If gravity fields create pressure at their center which creates thermal energy, then isn't a gravity field creating energy that otherwise wouldn't be there (breaking the first law of thermodynamics).
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3answers
581 views

Why use ultrasound for medical imaging? [closed]

What advantage does ultrasound have over sound between 20-20000Hz that it is used in medical imaging over sound in that frequency range?
3
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1answer
102 views

Is making nuclear bomb using natural uranium possible?

I know that the critical size of natural uranium metal with graphite moderator is about 27 tons "oak ridge graphite reactor". for comparison highly enriched uranium have only 17 kg critical size with ...
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0answers
20 views

power consumption of vacuum pump

I am using the vacuum pump from KNF Neuberger Inc to create a reduced pressure environment for the dryer. I want to calculate the energy consumption of this pump. The pump is removing nitrogen gas ...
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1answer
591 views

How is internal energy related to degree of freedom?

I have a doubt in understanding the concept of internal energy. The internal energy of a system is equal to U=f/2nrT, where T is the temperature. Now, also we know that dU=Q-W Then if heat is zero, ...