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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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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 ...
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22 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) (...
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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.
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20 views

Energy Functional vanishing to first order

I am trying to show the energy functional vanishes to first order: $$\int d^3r[\frac{\hbar^2}{2m}\nabla(\Psi+\delta\Psi)\cdot\nabla(\Psi+\delta\Psi)+V(\vec r)(\Psi+\delta\Psi)^2].$$ For the $\int\...
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2answers
29 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
47 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 ...
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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. ...
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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....
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0answers
42 views

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

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 ...
0
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1answer
35 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. ...
2
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1answer
46 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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2answers
30 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 ...
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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 ...
3
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3answers
178 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-...
0
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1answer
18 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 ...
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1answer
40 views

Work from string wrapped around cylinder

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 ...
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2answers
38 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 $...
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1answer
34 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
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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.) ...
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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
61 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 ...
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3answers
47 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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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
577 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?
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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
19 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
35 views

How do electrons gain energy in a cell?

When electrons reach the positive terminal, how does the cell do work on them? I’ve read in a book that states voltage is literally the push that the cell exerts on electrons, which thus does work on ...
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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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1answer
28 views

How much diesel fuel would you need to generate as much power as a kilogram of average nuclear fuel?

I'd like to be able to visualize the volumes associated with conventional fuels vs. nuclear fuels. Links to source material would be much appreciated.
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3answers
73 views

Is there a relation between work and energy?

I am in high school, and I know that work=forcedistancecos and i also know KE=0.5mv^2 and that PE=mgh and that ME=PE+KE , but my question is if there is a relation between work and mechanical energy(...
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1answer
22 views

Question about units in photonics calculation

My textbook has a photonics example problem, where the absorbed light energy density is calculated as $$\Psi = 25 \times 9.55 = 238.75 \text{mJ/cm$^3$} = 2.38 \times 10^5 \text{J/m}^3$$ I'm new to ...
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1answer
26 views

Thermistors and their resistance

I take GCSE physics and I'm having difficulty understanding how resistance and temperature link with one another. I've been taught that the greater a component's temperature, the greater the KE of its ...
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3answers
37 views

The order of excited states

The energy level of electron in an infinite square well in three dimensions is given by $E_{n_1 n_2 n_3} =\frac{ \hbar^2 \pi^2}{2mL^2}(n_1^2 + n_2^2 +n_3^2)$. It is understood that $E_{111}$ ...
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3answers
72 views

Equation of energy

We have $$E=E_K+E_P$$ What happens when $\frac{dE}{dt}=0$? Is the potential force conservative?
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50 views

Why there is $1/2$ in $KE=(1/2)mv^2$ and Etc [duplicate]

Hello I’m a high school student, my teacher asked the class for why and how there is a 1/2 in (KE=1/2mv^2)&(PE=1/2 k Δx^2)&(PE=1/2 c(Δv)^2)&(KE=1/2 I W). This is my first time on this ...
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2answers
71 views

The difference between optical phonons and acoustic phonons in terms of energy absorption

Why can the energy of a photon be directly absorbed by an optical phonon but not by an acoustic phonon?
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2answers
51 views

What happens energy-wise if you put a gyroscope at the north pole?

I had an idea for energy generation: put a gyroscope at the north or south pole connected to a dynamo or similar. The idea was that when the gyroscope resists the Earth's rotation, it would drive the ...
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3answers
227 views

Why are so many energies represented by $\frac{1}{2} ab^2$? [duplicate]

Why are so many energies in our universe mathematically represented by the equation $\frac{1}{2}ab^2$. For example: Kinetic energy $$\frac{1}{2}mv^2$$ Energy stored in a capacitor $$\frac{1}{2}CV^2$...
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0answers
27 views

Does the temperature of a rapidly moving macroscopic object higher than its stationary state? [duplicate]

I learned that the average kinetic energy has $$ E_k = \frac{3}{2}\ kT. $$ But if heat is in essence molecular motion, then is a rapidly moving ball different with a still one?
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2answers
55 views

Does the work-energy theorem assume that the particles of the system have time-independent masses?

$\newcommand{\force}{\mathbf{F}}$ $\newcommand{\velocity}{\mathbf{v}}$ $\newcommand{\position}{\mathbf{r}}$ Here's an incomplete proof of the work-energy theorem: $$W = \int_1^2\force \cdot d\...
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0answers
28 views

Why is negative energy allowed in classical systems despite the fact that negative energy density produces “negative spacetime curvature”? [duplicate]

How can a classical potential (gravitational or electrical) of a point charge be negative thus allowing negative net energy?
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1answer
65 views

Are energy and mass two forms of the same thing?

I know that energy can convert to mass and vice versa. So are we not looking at simply two forms of the same thing?
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1answer
112 views

Nernst Equation and Energy

I have a question regarding the Nernst equation and energy in a system. I know this is also a chemistry question, but I was interested in the thermodynamics (the energy of a system), so I decided to ...
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0answers
60 views

Can kinetic energy broken down into its components ? Even if it is scalar?

I'm confused while reading about degrees of freedom. According to what I read the number of degrees of freedom (DOF) of a monoatomic gas is 3 at low temperature. My question is: is that 1) because the ...
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0answers
32 views

Is it true that four-wheel drive is less efficient and if so, why? [migrated]

I have heard many times that four-wheel drive is less efficient than two-wheel drive. Many car producers have a normal drive mode where only two wheels are powered, and turn the four-wheel drive on ...
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0answers
57 views

Hamiltonian directly expressed in $(q,\dot{q})$ : how to find what is $p$?

I am reading a book about non relativistic quantization of E.M field. But first we do classical field theory. We directly wrote the Hamiltonian of our study, and a part of our Hamiltonian is the ...
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0answers
37 views

Imaginary energy integration

Is it possible to solve the integral: $$ \int_{-\infty}^{\infty} \frac{e^{ixs}}{k^2-s^2}ds $$ if it is known that $k^2$ has infinitesimal imaginary part (proportional to energy of a quantum ...
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0answers
24 views

Non relativistic Hamiltonian particle+EM field : where is the interacting part

I would like to properly understand how to find the Hamiltonian of a particle in a charged E.M field In my book, they say it is this one : $$H=\sum_a \frac{1}{2} m v_a^2(t) + \frac{\epsilon_0}{2} \...