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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What are simple comparisons of energy draws?

I have recently attended a Bitcoin mining conference. Some of the attendees were specialists in energy with their business being oriented around building data centres or catering to the needs of data ...
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Self-propelling water bottle [on hold]

The water bottle appears to propel itself. Note that the bottle did not hit the roof of the car, or did not contact the car in any manner. Why is the bottle doing this? https://youtu.be/oZYfyknvtxo
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Does linear momentum of a hydrogen atom-system remain constant during electronic transitions?

When electronic transitions(excited to ground) take place in a hydrogen atom, does the linear momentum of proton-electron system remain constant?
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Proof that $v=\frac{dE}{dp}$ at relativistic speeds?

This is a formula I have seen stated in two different places in a relativistic context, but I do not know the proof and seem unable to derive it on my own. $E$ is the energy of a free particle, $v$ is ...
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1answer
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Point with 0 electric field and non-zero potential

I'm quite new to physics and I don't totally understand the concepts of potential energy and work. I've been watching MIT courses and I got pretty confused, considering the following situation: We ...
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1answer
34 views

Force Applied but No Distance Travelled

Suppose I push on a wall with a constant force of 5 N for 10 s. The wall won't move and hence no work will be done on the wall. However, pushing requires energy. How can I find out how much energy I ...
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What quantity remains conserved in a calorimeter when two bodies at different temperatures are mixed?

This is the question: Two identical bodies at different temperatures are mixed in a calorimeter. Which of the following quantities remains conserved? 1) Sum of the temperatures of the two bodies 2) ...
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4answers
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The potential at a point

According to my book, 'The potential at a point is said to be 1 volt when 1 joule of work is done in bringing 1 coulomb charge from infinity to that point.' But I wonder how it is possible. As the ...
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Transferring Kinetic Energy - a worldbuilding physics question

I'm working on an RPG for some friends and I'm curious about a magic system I'm thinking of designing and how it would interact with physics. Please bear with me as I'm neither mathsy nor a physicist. ...
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Interpretation of equal absolute values of pressure and tension in the electric or magnetic field along Cartesian axes aligned with the field

For an electric or magnetic field along the $x$ axis, the stress-energy tensor in mixed covariant-contravariant form, in $(t,x,y,z)$ coordinates, is of the form $\operatorname{diag}(1,1,-1,-1)$ (...
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Is it Valid to Derive $E = pc$ From the Energy-Momentum Relationship for Photons?

Given a particle with mass $m$ moving at velocity $v$, total energy is: $$E^2 = (pc)^2 + (mc^2)^2$$ Note I am not using the relativistic - rest mass convention, as I was taught to think in ...
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The equilibrium era and energy density

In Weinberg's 'Cosmology', at page 109, he presents an era "of equilibrium" when "the temperature and density were sufficiently high so that radiation and matter were in thermal equilibrium. Then, ...
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Power of conveyor belt required to accelerate a mass

Assume the following scenario: A conveyor belt is driven at velocity v by a motor. Sand drops vertically onto the conveyor belt at a rate of $m$ kg/s. What is the power required to move the belt at ...
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Wind energy as a function of speed and temperature

My fellow windsurfers, most of whom have little knowledge of Physics, all agree that the force of the wind when sailing in the Northeast of Brazil, where the weather is warm, is much less than where ...
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What is the average energy of visible light photons, of what we perceive to be white light? (i.e. visible spectrum of 5500K radiation)

Visible light is anything between 1.65eV and 3.10eV, so the answer is somewhere between these values. Naively we could average this to 2.375 eV, but that would not be actual average of full spectrum ...
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Trouble with conceptualizing the “temperature” of a deck of cards

Entropy is quite often introduced with the help of a deck of cards. Because there are so many cards, and thus an enormous number of microstates (orderings of the cards), if we assume equipartition of "...
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Does a normal reaction do work too? [duplicate]

On backflipping off a wall, the wall applies an equal and opposite force that pushes the person in the opposite direction of the applied force. Or, on simply pushing a wall, the wall pushes back. Does ...
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Why is work equal to force times displacement?

