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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21 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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71 views

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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Do quantum fluctuations in the inflaton field lead to fluctuations in the potential energy density?

During inflation there are quantum fluctuations in the inflaton field. Do quantum fluctuations in the inflaton field lead to fluctuations in the energy density?
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4answers
46 views

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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31 views

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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157 views

Do the electrons arrive at the anode with some kinetic energy? Where does it go?

I'm studying electronics and circuits. I'm trying to understand how electrons behave as they move in a circuit, but there's something that still bothers me. Electrons have some potential energy (the ...
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24 views

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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3answers
434 views

Thermal expansion of solid body and calculation of work produced

From the information about thermal expansion on Wikipedia, I found that if I heat a steel rod that's 1-meter in length and 1-cm in diameter steel rod to 100 degrees Celsius, then it will expand by ...
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1answer
82 views

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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3answers
258 views

Net Force on System Not Equal Zero, but GPE increases and no change in Kinetic Energy

I am confused about changes in gravitational potential energy, net external force, and work-energy theorem. For a single particle system, I understand that a net external force results in a change 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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118 views

What is free energy in the context of a quantum field theory?

I was reading the papers Large $N$ behavior of mass deformed ABJM theory and New 3D ${\cal N}=2$ SCFT's with $N^{3/2}$ scaling. These papers talk about the free energy in the context of quantum field ...
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27 views

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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123 views

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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1answer
218 views

Can the minimum energy state of an Ising-Hamiltonian be negative?

I am simulating 1D many body (N>2) quantum chains, governed by Ising-Hamiltonians. More on the Ising-Hamiltonian. My simulation generates the Hamiltonian based on the following formula: $$\hat H = J\...
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71 views

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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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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1answer
40 views

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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7answers
86 views

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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2answers
465 views

Do gravitational waves carry energy?

We now know that gravitational waves are real. Do they carry energy? I mean thy are waves and every wave carries energy. If they carry energy, how do they do it? Is there any equation to describe ...
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1answer
19 views

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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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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829 views

Total energy of neutrons and protons

In a stable nucleus, are the total energies of neutrons and protons same?
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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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2answers
56 views

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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Bungee Jumping - Why do both of my methods get the same wrong answer? [closed]

An exciting outdoor activity is bungee jumping. Model the ropes as a spring with effective spring constant $k$. Assume you jump from a distance $h$ above the surface of the Earth, the unstretched ...
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32 views

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
51 views

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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64 views

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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244 views

The difference between Gibbs and grand canonical potentials

To my understanding, Helmholtz free energy is a measure of the amount of work a thermodynamical system can perform. Following the same reasoning Gibbs and Grand-canonical free energies measure the the ...
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1answer
21 views

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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2answers
39 views

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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2answers
519 views

At what angle do billiard balls scatter if they collide off center?

The angle defined by joining a line from the centers of the balls must be important. But do they follow this angle when viewed in the rest frame of one of the balls or in the CM frame? The spheres ...
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1answer
2k views

How many calories in a block of wood?

I was recently thinking about the human body and the energy we get from food. I understand we don't have the ability to properly digest certain things like stones and wood and grass, but if we did, I ...
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4answers
7k views

Change in internal energy is 0 in isothermal process

I am having trouble understanding why in an isothermal process, the change in internal energy is zero. I know that $\Delta U$ or $\Delta E=q+w$, and so in isothermal process $q=0$. But how does one ...
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0answers
43 views

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
193 views

oscillating piston shock waves

I am interesting to know whether there are analytical solutions for a piston gonging like a sine wave and generated shock wave and rarefaction. How the energy change during this process and how can we ...
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3answers
62 views

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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38 views

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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1answer
38 views

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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3answers
127 views

$\Delta K=\Delta U$ vs $\Delta K = -\Delta U$

While doing physics homework, I noticed that in some problems, the change in kinetic energy(KE) is equal to the change in potential energy(PE), even though I learned that conservation of energy shows ...
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2answers
2k views

What arrangement of sound waves would be needed to heat air in a typical sized room?

From what I understand, sound is simply the jostling of the molecules that make up the air in a specific pattern, widely known as waves. I also know that these are longitudinal waves. If we were to ...
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1answer
58 views

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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4answers
23k views

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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17 views

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
286 views

How much heat can be adsorbed in a solar parabolic trough?

How do I know the produced heat on the focal point of a solar parabolic trough per different radius if made from steel and also from glass?
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Is $E^2=(mc^2)^2+(pc)^2$ correct, or is $E=mc^2$ the correct one?

I have been having trouble distinguishing these two equations and figuring out which one is correct. I have watched a video that says that $E^2=(mc^2)^2+(pc)^2$ is correct, but I do not know why. It ...
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1answer
39 views

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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26 views

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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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 ...