This is how I think of what work is.I am sure I am wrong somewhere because I shouldn't be coming to the conclusion that I am coming to.It would be helpful if you would point out where this conceptual ...
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Invariant quantities? [duplicate]

Every phisical quantity is tensor quantity (special cases of tensors are vectors and scalars). There are transformation rules for tensors. For example for scalar quantity F transformation rule is F'(x'...
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What's the difference between using virial theorem and total energy?

What the difference between using $$E_{tot} = E_{grav} + U $$ and $$<T> = - \frac{1}{2} <U>$$
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Confusion about internal energy and temperature during a phase change

During a phase change, internal energy of a system will change because energy must be added or subtracted (depending on "direction" of change) from the system in the form of latent heat. However, the ...
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Derive canonical momentum in electromagnetic field without knowledge of Lagrangian and Hamiltonian mechanics

One of my friend is studying quantum mechanics and he asks me a lot. This time, he asked me about the momentum expression in Hamiltonian of charged particle in electromagnetic field. $$H = \frac{1}{...
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1answer
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Kinetic Energy in a non-inertial frame

This is the question I was solving: A ball of mass m is released from A inside a smooth wedge of mass m as shown in figure. What is the speed of the wedge when the ball reaches point B? This was ...
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2answers
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How does $p\cdot u$ relate to observed energy and momentum for a massive particle?

Gravitation by Charles W. Misner, Kip Throne and John Wheeler page 65 Exercise 2.5. The book defined "energy" for a photon $E=-\mathbf p\cdot \mathbf u$ for subsection 2.8, which later explained as a ...
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Would a graviton if it exists have energy?

If it has energy and comes from all matter would then matter lose mass as it releases these gravitons that have energy given that energy and mass can be converted to each other?
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How to get equations of state by differentiating a thermodynamic potential?

For an ideal gas in a closed system, the thermodynamic potential U, the internal energy, is given by $$ U(V,S) = U_0 \left( \frac{V_0}{V} \right)^{2/3} \text{e}^{\frac{S-S_0}{C_V}},\quad C_V = \frac{...
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How is energy conserved in terms of “Work”

Basic equation of work is given by $F\cdot s$. When work is done, the energy is stored either in form of potential or kinetic. My question arises when we look at a case of applying $m g$ of force ...
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Why does PV work consider total pressure instead of net pressure?

In physics, we define work as: $$W = F_{net}×d$$ However, in physical chemistry, PV work is calculated as: $$W = P×ΔV$$ Why is total pressure used instead of net pressure? For example, if a gas is ...
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Showing maximum entropy - minimum energy principles equivalence

I'm reading H. Callen's book on Thermodynamics and I don' t understand his proof that the maximum entropy principle implies the minimum energy principle. As a reminder (chapter 5), Entropy maximum ...
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Rolling ball while winding a tape [closed]

I've been trying to solve this for a hours but i can't build the equations related problem involving this question, could someone help me? a ball of mass $m$ and radius $r$ rolls down without sliding ...
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1answer
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Eigenfunctions in Spherically Symmetric Well

I am looking at a problem that has a potential $$ V(r) = \begin{cases} 0 & a<r<b\\ \infty & \text{elsewhere} \end{cases} $$ This is a modification of the infinite spherical well ...
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Circular motion of electrons around nucleus [duplicate]

We know that electrons are revolving around the nucleus because of the electrostatic force .But if the electron is moving in circular path then as acceleration has to be in inward direction due to a ...
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1answer
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Finding height and speed [closed]

A small puck of mass m glides along a smooth horizontal table at a speed v0 to a shiftable slide of mass 3m that is at rest, the puck up the slide with no friction and then glides down the slope in ...
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Pressure waves for sounds [closed]

Is it possible to mechanically create a pressure wave, with right amplitude, frequency, and other characteristics to recreate known sounds like train horns, chime bells, gun shot, etc. ?
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Calculating Simple Ground State Energy Of Hydrogen From Geminal Spin Function

I'm very new to quantum stuff and I'm experimenting, but I'd like a good explanation here if you can give it. Here's the circuit diagram and the results. I'm seeing from this paper that you can get ...
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3answers
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Energy loss in sticky collisions

A typical problem of sticky collisions involves an object colliding with an object at rest on frictionless surface, and the two move together. If conservation of momentum is applied we get $ v'=\frac{...
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Is every kind of energy quantised?

I know this is been asked on this site before but I felt unsatisfied by its one answer. I’m familiar with the concept of quantisation, I keep reading that “energy is quantised” but what do they mean ...
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1answer
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Does $E=mc^2$ apply to electron shells with different energies? [duplicate]

Is this statement true: Each electron shell has a different energy level, with those shells closest to the nucleus being lower in energy than those farther from the nucleus. Mass of electron $= 9....
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1answer
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Overall pressure and movement in Earth making heat because of gravity

There are three sources of heat in the Earth's core, impacts of the planet when it was created (impacts made heat), radioactive decay of elements, and the extreme amounts of pressure and friction ...
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Gravity creates heat?

There are three sources of heat in the Earth's core, impacts of the planet when it was created (impacts made heat), radioactive decay of elements, and the extreme amounts of pressure and friction ...
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1answer
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Electric Potential Outside of Two Infinite Sheets of Opposite charge density

If you have two infinite sheets of equal but opposite charge density, the electric field outside the sheets will be zero due to cancellation. This led me to initially think that the potential would ...
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1answer
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Computing total derivative of Kinetic Energy w.r.t time

I am confused as to how to take the total derivative $\frac{dKE}{dt}$, where $KE$ is the kinetic energy. I know that $KE = 1/2 *m * \dot{\vec r} \cdot \dot{\vec r}$. From here, if I take derivative ...
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force by an air particle when hitting another air particle

I just had this really weird question. On average, how much energy is lost, when one air molecules hits another air molecule? By air molecule I mean one molecule of the gases in the atmosphere. On ...
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Temperature relation to Gibbs free energy

The equation to calculate gibbs free energy includes the absolute temperature of the system. However isn't it the energy needed to raise temperature independent of the initial temperature? Why does ...
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1answer
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Energy or a piecewise defined wave function in an Infinite Square well

So I'm asked to determine the most probable measured energy value of a particle in an infinite square well with wave function $$\psi(x)=\begin{cases} Ax, & 0< x<\frac{a}{4} \\[1em] ...
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1answer
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Mass and energy of radiation

We know that mass can be  converted to energy and vice versa. So does the mass of molecules or atoms change when they absorb a radiation and go to the higher energy state?
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What is the proof for the relativistic expression for kinetic energy?

In his book on relativity, Albert Einstein states in chapter 15 that we now don't write kinetic energy as $K_e = \frac{1}{2}mv^2$ but as $$K_e = \frac{mc^2}{\sqrt{1- v^2/c^2}}-mc^2.$$ How is this ...
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Why does the interaction energy for the electric fields of two point charges vanish over any shell smaller than their separation?

A previous question, A triple integral in Spherical coordinates from Jackson's book on Electrodynamics, presents the fairly routine calculation of the interaction energy encoded in the cross-term ...
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How to calculate the energy of a mechanically produced wave?

I have a situation in mind, where at a number of points there is placed a material that positively strains in response to a stimuli, like pictured below, This is to produce a Rayleigh or surface ...
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2answers
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Infinite square well discrete energies meaning

In the problem of infinite square well we come out with quantized energies that an electron can have. And each energy level has its own wave function. The general solution is a linear combination of ...